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第30行： + − |title= Human Population and the Environmental Crisis+ − 人口与环境危机+ + + + + + + 第40行：
第47行： − + − 正反馈容易导致系统不稳定。当环增益为正且大于1时，通常会出现指数增长、振荡加剧、混沌行为或其它偏离平衡的现象。结果对起源过程的反馈可以是直接的，也可以是通过其他状态变量。因此，正反馈的一个关键特征就是小扰动变大。当系统发生变化时，正反馈会引起同一方向的进一步变化。+ − |author2 = David Jefferson+ − |name-list-style = amp − |publisher = Jones & Bartlett Learning + − causal functions.]]+ − [因果函数]+ − |year = 1996+ + − |isbn = 9780867209662+ − A simple feedback loop is shown in the diagram. If the loop gain AB is positive, then a condition of positive or regenerative feedback exists.+ − 一个简单的反馈环路如图所示。 如果环增益AB为正值，则存在正反馈或再生反馈的条件。+ + + + + + − |page = 42 − |url = https://books.google.com/books?id=a1gW4uV-q8EC&pg=PA42+ + − If the functions A and B are linear and AB is smaller than unity, then the overall system gain from the input to output is finite, but can be very large as AB approaches unity. In that case, it can be shown that the overall or "closed loop" gain from input to output is:+ − 如果函数A和B是线性的，且AB小于1，那么从输入到输出的整体系统增益是有限的，但当AB接近1时，系统增益可能非常大。 在这种情况下，可以证明从输入到输出的整体或 "闭环 "增益为：。+ + − |url-status = live + − |archiveurl = https://web.archive.org/web/20180106192002/https://books.google.com/books?id=a1gW4uV-q8EC&pg=PA42+ − + + + − [ math > g _ c = a/(1-AB)]+ − − |archivedate = 2018-01-06 − − }}</ref> That is, '''''A''' produces more of '''B''' which in turn produces more of '''A'''''.<ref name="culturalanthropology2nded">Keesing, R.M. (1981). Cultural anthropology: A contemporary perspective (2nd ed.) p.149. Sydney: Holt, Rinehard & Winston, Inc.</ref> In contrast, a system in which the results of a change act to reduce or counteract it has [[negative feedback]].<ref name=zuckerman/><ref name=theorymodelling/> Both concepts play an important role in science and engineering, including biology, chemistry, and [[cybernetics]] .<br> − − 也就是说，产生更多的B，进而产生更多的A。 − 相反，在一个系统中，变化的结果作用于减少或抵消变化，该系统具有负反馈。 − 这两个概念在科学和工程中发挥着重要作用，包括生物学、化学和控制论。 − − − − Mathematically, positive feedback is defined as a positive [[loop gain]] around a closed loop of cause and effect.<ref name=zuckerman/><ref name=theorymodelling> − 数学上，正反馈被定义为围绕一个闭合的因果循环的正环增益。 + − 因此，根据反馈，状态变化可以是收敛的，也可以是发散的。正反馈的结果是增加变化，因此小的扰动可能导致大的变化。 − − {{cite book − − |title = Theory of Modeling and Simulation: Integrating Discrete Event and Continuous Complex Dynamic Systems − 建模与仿真理论:离散事件与连续复杂动态系统的集成 + − 一个处于平衡状态的系统，其当前状态的任何变化都有正反馈，这个系统可能是不稳定的，在这种情况下，该系统被称为处于不稳定平衡状态。使这种系统偏离其平衡状态的力的大小，是状态与平衡状态之间的“距离”的递增函数。+ − − |author1 = Bernard P. Zeigler − − |author2 = Herbert Praehofer − − − |author3 = Tag Gon Kim Section+ + − |publisher = Academic Press+ − |year = 2000+ − − |isbn = 9780127784557 − − |page = 55 − − |section = 3.3.2 Feedback in continuous systems+ − |url = https://books.google.com/books?id=REzmYOQmHuQC&pg=PA55+ − In a [[Schmitt trigger circuit, feedback to the non-inverting input of an amplifier pushes the output directly away from the applied voltage towards the maximum or minimum voltage the amplifier can generate.]]+ − 在施密特触发电路中，对放大器的非反相输入的反馈直接将输出从外加电压推向放大器所能产生的最大或最小电压。+ − |quote = A positive feedback loop is one with an even number of negative influences <nowiki>[around the loop]</nowiki>.+ − 正反馈循环是指负面影响为偶数的循环 − |url-status = live+ − − In the real world, positive feedback loops typically do not cause ever-increasing growth, but are modified by limiting effects of some sort. According to Donella Meadows: − − 在现实世界中，正反馈循环通常不会导致不断增长，而是通过某种限制性作用来改变。根据多内拉 · 梅多斯 Donella Meadows的说法: − − |archiveurl = https://web.archive.org/web/20170103061121/https://books.google.com/books?id=REzmYOQmHuQC&pg=PA55 − − |archivedate = 2017-01-03 − − − }}</ref>+ − − That is, ’’’<font color=”#32CD32”><positive feedback is [[Phase (waves)|in phase with]] the input></font>’’’, in the sense that it adds to make the input larger.<ref> − − 也就是说， ’’’<font color=”#32CD32”><正反馈是相位(波)|与输入的相位></font>’’’，在某种意义上，它增加了并使输入变大。 第167行：
第140行： − {{cite book+ − − |title = Newnes Dictionary of Electronics − 纽恩斯电子词典 − − − |edition = 4th+ − + − |author1 = S W Amos − − |author2 = R W Amos 第190行：
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第167行： − + − 根据此后不久出现的术语混淆:+ − + + + + + + + + + + + + + + + + − “ ... ... Friis和Jensen对Black在 "正反馈 "和 "负反馈 "之间的区分是一样的，不是基于反馈本身的符号，而是基于它对放大器增益的影响。相反，Nyquist和Bode在Black的工作基础上，将负反馈称为符号相反的反馈。Black难以说服其他人相信他的发明的实用性，部分原因是在基本的定义问题上存在混乱。"用于放大和接收非常弱的无线电信号。仔细控制单个晶体管放大器周围的正反馈，可以使其增益增加1000倍或更多。因此，一个信号可以在一个阶段被放大20000甚至100000倍，通常情况下，该信号的增益只有20到50。再生放大器在这些非常高的增益下工作的问题是，它们很容易变得不稳定，开始振荡。无线电操作员必须准备相当连续地调整反馈量，以获得良好的接收效果。现代无线电接收机采用超异构设计，多了许多放大级，但工作更稳定，没有正反馈。+ + + − |url-status = live − |archiveurl = https://web.archive.org/web/20170329045057/https://books.google.com/books?id=lROa-MpIrucC&pg=PA247+ − − |archivedate = 2017-03-29 第227行：
第206行： − − {{cite book − − |title = Modern Dictionary of Electronics − 现代电子词典 第265行：
第239行： − + − 也不是带正反馈的闸门。红色和黑色分别代表逻辑上的’1’和’0’。]+ 第278行：
第252行： − − Positive feedback tends to cause [[Control theory#Stability|system instability]]. When the loop gain is positive and above 1, there will typically be [[exponential growth]], increasing [[oscillation]]s, [[Chaos theory|chaotic behavior]] or other divergences from [[wikt:equilibrium|equilibrium]].<ref name=theorymodelling/> System parameters will typically accelerate towards extreme values, which may damage or destroy the system, or may end with the system [[Latch (electronics)|latched]] into a new stable state. Positive feedback may be controlled by signals in the system being [[Filter (signal processing)|filtered]], [[Damping|damped]], or [[Maxima and minima|limited]], or it can be cancelled or reduced by adding negative feedback. − − 正反馈往往会引起系统不稳定性。当环路增益为正且高于1时，通常会出现指数增长、增加振荡、混沌行为或其他偏离平衡的现象。 − 系统参数通常会向极端值加速，这可能会损坏或破坏系统，或者可能以系统锁存进入新的稳定状态而结束。正反馈可以通过系统中的信号被滤波器过滤、阻尼或最大值和最小值限制来控制，也可以通过增加负反馈来取消或减少。 第288行：
第257行： + + + − Positive feedback is used in [[digital electronics]] to force voltages away from intermediate voltages into '0' and '1' states. On the other hand, [[thermal runaway]] is a type of positive feedback that can destroy [[p–n junction|semiconductor junctions]]. Positive feedback in [[chemical reactions]] can increase the rate of reactions, and in some cases can lead to [[explosives|explosions]]. Positive feedback in mechanical design causes [[Tipping point (physics)|tipping-point]], or 'over-centre', mechanisms to snap into position, for example in [[Miniature snap-action switch|switches]] and [[locking pliers]]. Out of control, it can cause [[Tacoma Narrows Bridge (1940)|bridges to collapse]]. Positive feedback in economic systems can cause [[Economic boom|boom-then-bust cycles]]. A familiar example of positive feedback is the loud squealing or howling sound produced by [[audio feedback]] in [[public address|public address systems]]: the microphone picks up sound from its own loudspeakers, amplifies it, and sends it through the speakers again.+ − 正反馈用于数字电子，以迫使电压远离中间电压，进入'0'和'1'状态。另一方面，热失控是一种可以破坏p-n结构的正反馈。在化学反应中的正反馈可以增加反应速度，在某些情况下可以导致爆炸物爆炸。机械设计中的正反馈会导致倾点（物理学），或 "过中心"，机械装置突然就位，例如在微型快动开关和锁紧钳中。失去控制，会导致塔科马窄桥（1940）桥梁倒塌。经济系统中的正反馈会造成经济繁荣|繁荣-再繁荣-再萧条的循环。 正反馈的一个熟悉的例子是公共广播中的音频反馈产生的响亮的尖叫声或嚎叫声：麦克风从自己的扬声器中拾取声音，将其放大，然后再通过扬声器发送。+ − A phonograph turntable is prone to acoustic feedback.+ + − 留声机转盘容易受到声反馈的影响。+ + + − [[File:Positive Feedback Diagram (2).svg|thumb|[[Platelet]] clotting demonstrates positive feedback. The damaged blood vessel wall releases chemicals that initiate the formation of a blood clot through platelet congregation. As more platelets gather, more chemicals are released that speed up the process. The process gets faster and faster until the blood vessel wall is completely sealed and the positive feedback loop has ended. The exponential form of the graph illustrates the positive feedback mechanism. ]]+ − − 血小板凝血表现为正反馈。受损的血管壁会释放化学物质，通过血小板聚集启动血栓的形成。随着越来越多的血小板聚集，更多的化学物质被释放出来，加快了这个过程。这个过程越来越快，直到血管壁完全密封，正反馈循环结束。 图中的指数形式说明了正反馈机制。 − Audio and video systems can demonstrate positive feedback. If a microphone picks up the amplified sound output of loudspeakers in the same circuit, then howling and screeching sounds of audio feedback (at up to the maximum power capacity of the amplifier) will be heard, as random noise is re-amplified by positive feedback and filtered by the characteristics of the audio system and the room. − 音频和视频系统可以表现出正反馈。如果麦克风拾取同一电路中扬声器的放大声音输出，那么就会听到音频反馈的嚎叫和尖叫声（在放大器的最大功率容量下），因为随机噪声被正反馈重新放大，并被音频系统和房间的特性所过滤。+ − == Overview ==+ − 总览 − Positive feedback enhances or amplifies an effect by it having an influence on the process which gave rise to it. For example, when part of an electronic output signal returns to the input, and is in phase with it, the system [[Gain (electronics)|gain]] is increased.<ref>{{cite web|title=Positive feedback|url=http://www.oxforddictionaries.com/definition/english/positive-feedback|work=Oxford English Dictionary|publisher=Oxford University Press|accessdate=15 April 2014|url-status=live|archiveurl=https://web.archive.org/web/20140302160045/http://www.oxforddictionaries.com/definition/english/positive-feedback|archivedate=2 March 2014}}<br> − 正反馈通过对产生该效应的过程产生影响来增强或放大该效应。例如，当部分电子输出信号返回到输入端，并与输入端同相，系统增益就会增加。+ + − </ref> The feedback from the outcome to the originating process can be direct, or it can be via other state variables.<ref name=theorymodelling/> Such systems can give rich qualitative behaviors, but whether the feedback is instantaneously positive or negative in sign has an extremely important influence on the results. − 结果对始发过程的反馈可以是直接的，也可以通过其他状态变量。 − 这样的系统可以给出丰富的定性行为，但反馈的信号是瞬时的正向还是负向，对结果有极其重要的影响。 − <ref name=theorymodelling/> Positive feedback reinforces and negative feedback moderates the original process. ''Positive'' and ''negative'' in this sense refer to loop gains greater than or less than zero, and do not imply any [[value judgement]]s as to the desirability of the outcomes or effects.<ref>{{cite web|title=Feedback|url=http://metadesigners.org/Feedback-Glossary|work=Glossary|publisher=Metadesigners Network|accessdate=15 April 2014|url-status=live|archiveurl=https://web.archive.org/web/20140416183720/http://metadesigners.org/Feedback-Glossary|archivedate=16 April 2014}}</ref> A key feature of positive feedback is thus that small disturbances get bigger. When a change occurs in a system, positive feedback causes further change, in the same direction. − 正反馈强化原过程，负反馈调节原过程。''正''和''负''在这个意义上指的是大于或小于零的循环收益，并不意味着对结果或效果的可取性有任何价值判断。+ − 因此，正反馈的一个重要特点是小扰动变大。当系统发生变化时，正反馈会引起进一步的变化，而且是同方向的变化。+ − Audio feedback (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive feedback which occurs when a sound loop exists between an audio input (for example, a microphone or guitar pickup) and an audio output (for example, a loudly-amplified loudspeaker). In this example, a signal received by the microphone is amplified and passed out of the loudspeaker. The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again. The frequency of the resulting sound is determined by resonance frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them. For small PA systems the sound is readily recognized as a loud squeal or screech.+ − 音频反馈（也称为声反馈，简称反馈，或拉森效应）是一种特殊的正反馈，当音频输入（例如，麦克风或吉他拾音器）和音频输出（例如，大声放大的扬声器）之间存在声音回路时，就会出现这种反馈。在这个例子中，麦克风接收到的信号被放大并从扬声器传出。然后，来自扬声器的声音可以再次被麦克风接收，进一步放大，然后再次通过扬声器传递出去。 所产生的声音的频率由传声器、放大器和扬声器的共振频率、房间的声学特性、传声器和扬声器的定向拾音和发射模式以及它们之间的距离决定。对于小型的扩声系统来说，这种声音很容易被识别为响亮的吱吱声或尖叫声。+ + + − === Basic ===+ − 基础 − Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a sound reinforcement system or PA system. Audio engineers use various electronic devices, such as equalizers and, since the 1990s, automatic feedback detection devices to prevent these unwanted squeals or screeching sounds, which detract from the audience's enjoyment of the event. On the other hand, since the 1960s, electric guitar players in rock music bands using loud guitar amplifiers and distortion effects have intentionally created guitar feedback to create a desirable musical effect. "I Feel Fine" by the Beatles marks one of the earliest examples of the use of feedback as a recording effect in popular music. It starts with a single, percussive feedback note produced by plucking the A string on Lennon's guitar. Artists such as the Kinks and the Who had already used feedback live, but Lennon remained proud of the fact that the Beatles were perhaps the first group to deliberately put it on vinyl. In one of his last interviews, he said, "I defy anybody to find a record—unless it's some old blues record in 1922—that uses feedback that way."+ − 在使用扩声系统或扩音系统的活动中，当歌手或公众演讲者的麦克风发生反馈时，几乎总是被认为是不受欢迎的。音频工程师使用各种电子设备，如均衡器，自20世纪90年代以来，自动反馈检测设备，以防止这些不需要的尖叫声或尖叫声，这些声音影响了观众对活动的享受。另一方面，自20世纪60年代以来，摇滚乐队中的电吉他手使用大音量的吉他放大器和失真效果，有意制造吉他反馈，以创造理想的音乐效果。 披头士乐队的 "I Feel Fine "标志着流行音乐中最早使用反馈作为录音效果的例子之一。它的开头是由Lennon拨动吉他上的A弦产生的一个单一的、有冲击力的反馈音。像 Kinks 和 Who 等艺术家已经在现场使用了反馈，但是Lennon仍然为披头士乐队可能是第一个特意把它放在黑胶唱片上的乐队而感到骄傲。在他最后的一次采访中，他说，“我认为任何人都不可能找到一张唱片——除非它是1922年的一张旧布鲁斯唱片——那样使用反馈。”+ + − [[File:Ideal feedback model.svg|thumb|A basic feedback system can be represented by this block diagram. In the diagram the + symbol is an adder and A and B are arbitrary [[causal system|causal]] functions.]] − 一个基本的反馈系统可以用这个框图来表示。在图中，+号是加法器，A和B是任意因果函数。 − The principles of audio feedback were first discovered by Danish scientist Søren Absalon Larsen. Microphones are not the only transducers subject to this effect. Record deck pickup cartridges can do the same, usually in the low frequency range below about 100 Hz, manifesting as a low rumble. Jimi Hendrix was an innovator in the intentional use of guitar feedback in his guitar solos to create unique sound effects. He helped develop the controlled and musical use of audio feedback in electric guitar playing, and later Brian May was a famous proponent of the technique.+ − 音频反馈的原理是由丹麦科学家Søren Absalon Larsen首先发现的。麦克风并不是唯一受此影响的换能器。录音板拾音器也可以做到这一点，通常在100赫兹以下的低频范围内，表现为低沉的轰鸣声。Jimi Hendrix是一个创新者，在他的吉他独奏中有意使用吉他反馈来创造独特的声音效果。他帮助开发了电吉他演奏中音频反馈的可控性和音乐性，后来Brian May也是这种技术的著名支持者。 − A simple feedback loop is shown in the diagram. If the loop gain AB is positive, then a condition of ''positive'' or ''regenerative'' feedback exists.+ − 图中是一个简单的反馈回路。 如果环增益AB为正值，则存在'正'或'再生'反馈的条件。 + + − [[Video feedback.]] − [[视频反馈] − If the functions A and B are linear and AB is smaller than unity, then the overall system gain from the input to output is finite, but can be very large as AB approaches unity.<ref name=smith> Electronics circuits and devices second edition. Ralph J. Smith</ref> In that case, it can be shown that the overall or "closed loop" gain from input to output is:+ − 如果函数A和B是线性的，且AB小于1，那么系统从输入到输出的整体增益是有限的，但当AB接近1时，系统的增益可以非常大。 − 在这种情况下，可以表明从输入到输出的整体或 "闭环 "增益为。+ − :<math>G_c = A/(1-AB)</math>+ − Similarly, if a video camera is pointed at a monitor screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to the first ten series of the television program Doctor Who.+ − 同样，如果一台摄像机对准一个正在显示摄像机自身信号的监控屏幕，那么通过正反馈就可以在屏幕上形成重复的图案。这种视频反馈效果在电视节目《神秘博士》前十集的开场白中就被使用了。+ + + − When AB > 1, the system is unstable, so does not have a well-defined gain; the gain may be called infinite.+ − 当AB>1时，系统是不稳定的，所以不具有明确的增益，增益可称为无限。+ + − In electrical switches, including bimetallic strip based thermostats, the switch usually has hysteresis in the switching action. In these cases hysteresis is mechanically achieved via positive feedback within a tipping point mechanism. The positive feedback action minimises the length of time arcing occurs for during the switching and also holds the contacts in an open or closed state.+ − 在电气开关中，包括双金属带式温控器，开关的开关动作通常具有滞后性。在这些情况下，迟滞是通过一个临界点机构内的正反馈来实现的。正反馈作用可最大限度地减少开关过程中发生电弧的时间，并使触头保持在打开或关闭状态。 − Thus depending on the feedback, state changes can be convergent, or divergent. The result of positive feedback is to [[wikt:augment|augment]] changes, so that small perturbations may result in big changes.+ − 因此根据反馈，状态变化可以是收敛的，也可以是发散的。 正反馈的结果是增强变化，因此小的扰动可能导致大的变化。 + − A system in equilibrium in which there is positive feedback to any change from its current state may be unstable, in which case the system is said to be in an [[unstable equilibrium]]. The magnitude of the forces that act to move such a system away from its equilibrium are an [[increasing function]] of the "distance" of the state from the equilibrium.+ − 一个处于平衡状态的系统，其当前状态的任何变化都有正反馈，它可能是不稳定的，在这种情况下，系统被称为不稳定平衡。使这种系统远离其平衡状态的力的大小是状态与平衡状态之间的距离的递增函数。 + − Positive feedback is the amplification of a body's response to a stimulus. For example, in childbirth, when the head of the fetus pushes up against the cervix (1) it stimulates a nerve impulse from the cervix to the brain (2). When the brain is notified, it signals the pituitary gland to release a hormone called [[oxytocin(3). Oxytocin is then carried via the bloodstream to the uterus (4) causing contractions, pushing the fetus towards the cervix eventually inducing childbirth.]] − 正反馈是指身体对刺激的反应的放大。例如，在分娩过程中，当胎儿的头顶到子宫颈时(1)，它刺激了从子宫颈到大脑的神经冲动(2)。大脑接到通知后，会向脑垂体发出信号，释放一种叫做催产素(3)的激素。催产素随后通过血液输送到子宫(4)，引起宫缩，将胎儿推向子宫颈，最终诱发分娩。+ − Positive feedback does not necessarily imply instability of an equilibrium, for example stable ''on'' and ''off'' states may exist in positive-feedback architectures.<ref name="ReferenceA">{{cite journal|last1=Lopez-Caamal|first1=Fernando|last2=Middleton|first2=Richard H.|last3=Huber|first3=Heinrich|title=Equilibria and stability of a class of positive feedback loops|journal=Journal of Mathematical Biology|date=February 2014|pages=609–645|doi = 10.1007/s00285-013-0644-z|pmid=23358701|volume=68|issue=3}}</ref>+ − 正反馈并不一定意味着平衡的不稳定性，例如，在正反馈结构中可能存在稳定的开关状态。 − + − 磁滞现象+ − {{main|Hysteresis}} − A number of examples of positive feedback systems may be found in physiology. − 在生理学中可以找到许多正反馈系统的例子。+ + + + + + − [[File:Hysteresis sharp curve.svg|thumb|Hysteresis causes the output value to depend on the history of the input]] − 迟滞会导致输出值取决于输入的历史记录 − + − 在施密特触发器电路中，对放大器的非反相输入的反馈直接将输出从外加电压推向放大器所能产生的最大或最小电压。+ − In the real world, positive feedback loops typically do not cause ever-increasing growth, but are modified by limiting effects of some sort. According to [[Donella Meadows]]:+ − 在现实世界中，正反馈循环通常不会引起不断增长，而是通过某种限制效应来改变。根据Donella Meadows的说法：+ − A cytokine storm, or hypercytokinemia is a potentially fatal immune reaction consisting of a positive feedback loop between cytokines and immune cells, with highly elevated levels of various cytokines. In normal immune function, positive feedback loops can be utilized to enhance the action of B lymphocytes. When a B cell binds its antibodies to an antigen and becomes activated, it begins releasing antibodies and secreting a complement protein called C3. Both C3 and a B cell's antibodies can bind to a pathogen, and when a B cell has its antibodies bind to a pathogen with C3, it speeds up that B cell's secretion of more antibodies and more C3, thus creating a positive feedback loop.+ − 细胞因子风暴，或称高细胞因子血症，是一种潜在的致命性免疫反应，由细胞因子和免疫细胞之间的正反馈循环组成，各种细胞因子水平高度升高。在正常的免疫功能中，可以利用正反馈循环来增强B淋巴细胞的作用。当B细胞将其抗体与抗原结合并被激活后，就开始释放抗体，并分泌一种补体蛋白，即C3。C3和B细胞的抗体都可以与病原体结合，当B细胞的抗体与C3结合后，就会加快该B细胞分泌更多的抗体和更多的C3，从而形成一个正反馈循环。+ + + − ::"Positive feedback loops are sources of growth, explosion, erosion, and collapse in systems. A system with an unchecked positive loop ultimately will destroy itself. That’s why there are so few of them. Usually a negative loop will kick in sooner or later."<ref name=meadows> − “正反馈循环是系统增长、爆炸、侵蚀和崩溃的源头。如果一个系统的正循环不受控制，最终将会自我毁灭。这就是为什么很少有这样的系统。通常情况下，负面循环迟早会发生。” − Donella Meadows, [http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf ''Leverage Points: Places to Intervene in a System''] {{webarchive|url=https://web.archive.org/web/20131008160618/http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf |date=2013-10-08 }}, 1999</ref>+ + + + − + + − Hysteresis, in which the starting point affects where the system ends up, can be generated by positive feedback. When the gain of the feedback loop is above 1, then the output moves away from the input: if it is above the input, then it moves towards the nearest positive limit, while if it is below the input then it moves towards the nearest negative limit. − 磁滞现象，即起点影响系统的终点，可以通过正反馈产生。当反馈循环的增益高于1时，那么输出就会远离输入：如果高于输入，则向最近的正极限移动，而如果低于输入，则向最近的负极限移动。+ − + − Once it reaches the limit, it will be stable. However, if the input goes past the limit,{{clarify|date=June 2012}} then the feedback will change sign{{dubious|date=June 2012}} and the output will move in the opposite direction until it hits the opposite limit. The system therefore shows [[bistability|bistable]] behaviour. − 一旦达到极限，它就会稳定下来。但是，如果输入超过极限，那么反馈将改变符号，输出将向相反的方向移动，直到达到相反的极限。因此，该系统表现出双稳态行为。 − + − + − + − 术语学+ − The terms ''positive'' and ''negative'' were first applied to feedback before [[World War II]]. The idea of positive feedback was already current in the 1920s with the introduction of the [[regenerative circuit]].<ref name=mindell> − {{Cite book − According to the theory of reflexivity advanced by George Soros, price changes are driven by a positive feedback process whereby investors' expectations are influenced by price movements so their behaviour acts to reinforce movement in that direction until it becomes unsustainable, whereupon the feedback drives prices in the opposite direction.+ + − 根据乔治•索罗斯 George Soros提出的反射性理论，价格变化是由一个正反馈过程驱动的，即投资者的预期受到价格变动的影响，因此他们的行为会强化这个方向的运动，直到它变得不可持续，于是反馈推动价格向相反的方向发展。+ + − |first = David A.+ + − |last = Mindell+ − |title = Between Human and Machine : Feedback, Control, and Computing before Cybernetics+ − |date = 2002+ + 第489行：
第457行： − + − + − |archiveurl = https://web.archive.org/web/20180106192002/https://books.google.com/books?id=sExvSbe9MSsC − − |archivedate = 2018-01-06 − − |isbn = 9780801868955 第508行：
第471行： − − {{harvtxt|Friis|Jensen|1924}} described regeneration in a set of electronic amplifiers as a case where ''the "feed-back" action is positive'' in contrast to negative feed-back action, which they mention only in passing.<ref name="friis">{{Citation |last1=Friis |first1=H. T. |first2=A. G. |last2=Jensen |title=High Frequency Amplifiers |journal=Bell System Technical Journal |volume=3 |issue= 2|date=April 1924 |pages=181–205 |doi=10.1002/j.1538-7305.1924.tb01354.x}}</ref> [[Harold Stephen Black]]'s classic 1934 paper first details the use of negative feedback in electronic amplifiers. According to Black: 第515行：
第476行： − + − ::"Positive feed-back increases the gain of the amplifier, negative feed-back reduces it."<ref name=black> − − 正反馈会增加放大器的增益，负反馈会降低增益 − − {{Citation |first=H. S. |last=Black |title=Stabilized feed-back amplifiers |journal=Electrical Engineering |volume=53 |issue= |pages=114–120 |date=January 1934 |doi=10.1109/ee.1934.6540374}}</ref> − − According to {{harvtxt|Mindell|2002}} confusion in the terms arose shortly after this: − − 根据 harvtxt Mindell（2002）的说法，这之后不久就出现了术语上的混乱。 − − ::"...Friis and Jensen had made the same distinction Black used between 'positive feed-back' and 'negative feed-back', based not on the sign of the feedback itself but rather on its effect on the amplifier’s gain. In contrast, Nyquist and Bode, when they built on Black’s work, referred to negative feedback as that with the sign reversed. Black had trouble convincing others of the utility of his invention in part because confusion existed over basic matters of definition."<ref name=mindell/>{{rp|page=121}} − − “ ... ... Friis和Jensen对Black在 "正反馈 "和 "负反馈 "之间的区分是一样的，不是基于反馈本身的符号，而是基于它对放大器增益的影响。相反，Nyquist和Bode在Black的工作基础上，将负反馈称为符号相反的反馈。Black难以说服其他人相信他的发明的实用性，部分原因是在基本的定义问题上存在混乱。 第535行：
第483行： − + − − 实例与应用 − − − − 在电子领域 第547行：
第489行： − + − − [[File:Regenerartive Receiver-S7300056.JPG|thumb|right|A vintage style regenerative radio receiver. Due to the controlled use of positive feedback, sufficient amplification can be derived from a single [[vacuum tube]] or valve (centre).]] − − − − [[Regenerative circuit]]s were invented and patented in 1914<ref>{{cite patent |inventor-last=Armstrong |inventor-first=E. H. |country-code=US |patent-number=1113149 |title=Wireless receiving system |date=1914}}</ref> for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single [[transistor]] amplifier can multiply its [[Gain (electronics)|gain]] by 1,000 or more.<ref>{{cite web|last=Kitchin|first=Charles|title=A Short Wave Regenerative Receiver Project|url=http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|accessdate=23 September 2010|url-status=live|archiveurl=https://web.archive.org/web/20100710100031/http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|archivedate=10 July 2010}}</ref> Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the [[superheterodyne]] design, with many more amplification stages, but much more stable operation and no positive feedback. − − "用于放大和接收非常弱的无线电信号。仔细控制单个晶体管放大器周围的正反馈，可以使其增益增加1000倍或更多。因此，一个信号可以在一个阶段被放大20000甚至100000倍，通常情况下，该信号的增益只有20到50。再生放大器在这些非常高的增益下工作的问题是，它们很容易变得不稳定，开始振荡。无线电操作员必须准备相当连续地调整反馈量，以获得良好的接收效果。现代无线电接收机采用超异构设计，多了许多放大级，但工作更稳定，没有正反馈。 − − − The oscillation that can break out in a regenerative radio circuit is used in [[electronic oscillator]]s. By the use of [[tuned circuit]]s or a [[piezoelectricity|piezoelectric]] [[crystal]] (commonly [[quartz]]), the signal that is amplified by the positive feedback remains linear and [[Sine wave|sinusoidal]]. There are several designs for such [[harmonic oscillator]]s, including the [[Armstrong oscillator]], [[Hartley oscillator]], [[Colpitts oscillator]], and the [[Wien bridge oscillator]]. They all use positive feedback to create oscillations.<ref>{{cite web|title=Sinewave oscillators|url=http://www.educypedia.be/electronics/analogosciltypes.htm|work=EDUCYPEDIA - electronics|accessdate=23 September 2010|url-status=dead|archiveurl=https://web.archive.org/web/20100927094330/http://www.educypedia.be/electronics/analogosciltypes.htm|archivedate=27 September 2010}}</ref> − − − − Many electronic circuits, especially amplifiers, incorporate [[negative feedback]]. This reduces their gain, but improves their linearity, [[input impedance]], [[output impedance]], and [[Bandwidth (signal processing)|bandwidth]], and stabilises all of these parameters, including the closed-loop gain. These parameters also become less dependent on the details of the amplifying device itself, and more dependent on the feedback components, which are less likely to vary with manufacturing tolerance, age and temperature. The difference between positive and negative feedback for [[Alternating current|AC]] signals is one of [[Phase (waves)|phase]]: if the signal is fed back out of phase, the feedback is negative and if it is in phase the feedback is positive. One problem for amplifier designers who use negative feedback is that some of the components of the circuit will introduce [[Phase (waves)#Phase shift|phase shift]] in the feedback path. If there is a frequency (usually a high frequency) where the phase shift reaches 180°, then the designer must ensure that the amplifier gain at that frequency is very low (usually by [[low-pass filter]]ing). If the [[loop gain]] (the product of the amplifier gain and the extent of the positive feedback) at any frequency is greater than one, then the amplifier will oscillate at that frequency ([[Barkhausen stability criterion]]). Such oscillations are sometimes called [[parasitic oscillation]]s. An amplifier that is stable in one set of conditions can break into parasitic oscillation in another. This may be due to changes in temperature, supply voltage, adjustment of front-panel controls, or even the proximity of a person or other conductive item. 第572行：
第499行： + + − Amplifiers may oscillate gently in ways that are hard to detect without an [[oscilloscope]], or the oscillations may be so extensive that only a very distorted or no required signal at all gets through, or that damage occurs. Low frequency parasitic oscillations have been called 'motorboating' due to the similarity to the sound of a low-revving exhaust note.<ref>{{cite book|last=Self|first=Douglas|title=Audio Power Amplifier Design Handbook|year=2009|publisher=Focal Press|isbn=978-0-240-52162-6|pages=254–255|url=https://books.google.com/books?id=Qpmi4ia2nhcC&pg=PA254&lpg=PA254#v=onepage|url-status=live|archiveurl=https://web.archive.org/web/20140129111458/http://books.google.com/books?id=Qpmi4ia2nhcC&pg=PA254&lpg=PA254#v=onepage&q&f=false|archivedate=2014-01-29}}</ref>+ − + − [[File:Smitt hysteresis graph.svg|thumb|right|The effect of using a Schmitt trigger (B) instead of a comparator (A)]]+ + + − Many common [[Digital electronics|digital electronic]] circuits employ positive feedback. While normal simple boolean [[logic gate]]s usually rely simply on gain to push digital signal voltages away from intermediate values to the values that are meant to represent [[Boolean logic|boolean]] '0' and '1', but many more complex gates use feedback. When an input voltage is expected to vary in an [[Analogue electronics|analogue]] way, but sharp thresholds are required for later digital processing, the [[Schmitt trigger]] circuit uses positive feedback to ensure that if the input voltage creeps gently above the threshold, the output is forced smartly and rapidly from one logic state to the other. One of the corollaries of the Schmitt trigger's use of positive feedback is that, should the input voltage move gently down again past the same threshold, the positive feedback will hold the output in the same state with no change. This effect is called [[hysteresis]]: the input voltage has to drop past a different, lower threshold to 'un-latch' the output and reset it to its original digital value. By reducing the extent of the positive feedback, the hysteresis-width can be reduced, but it can not entirely be eradicated. The Schmitt trigger is, to some extent, a [[Latch (electronics)|latching]] circuit.<ref>{{cite web|title=CMOS Schmitt Trigger—A Uniquely Versatile Design Component|url=http://www.fairchildsemi.com/an/AN/AN-140.pdf|work=Fairchild Semiconductor Application Note 140|publisher=Fairchild Semiconductors|accessdate=29 September 2010|year=1975|url-status=live|archiveurl=https://web.archive.org/web/20101122185614/http://fairchildsemi.com/an/AN/AN-140.pdf|archivedate=22 November 2010}}</ref>+ 第588行：
第519行： − + − − [[File:Positive feedback bistable switch.svg|thumb|Positive feedback is a mechanism by which an output is enhanced, such as protein levels. However, in order to avoid any fluctuation in the protein level, the mechanism is inhibited stochastically (I), therefore when the concentration of the activated protein (A) is past the threshold ([I]), the loop mechanism is activated and the concentration of A increases exponentially if d[A]=k [A]]] − − − − [[File:R-S mk2.gif|thumb|right|Illustration of an R-S ('reset-set') flip-flop made from two digital [[NOR gate|nor]] gates with positive feedback. Red and black mean logical '1' and '0', respectively.]] − − − − An electronic [[flip-flop (electronics)|flip-flop]], or "latch", or "bistable [[multivibrator]]", is a circuit that due to high positive feedback is not stable in a balanced or intermediate state. Such a bistable circuit is the basis of one [[bit]] of electronic [[Computer memory|memory]]. The flip-flop uses a pair of amplifiers, transistors, or logic gates connected to each other so that positive feedback maintains the state of the circuit in one of two unbalanced stable states after the input signal has been removed, until a suitable alternative signal is applied to change the state.<ref>{{cite web|last=Strandh|first=Robert|title=Latches and flip-flops|url=http://www.labri.fr/perso/strandh/Teaching/AMP/Common/Strandh-Tutorial/flip-flops.html|publisher=Laboratoire Bordelais de Recherche en Informatique|accessdate=4 November 2010|url-status=live|archiveurl=https://web.archive.org/web/20110716085637/http://www.labri.fr/perso/strandh/Teaching/AMP/Common/Strandh-Tutorial/flip-flops.html|archivedate=16 July 2011}}</ref> Computer [[random access memory]] (RAM) can be made in this way, with one latching circuit for each bit of memory.<ref>{{cite web|last=Wayne|first=Storr|title=Sequential Logic Basics: SR Flip-Flop|url=http://www.electronics-tutorials.ws/sequential/seq_1.html|publisher=Electronics-Tutorials.ws|accessdate=29 September 2010|url-status=live|archiveurl=https://web.archive.org/web/20100916114700/http://www.electronics-tutorials.ws/sequential/seq_1.html|archivedate=16 September 2010}}</ref> 第608行：
第529行： − + − − [[Thermal runaway]] occurs in electronic systems because some aspect of a circuit is allowed to pass more current when it gets hotter, then the hotter it gets, the more current it passes, which heats it some more and so it passes yet more current. The effects are usually catastrophic for the device in question. If devices have to be used near to their maximum power-handling capacity, and thermal runaway is possible or likely under certain conditions, improvements can usually be achieved by careful design.<ref>{{cite web|last=Sharma|first=Bijay Kumar|title=Analog Electronics Lecture 4 Part C RC coupled Amplifier Design Procedure|url=http://cnx.org/content/m31058/latest/|accessdate=29 September 2010|year=2009}}</ref> − − 热失控在电子系统中发生的原因是，当电路的某些方面变得更热时，允许它通过更多的电流，然后它越热，通过的电流就越多，这就使它更热一些，所以它又通过更多的电流。这种影响对有关器件来说通常是灾难性的。如果设备必须在接近其最大功率处理能力的情况下使用，并且在某些条件下可能或可能出现热失控，通常可以通过精心设计来改进。 − − − [[File:Technics SL-1210MK2.jpg|thumb|left|A phonograph turntable is prone to acoustic feedback.]] − − 留声机转盘容易发生声响，因此，在使用过程中要注意以下几点 第623行：
第535行： + − − [[Sound recording and reproduction|Audio]] and [[video]] systems can demonstrate positive feedback. If a [[microphone]] picks up the amplified sound output of [[loudspeaker]]s in the same circuit, then howling and screeching sounds of [[audio feedback]] (at up to the maximum power capacity of the amplifier) will be heard, as random noise is re-amplified by positive feedback and [[Filter (signal processing)|filtered]] by the characteristics of the audio system and the room. − − 声音记录和重现音频和视频系统可以展示正反馈。如果传声器接收到同一电路中扬声器的放大声音输出，那么音频反馈的嚎叫和尖叫声就会出现。(在放大器的最大功率容量下)会听到，因为随机噪声被正反馈和滤波器(信号处理)过滤的音频系统和房间的特性重新放大。 − − − ===Audio and live music=== − 音频和现场音乐 − 许多野生动物被猎杀的原因是它们的部位可能相当有价值。目标物种越是接近灭绝，其部位的价格就越高。这就是一个正反馈的例子。+ − − [[Audio feedback]] (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive feedback which occurs when a sound loop exists between an audio input (for example, a [[microphone]] or [[guitar pickup]]) and an audio output (for example, a loudly-amplified [[loudspeaker]]). In this example, a signal received by the microphone is [[Amplifier|amplified]] and passed out of the loudspeaker. The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again. The [[frequency]] of the resulting sound is determined by resonance frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them. For small [[PA system]]s the sound is readily recognized as a loud squeal or screech. − − 音频反馈(也称为声反馈，简称为反馈，或拉森效应)是一种特殊的正反馈，当音频输入(例如麦克风或吉他拾音器)和音频输出(例如经过大声放大的扬声器)之间存在声音回路时，就会出现这种反馈。在这个例子中，麦克风接收到的信号被放大器放大并从扬声器中传出。然后，来自扬声器的声音可以再次被传声器接收，进一步放大，然后再次通过扬声器传递出去。 所产生的声音的频率由传声器、放大器和扬声器的共振频率、房间的声学特性、传声器和扬声器的定向拾取和发射模式以及它们之间的距离决定。对于小型扩音系统来说，这种声音很容易被识别为响亮的尖叫声或尖锐的声音。 − − − − Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a [[sound reinforcement system]] or [[PA system]]. [[Audio engineer]]s use various electronic devices, such as equalizers and, since the 1990s, automatic feedback detection devices to prevent these unwanted squeals or screeching sounds, which detract from the audience's enjoyment of the event. On the other hand, since the 1960s, [[electric guitar]] players in [[rock music]] bands using loud [[guitar amplifier]]s and [[distortion (music)|distortion]] effects have intentionally created guitar feedback to create a desirable musical effect. "I Feel Fine" by the Beatles marks one of the earliest examples of the use of feedback as a recording effect in popular music. It starts with a single, percussive [[audio feedback|feedback]] note produced by plucking the A string on Lennon's guitar. Artists such as the Kinks and the Who had already used feedback live, but Lennon remained proud of the fact that the Beatles were perhaps the first group to deliberately put it on vinyl. In one of his last interviews, he said, "I defy anybody to find a record—unless it's some old blues record in 1922—that uses feedback that way."<ref>{{cite book |last=Sheff |first=David |date=2000 |title=All We Are Saying |location=New York, New York |publisher=St. Martin's Press |page=[https://archive.org/details/allwearesayingla00lenn/page/173 173] |isbn=978-0-312-25464-3 |url=https://archive.org/details/allwearesayingla00lenn/page/173 }}</ref> − − 在使用扩声系统或扩音系统的活动中，当歌手或公众演讲者的麦克风发生反馈时，几乎总是被认为是不受欢迎的。音频工程师使用各种电子设备，如均衡器，自20世纪90年代以来，自动反馈检测设备，以防止这些不需要的尖叫声或尖叫声，这些声音影响了观众对活动的享受。另一方面，自20世纪60年代以来，摇滚乐队中的电吉他手使用大音量的吉他放大器和失真效果，有意制造吉他反馈，以创造理想的音乐效果。 披头士乐队的 "I Feel Fine "标志着流行音乐中最早使用反馈作为录音效果的例子之一。它的开头是由Lennon拨动吉他上的A弦产生的一个单一的、有冲击力的反馈音。像 Kinks 和 Who 等艺术家已经在现场使用了反馈，但是Lennon仍然为披头士乐队可能是第一个特意把它放在黑胶唱片上的乐队而感到骄傲。在他最后的一次采访中，他说，“我认为任何人都不可能找到一张唱片——除非它是1922年的一张旧布鲁斯唱片——那样使用反馈。” − − − − The principles of audio feedback were first discovered by Danish scientist [[Søren Absalon Larsen]]. Microphones are not the only transducers subject to this effect. [[Phonograph|Record deck]] [[Magnetic cartridge|pickup cartridges]] can do the same, usually in the low frequency range below about 100 Hz, manifesting as a low rumble. [[Jimi Hendrix]] was an innovator in the intentional use of guitar feedback in his [[guitar solo]]s to create unique sound effects. He helped develop the controlled and musical use of audio feedback in [[electric guitar]] playing,<ref>{{cite book|last = Shadwick|first = Keith|title = Jimi Hendrix, Musician|publisher = [[Backbeat Books]]|year = 2003|page = 92|isbn = 978-0-87930-764-6}}</ref> and later [[Brian May]] was a famous proponent of the technique.<ref>{{cite web|last=May|first=Brian|title=Burns Brian May Tri-Sonic Pickups|url=http://www.brianmayguitars.co.uk/accessories/19|publisher=House Music & Duck Productions|accessdate=2 February 2011|url-status=live|archiveurl=https://web.archive.org/web/20101120063431/http://brianmayguitars.co.uk/accessories/19|archivedate=20 November 2010}}</ref> − − 音频反馈的原理是由丹麦科学家Søren Absalon Larsen首先发现的。麦克风并不是唯一受此影响的换能器。录音板拾音器也可以做到这一点，通常在100赫兹以下的低频范围内，表现为低沉的轰鸣声。Jimi Hendrix是一个创新者，在他的吉他独奏中有意使用吉他反馈来创造独特的声音效果。他帮助开发了电吉他演奏中音频反馈的可控性和音乐性，后来Brian May也是这种技术的著名支持者。 − − − [[File:Adam Savage HOPE.jpg|thumb|right|220px|[[Video feedback]].]] − − − − ===Video=== − 视频 − − Similarly, if a [[video camera]] is pointed at a [[Video monitor|monitor]] screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to the [[Doctor Who (season 1)|first]] [[Doctor Who (season 10)|ten]] series of the television program ''[[Doctor Who]]''. − − 同样，如果一台摄像机对准一个正在显示摄像机自身信号的监控屏幕，那么通过正反馈就可以在屏幕上形成重复的图案。这种视频反馈效果在电视节目《神秘博士》前十季的开场白中就被使用了。 − − − === Switches === − 开关 − − In [[electrical switch]]es, including [[bimetallic strip]] based thermostats, the switch usually has hysteresis in the switching action. In these cases hysteresis is mechanically achieved via positive feedback within a tipping point mechanism. The positive feedback action minimises the length of time arcing occurs for during the switching and also holds the contacts in an open or closed state.<ref>{{cite web|title=Positive Feedback and Bistable Systems|url=http://sbw.kgi.edu/sbwwiki/_media/sysbio/labmembers/hsauro/bistablesystems.pdf|publisher=University of Washington|quote=* Non-Hysteretic Switches, Memoryless Switches: These systems have no memory, that is, once the input signal is removed, the system returns to its original state. * Hysteretic Switches, Bistability: Bistable systems, in contrast, have memory. That is, when switched to one state or another, these systems remain in that state unless forced to change back. The light switch is a common example of a bistable system from everyday life. All bistable systems are based around some form of positive feedback loop.|url-status=live|archiveurl=https://web.archive.org/web/20150413020657/http://sbw.kgi.edu/sbwwiki/_media/sysbio/labmembers/hsauro/bistablesystems.pdf|archivedate=2015-04-13}}</ref> − − 在电气开关中，包括双金属条型恒温器，开关通常在开关动作中具有滞后性。在这些情况下，滞后是通过一个临界点机构内的正反馈来实现的。正反馈作用可最大限度地减少开关过程中发生电弧的时间，并使触点保持在断开或闭合状态。 − − 开关，无记忆开关。这些系统没有记忆，也就是说，一旦输入信号被移除，系统就会回到原来的状态。* 滞性开关，双稳态：与此相反，双稳态系统是有记忆的。也就是说，当切换到一个状态或另一个状态时，这些系统会保持在该状态，除非被迫变回来。电灯开关是日常生活中常见的双稳态系统的例子。所有的双稳态系统都是围绕着某种形式的正反馈环路。 − − === In biology === − 在生物学中 − − − − [[File:Positive Feedback- Childbirth (1).svg|thumb|Positive feedback is the amplification of a body's response to a stimulus. For example, in childbirth, when the head of the fetus pushes up against the cervix (1) it stimulates a nerve impulse from the cervix to the brain (2). When the brain is notified, it signals the pituitary gland to release a hormone called [[oxytocin]](3). Oxytocin is then carried via the bloodstream to the [[uterus]] (4) causing contractions, pushing the fetus towards the cervix eventually inducing childbirth.]] − − 正反馈是指身体对刺激的反应的放大。例如，在分娩过程中，当胎儿的头顶到子宫颈时(1)，会刺激神经冲动从子宫颈到大脑(2)。大脑接到通知后，会向脑垂体发出信号，释放一种叫做[催产素]]的激素(3)。 催产素然后通过血液流向[[子宫]]。(4)引起宫缩，将胎儿推向宫颈，最终分娩。 − − − ==== In physiology ==== − 在生理学中 − − − − A number of examples of positive feedback systems may be found in [[physiology]]. − − 在生理学中可以找到一些正反馈系统的例子。 − − * One example is the onset of [[Contraction (childbirth)|contractions]] in [[childbirth]], known as the [[Ferguson reflex]]. When a contraction occurs, the hormone [[oxytocin]] causes a nerve stimulus, which stimulates the [[hypothalamus]] to produce more oxytocin, which increases uterine contractions. This results in contractions increasing in [[amplitude]] and [[frequency]].<ref name=Guyton1991>Guyton, Arthur C. (1991) ''Textbook of Medical Physiology''. (8th ed). Philadelphia: W.B. Saunders. {{ISBN|0-7216-3994-1}}</ref>{{rp|pages=924–925}} − − 一个例子是分娩中宫缩的发生，称为弗格森反射。当宫缩发生时，激素催产素引起神经刺激，刺激下丘脑产生更多的催产素，从而增加子宫收缩。这将导致振幅和频率的收缩增加。 − − * Another example is the process of [[Coagulation|blood clotting]]. The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot.<ref name=Guyton1991/>{{rp|pages=392–394}} − − 另一个例子是凝血的过程。当受伤的组织释放出信号化学物质，激活血液中的血小板时，这个循环就启动了。激活的血小板释放化学物质激活更多的血小板，导致快速的级联和血栓的形成。 − − * [[Lactation]] also involves positive feedback in that as the baby suckles on the nipple there is a nerve response into the spinal cord and up into the hypothalamus of the brain, which then stimulates the [[pituitary]] gland to produce more [[prolactin]] to produce more milk.<ref name=Guyton1991/>{{rp|page=926}} − − 泌乳还包括正反馈，当婴儿吸吮乳头时，会有神经反应进入脊髓，并上传到大脑的下丘脑，然后刺激垂体腺产生更多的催乳素以产生更多的乳汁。 − − * A spike in [[estrogen]] during the [[follicular phase]] of the menstrual cycle causes [[ovulation]].<ref name=Guyton1991/>{{rp|page=907}} − − 在月经周期的卵泡期期间，雌激素的飙升会导致排卵。 − − {{reflist|30em|refs= − − {通货再膨胀 | 30em | refs = − − * The generation of [[nerve signal]]s is another example, in which the membrane of a nerve fibre causes slight leakage of sodium ions through sodium channels, resulting in a change in the membrane potential, which in turn causes more opening of channels, and so on ([[Hodgkin cycle]]). So a slight initial leakage results in an explosion of sodium leakage which creates the nerve [[action potential]].<ref name=Guyton1991/>{{rp|page=59}} − − 神经信号的产生是另一个例子，神经纤维的膜使钠离子通过钠通道轻微漏出，导致膜电位的变化，进而引起更多的通道开放，等等（霍奇金循环)。所以，最初的轻微渗漏导致钠渗漏的爆发，从而产生神经动作电位。 − − * In [[excitation–contraction coupling]] of the heart, an increase in intracellular calcium ions to the cardiac myocyte is detected by ryanodine receptors in the membrane of the sarcoplasmic reticulum which transport calcium out into the cytosol in a positive feedback physiological response. − − 在心脏的兴奋收缩耦合中，肌浆网膜中的莱丹碱受体检测到心肌细胞内钙离子的增加，该受体以正反馈生理反应将钙运出到细胞质中。 − − − In most cases, such feedback loops culminate in counter-signals being released that suppress or break the loop. Childbirth contractions stop when the baby is out of the mother's body. Chemicals break down the blood clot. Lactation stops when the baby no longer nurses.<ref name=Guyton1991/> − − 在大多数情况下，这种反馈回路最终会释放出反信号，从而抑制或打破回路。分娩宫缩在宝宝离开母体时停止。化学物质分解血块。当婴儿不再需要被哺乳时，泌乳停止。 − − − ==== In gene regulation ==== − 基因调控 − − − − Positive feedback is a well studied phenomenon in gene regulation, where it is most often associated with [[bistability]]. Positive feedback occurs when a gene activates itself directly or indirectly via a double negative feedback loop. Genetic engineers have constructed and tested simple positive feedback networks in bacteria to demonstrate the concept of bistability.<ref name=Hasty2002/> A classic example of positive feedback is the [[lac operon]] in ''E. coli''. Positive feedback plays an integral role in cellular differentiation, development, and cancer progression, and therefore, positive feedback in gene regulation can have significant physiological consequences. Random motions in [[molecular dynamics]] coupled with positive feedback can trigger interesting effects, such as create population of phenotypically different cells from the same parent cell.<ref name=Veening2008/> This happens because noise can become amplified by positive feedback. Positive feedback can also occur in other forms of [[cell signaling]], such as enzyme kinetics or metabolic pathways.<ref name=Christoph2001/> − − 正反馈是基因调控中研究得很好的一种现象，其中最常见的是与双稳态有关。当一个基因通过双负反馈循环直接或间接激活自身时，就会出现正反馈。遗传工程师已经在细菌中构建并测试了简单的正反馈网络，以证明双稳态的概念。 − 正反馈的一个典型例子是''大肠杆菌''中的乳糖操纵子。正反馈在细胞分化、发育和癌症进展中起着不可或缺的作用，因此，基因调控中的正反馈可以产生显著的生理后果。分子动力学中的随机运动加上正反馈可以引发有趣的效应，例如从同一母细胞中产生表型不同的细胞群。 − 这种情况的发生是因为噪声会被正反馈放大。正反馈也可以发生在细胞信号的其他形式中，如酶动力学或代谢途径。 − − ==== In evolutionary biology ==== − 在进化生物学中 − − − − Positive feedback loops have been used to describe aspects of the dynamics of change in biological [[evolution]]. For example, beginning at the macro level, [[Alfred J. Lotka]] (1945) argued that the evolution of the species was most essentially a matter of selection that fed back energy flows to capture more and more energy for use by living systems.<ref name=Lotka1945/> At the human level, [[Richard D. Alexander]] (1989) proposed that social competition between and within human groups fed back to the selection of intelligence thus constantly producing more and more refined human intelligence. <ref name=Alexander1989/> [[Bernard Crespi|Crespi]] (2004) discussed several other examples of positive feedback loops in evolution.<ref name=Crespi2004/> The analogy of [[Evolutionary arms race]]s provide further examples of positive feedback in biological systems.<ref name=Blindwatchmaker/> − − 正反馈循环被用来描述生物进化中变化动态的各个方面。 例如，从宏观层面开始，阿尔弗雷德•J•洛特卡Alfred J. Lotka(1945)认为，物种的进化最本质上是一个选择问题，它反馈了能量流，以获取越来越多的能量供生物系统使用。在人类层面，理查德•D•亚历山大 Richard D. Alexander(1989)提出，人类群体之间和群体内部的社会竞争反馈到智力的选择上，从而不断产生更多、更完善的人类智力。 Bernard Crespi(2004)讨论了进化中正反馈循环的其他几个例子。进化军备竞赛的类比提供了生物系统中正反馈的进一步例子。 − − − [[File:Phanerozoic Biodiversity.svg|300px|right|thumb|During the Phanerozoic the [[biodiversity]] shows a steady but not monotonic increase from near zero to several thousands of genera.]] − − 在显生宙，生物多样性显示出稳定但不单调的增加，从接近零到几千属。 − − It has been shown that changes in [[biodiversity]] through the [[Phanerozoic]] correlate much better with hyperbolic model (widely used in [[demography]] and [[macrosociology]]) than with [[Exponential growth|exponential]] and [[Logistic function|logistic]] models (traditionally used in [[population biology]] and extensively applied to [[fossil]] [[biodiversity]] as well). The latter models imply that changes in diversity are guided by a first-order positive feedback (more ancestors, more descendants) and/or a [[negative feedback]] arising from resource limitation. Hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of the [[world population growth]] has been demonstrated (see below) to arise from a second-order positive feedback between the population size and the rate of [[technological growth]]. The hyperbolic character of biodiversity growth can be similarly accounted for by a positive feedback between the diversity and community structure complexity. It has been suggested that the similarity between the curves of [[biodiversity]] and human population probably comes from the fact that both are derived from the interference of the hyperbolic trend (produced by the positive feedback) with cyclical and stochastic dynamics.<ref>Markov A., [[Andrey Korotayev|Korotayev A.]] [https://archive.today/20120630063924/http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B83WC-4N0HJMK-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=74a80d7c55ff987c9fc8d9c7963feab9 "Phanerozoic marine biodiversity follows a hyperbolic trend." [[Palaeoworld]]. Volume 16, Issue 4, December 2007, Pages 311-318]</ref><ref>{{cite journal | last1 = Markov | first1 = A. | last2 = Korotayev | first2 = A. | year = 2008 | title = Hyperbolic growth of marine and continental biodiversity through the Phanerozoic and community evolution | url = http://elementy.ru/genbio/abstracts?artid=177 | journal = Journal of General Biology | volume = 69 | issue = 3 | pages = 175–194 | pmid = 18677962 | url-status = live | archiveurl = https://web.archive.org/web/20091225000305/http://elementy.ru/genbio/abstracts?artid=177 | archivedate = 2009-12-25 }}</ref> − − 研究表明，生物多样性在显生宙的变化与双曲模型(广泛用于人口学和宏观社会学)的关系比与指数增长和逻辑函数模型(传统上用于种群生物学并广泛用于生物多样性化石)的关系好得多。 后一种模式意味着多样性的变化是由一阶正反馈（更多的祖先，更多的后代）和/或由资源限制产生的负反馈引导的。双曲线模型意味着二阶正反馈。已经证明世界人口增长的双曲线模式（见下文）是由于人口数量与技术增长速度之间的二阶正反馈产生的。生物多样性增长的双曲线特征同样可以由多样性和群落结构复杂性之间的正反馈来解释。有人认为，生物多样性和人口曲线之间的相似性可能来自这样一个事实，即两者都是由双曲趋势(由正反馈产生)与周期性和随机动态的干扰而产生的。 − − − ==== Immune system ==== − 免疫系统 − − − − A [[cytokine storm]], or '''hypercytokinemia''' is a potentially fatal immune reaction consisting of a positive feedback loop between [[cytokine]]s and [[immune cell]]s, with highly elevated levels of various cytokines.<ref name="osterholm">{{cite journal | last = Osterholm | first = Michael T. | author-link = Michael Osterholm |title = Preparing for the Next Pandemic | journal = The New England Journal of Medicine | volume = 352 | issue = 18 | pages = 1839–1842 | date = 2005-05-05 | url = | doi = 10.1056/NEJMp058068 | pmid = 15872196 | citeseerx = 10.1.1.608.6200 }}</ref> In normal immune function, positive feedback loops can be utilized to enhance the action of B lymphocytes. When a B cell binds its antibodies to an antigen and becomes activated, it begins releasing antibodies and secreting a complement protein called C3. Both C3 and a B cell's antibodies can bind to a pathogen, and when a B cell has its antibodies bind to a pathogen with C3, it speeds up that B cell's secretion of more antibodies and more C3, thus creating a positive feedback loop.<ref>{{cite journal|last=Paul|first=William E.|title=Infectious Diseases and the Immune System|journal=Scientific American|volume=269|issue=3|date=September 1993|page=93|bibcode=1993SciAm.269c..90P|doi=10.1038/scientificamerican0993-90|pmid=8211095}}</ref> − − 细胞因子风暴，或''高细胞因子血症''是由细胞因子与免疫细胞之间的正反馈循环组成的一种潜在的致命性免疫反应，各种细胞因子的水平高度升高。 − 在正常的免疫功能中，可以利用正反馈环路来增强B淋巴细胞的作用。当B细胞将其抗体与抗原结合并被激活后，就会开始释放抗体，并分泌一种称为C3的补体蛋白。C3和B细胞的抗体都可以与病原体结合，当B细胞的抗体与C3结合后，就会加快该B细胞分泌更多的抗体和更多的C3，从而形成一个正反馈循环。 − − ==== Cell death ==== − 细胞死亡 − − }} − − }} − − − − [[Apoptosis]] is a [[caspase]]-mediated process of cellular death, whose aim is the removal of long-lived or damaged cells. A failure of this process has been implicated in prominent conditions such as [[cancer]] or [[Parkinson's disease]]. The very core of the apoptotic process is the auto-activation of caspases, which may be modeled via a positive-feedback loop. This positive feedback exerts an auto-activation of the [[effector caspase]] by means of intermediate caspases. When isolated from the rest of apoptotic pathway, this positive-feedback presents only one stable steady state, regardless of the number of intermediate activation steps of the effector caspase.<ref name="ReferenceA"/> When this core process is complemented with inhibitors and enhancers of caspases effects, this process presents bistability, thereby modeling the alive and dying states of a cell.<ref>{{cite journal|last=Eissing|first=Thomas |doi=10.1074/jbc.M404893200 |title=Bistability analyses of a caspase activation model for receptor-induced apoptosis|journal=Journal of Biological Chemistry|volume=279 |issue=35 |date=2014|pages=36892–36897|pmid=15208304 |doi-access=free}}</ref> − − 细胞凋亡是一种由酪蛋白酶介导的细胞死亡过程，其目的是清除长寿或受损的细胞。这一过程的失败与癌症或帕金森氏病等著名疾病有关。细胞凋亡过程的核心是半胱氨酸蛋白酶的自动激活，它可以通过一个正反馈循环来建模。这种正反馈通过中间胱天蛋白酶使效应子胱天蛋白酶自动活化。当从凋亡途径的其他部分分离出来时，无论效应子胱天蛋白酶的中间激活步骤的数量如何，这种正反馈仅呈现一种稳定的稳态。 − 当该核心过程与胱天蛋白酶作用的抑制剂和增强剂相辅相成时，该过程呈现双稳态，从而模拟细胞的存活和死亡状态。 − − − === In psychology === − 在心理学上 − − − − Winner (1996) described gifted children as driven by positive feedback loops involving setting their own learning course, this feeding back satisfaction, thus further setting their learning goals to higher levels and so on.<ref name=Winner1996/> Winner termed this positive feedback loop as a "rage to master." Vandervert (2009a, 2009b) proposed that the [[child prodigy]] can be explained in terms of a positive feedback loop between the output of thinking/performing in [[working memory]], which then is fed to the [[cerebellum]] where it is streamlined, and then fed back to working memory thus steadily increasing the quantitative and qualitative output of working memory.<ref name=Vandervert2009a/><ref name=Vandervert2009b/> Vandervert also argued that this working memory/cerebellar positive feedback loop was responsible for [[language]] evolution in working memory. − − 温纳（Winner，1996）将有天赋的孩子描述为受到正反馈循环的驱动，这些反馈循环涉及设置自己的学习课程，反馈自己的满意程度，从而进一步将他们的学习目标提高到更高水平等等。 − Winner将这种正反馈循环称为 "愤怒的掌握"。 Vandervert(2009a，2009b)提出，神童可以用工作记忆中的思维/表现的输出之间的正反馈回路来解释，工作记忆中的思维/表现的输出被反馈到小脑，在那里被精简，然后再反馈到工作记忆中，从而稳定地增加工作记忆的数量和质量输出。 − − === In economics === − 在经济学中 − − − − ====Markets with social influence==== − 具有社会影响力的市场 − − Product recommendations and information about past purchases have been shown to influence consumers choices significantly whether it is for music, movie, book, technological, and other type of products. Social influence often induces a rich-get-richer phenomenon ([[Matthew effect]]) where popular products tend to become even more popular.<ref name="altszyler2017">{{cite journal | title= Transient dynamics in trial-offer markets with social influence: Trade-offs between appeal and quality. | author1= Altszyler, E | author2= Berbeglia, F. | author3= Berbeglia, G. | author4= Van Hentenryck, P. | journal= PLOS ONE | year= 2017 | volume= 12 | issue= 7 | df= | doi=10.1371/journal.pone.0180040 |pmid = 28746334| pmc= 5528888 | page=e0180040| bibcode= 2017PLoSO..1280040A }}</ref> − − 事实证明，产品推荐和过去购买的信息对消费者的选择影响很大，无论是音乐、电影、书籍、技术还是其他类型的产品。社会影响往往会诱发一种 "富者越富 "的现象（马太效应），即热门产品往往会变得更加受欢迎。 − − − Category:Classical control theory − − 范畴: 经典控制理论 − − ====Market dynamics==== − 市场动态 − − Category:Cybernetics − − 类别: 控制论 − − According to the theory of [[reflexivity (social theory)|reflexivity]] advanced by [[George Soros]], price changes are driven by a positive feedback process whereby investors' expectations are influenced by price movements so their behaviour acts to reinforce movement in that direction until it becomes unsustainable, whereupon the feedback drives prices in the opposite direction.<ref>{{citation |title=Behavioural Technical Analysis |first=Paul V. |last=Azzopardi |publisher=Harriman House Limited |year=2010 |page=116 |isbn=9780857190680 |url=https://books.google.com/books?id=04Ay8qviuwgC&pg=PA116&lpg=PA116 |url-status=live |archiveurl=https://web.archive.org/web/20170329103058/https://books.google.com/books?id=04Ay8qviuwgC&pg=PA116&lpg=PA116&source=bl&hl=en&sa=X&f=false |archivedate=2017-03-29 }}</ref> − − 根据乔治•索罗斯 George Soros提出的反射性理论，价格变化是由一个正反馈过程驱动的，即投资者的预期受到价格变动的影响，因此他们的行为会强化这个方向的运动，直到它变得不可持续，于是反馈推动价格向相反的方向发展。 − − Category:Electronic feedback − − 类别: 电子反馈 − − − − Category:Feedback − − 分类: 反馈 − − ==== Systemic risk ==== − 系统性风险 − − − fr:Rétroaction+ + + + + + + + + + + + + + + + + + + + − fr:Rétroaction − <noinclude>+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + − <small>This page was moved from [[wikipedia:en:Positive feedback]]. Its edit history can be viewed at [[正反馈/edithistory]]</small></noinclude> − [[Category:待整理页面]]+ + +
无编辑摘要
正反馈(加剧反馈，自我强化反馈) 是指在反馈循环中发生的一个过程，它加剧了小扰动的影响。即一个扰动对系统的影响包括扰动幅度的增大。也就是说，A会产生更多的B，而B又会产生更多的A，相反，一个系统中，变化的结果会减少或抵消变化的结果，这就是负反馈。
正反馈(加剧反馈，自我强化反馈) 是指在反馈循环中发生的一个过程，它加剧了小扰动的影响。即一个扰动对系统的影响包括扰动幅度的增大。也就是说，A会产生更多的B，而B又会产生更多的A，相反，一个系统中，变化的结果会减少或抵消变化的结果，这就是负反馈。
<ref name=theorymodelling/> Both concepts play an important role in science and engineering, including biology, chemistry, and [[cybernetics]] .<br>
Both concepts play an important role in science and engineering, including biology, chemistry, and cybernetics.
这两个概念在科学和工程中发挥着重要作用，包括生物学、化学和控制论。
Mathematically, positive feedback is defined as a positive [[loop gain]] around a closed loop of cause and effect.<ref name=zuckerman/><ref name=theorymodelling>
数学上，正反馈被定义为围绕一个闭合的因果循环的正环增益。
That is, positive feedback is in phase with the input, in the sense that it adds to make the input larger.
That is, positive feedback is in phase with the input, in the sense that it adds to make the input larger.
|author1 = Ben Zuckerman
|author1 = Ben Zuckerman
Positive feedback tends to cause system instability. When the loop gain is positive and above 1, there will typically be exponential growth, increasing oscillations, chaotic behavior or other divergences from equilibrium. The feedback from the outcome to the originating process can be direct, or it can be via other state variables. A key feature of positive feedback is thus that small disturbances get bigger. When a change occurs in a system, positive feedback causes further change, in the same direction.
Positive feedback tends to cause system instability. When the loop gain is positive and above 1, there will typically be exponential growth, increasing oscillations, chaotic behavior or other divergences from equilibrium. System parameters will typically accelerate towards extreme values, which may damage or destroy the system, or may end with the system latched into a new stable state. Positive feedback may be controlled by signals in the system being filtered，damped or limited，or it can be cancelled or reduced by adding negative feedback.
正反馈容易导致系统不稳定。当环增益为正且大于1时，通常会出现指数增长、振荡加剧、混沌行为或其它偏离平衡的现象。系统参数通常会向极端值加速，这可能会损坏或破坏系统，或者可能以系统锁存进入新的稳定状态而结束。正反馈可以通过系统中的信号被滤波器过滤、阻尼或最大值和最小值限制来控制，也可以通过增加负反馈来取消或减少。
Positive feedback is used in digital electronics to force voltages away from intermediate voltages into '0' and '1' states. On the other hand, thermal runaway is a type of positive feedback that can destroy semiconductor junctions. Positive feedback in chemical reactions can increase the rate of reactions, and in some cases can lead to explosions. Positive feedback in mechanical design causes tipping-point, or 'over-centre', mechanisms to snap into position, for example in switches and locking pliers. Out of control, it can cause bridges to collapse. Positive feedback in economic systems can cause boom-then-bust cycles. A familiar example of positive feedback is the loud squealing or howling sound produced by audio feedback in public address systems: the microphone picks up sound from its own loudspeakers, amplifies it, and sends it through the speakers again.
[[File:Positive Feedback Diagram (2).svg|thumb|[[Platelet]] clotting demonstrates positive feedback. The damaged blood vessel wall releases chemicals that initiate the formation of a blood clot through platelet congregation. As more platelets gather, more chemicals are released that speed up the process. The process gets faster and faster until the blood vessel wall is completely sealed and the positive feedback loop has ended. The exponential form of the graph illustrates the positive feedback mechanism. ]]
Platelet clotting demonstrates positive feedback. The damaged blood vessel wall releases chemicals that initiate the formation of a blood clot through platelet congregation. As more platelets gather, more chemicals are released that speed up the process. The process gets faster and faster until the blood vessel wall is completely sealed and the positive feedback loop has ended. The exponential form of the graph illustrates the positive feedback mechanism.
血小板凝血表现为正反馈。受损的血管壁会释放化学物质，通过血小板聚集启动血栓的形成。随着越来越多的血小板聚集，更多的化学物质被释放出来，加快了这个过程。这个过程越来越快，直到血管壁完全密封，正反馈循环结束。 图中的指数形式说明了正反馈机制。
== Overview ==
总览
Positive feedback enhances or amplifies an effect by it having an influence on the process which gave rise to it. For example, when part of an electronic output signal returns to the input, and is in phase with it, the system [[Gain (electronics)|gain]] is increased.<ref>{{cite web|title=Positive feedback|url=http://www.oxforddictionaries.com/definition/english/positive-feedback|work=Oxford English Dictionary|publisher=Oxford University Press|accessdate=15 April 2014|url-status=live|archiveurl=https://web.archive.org/web/20140302160045/http://www.oxforddictionaries.com/definition/english/positive-feedback|archivedate=2 March 2014}}<br>
正反馈通过对产生该效应的过程产生影响来增强或放大该效应。例如，当部分电子输出信号返回到输入端，并与输入端同相，系统增益就会增加。
</ref> The feedback from the outcome to the originating process can be direct, or it can be via other state variables.<ref name=theorymodelling/> Such systems can give rich qualitative behaviors, but whether the feedback is instantaneously positive or negative in sign has an extremely important influence on the results.
结果对始发过程的反馈可以是直接的，也可以通过其他状态变量。
这样的系统可以给出丰富的定性行为，但反馈的信号是瞬时的正向还是负向，对结果有极其重要的影响。
<ref name=theorymodelling/> Positive feedback reinforces and negative feedback moderates the original process. ''Positive'' and ''negative'' in this sense refer to loop gains greater than or less than zero, and do not imply any [[value judgement]]s as to the desirability of the outcomes or effects.<ref>{{cite web|title=Feedback|url=http://metadesigners.org/Feedback-Glossary|work=Glossary|publisher=Metadesigners Network|accessdate=15 April 2014|url-status=live|archiveurl=https://web.archive.org/web/20140416183720/http://metadesigners.org/Feedback-Glossary|archivedate=16 April 2014}}</ref> A key feature of positive feedback is thus that small disturbances get bigger. When a change occurs in a system, positive feedback causes further change, in the same direction.
正反馈强化原过程，负反馈调节原过程。''正''和''负''在这个意义上指的是大于或小于零的循环收益，并不意味着对结果或效果的可取性有任何价值判断。
因此，正反馈的一个重要特点是小扰动变大。当系统发生变化时，正反馈会引起进一步的变化，而且是同方向的变化。
=== Basic ===
基础
[[File:Ideal feedback model.svg|thumb|A basic feedback system can be represented by this block diagram. In the diagram the + symbol is an adder and A and B are arbitrary [[causal system|causal]] functions.]]
A basic feedback system can be represented by this block diagram. In the diagram the + symbol is an adder and A and B are arbitrary causal functions.
一个基本的反馈系统可以用这个框图来表示。在图中，+号是加法器，A和B是任意因果函数。
A simple feedback loop is shown in the diagram. If the loop gain AB is positive, then a condition of ''positive'' or ''regenerative'' feedback exists.
图中是一个简单的反馈回路。 如果环增益AB为正值，则存在'正'或'再生'反馈的条件。
<math>G_c = A/(1-AB)</math>
If the functions A and B are linear and AB is smaller than unity, then the overall system gain from the input to output is finite, but can be very large as AB approaches unity.<ref name=smith> Electronics circuits and devices second edition. Ralph J. Smith</ref> In that case, it can be shown that the overall or "closed loop" gain from input to output is:
如果函数A和B是线性的，且AB小于1，那么系统从输入到输出的整体增益是有限的，但当AB接近1时，系统的增益可以非常大。
在这种情况下，可以表明从输入到输出的整体或 "闭环 "增益为：
:<math>G_c = A/(1-AB)</math>
When AB > 1, the system is unstable, so does not have a well-defined gain; the gain may be called infinite.
When AB > 1, the system is unstable, so does not have a well-defined gain; the gain may be called infinite.
当AB>1时，系统是不稳定的，所以不具有明确的增益，增益可称为无限。
当AB>1时，系统是不稳定的，所以不具有明确的增益，增益可称为无限。
Thus depending on the feedback, state changes can be convergent, or divergent. The result of positive feedback is to augment changes, so that small perturbations may result in big changes.
Thus depending on the feedback, state changes can be convergent, or divergent. The result of positive feedback is to augment changes, so that small perturbations may result in big changes.
因此根据反馈，状态变化可以是收敛的，也可以是发散的。 正反馈的结果是增强变化，因此小的扰动可能导致大的变化。
A system in equilibrium in which there is positive feedback to any change from its current state may be unstable, in which case the system is said to be in an unstable equilibrium. The magnitude of the forces that act to move such a system away from its equilibrium are an increasing function of the "distance" of the state from the equilibrium.
A system in equilibrium in which there is positive feedback to any change from its current state may be unstable, in which case the system is said to be in an unstable equilibrium. The magnitude of the forces that act to move such a system away from its equilibrium are an increasing function of the "distance" of the state from the equilibrium.
一个处于平衡状态的系统，其当前状态的任何变化都有正反馈，它可能是不稳定的，在这种情况下，系统被称为不稳定平衡。使这种系统远离其平衡状态的力的大小是状态与平衡状态之间的距离的递增函数。
Positive feedback does not necessarily imply instability of an equilibrium, for example stable ''on'' and ''off'' states may exist in positive-feedback architectures.<ref name="ReferenceA">{{cite journal|last1=Lopez-Caamal|first1=Fernando|last2=Middleton|first2=Richard H.|last3=Huber|first3=Heinrich|title=Equilibria and stability of a class of positive feedback loops|journal=Journal of Mathematical Biology|date=February 2014|pages=609–645|doi = 10.1007/s00285-013-0644-z|pmid=23358701|volume=68|issue=3}}</ref>
Positive feedback does not necessarily imply instability of an equilibrium, for example stable on and off states may exist in positive-feedback architectures.
正反馈并不一定意味着平衡的不稳定性，例如，在正反馈结构中可能存在稳定的开关状态。
正反馈并不一定意味着平衡的不稳定性，例如，在正反馈结构中可能存在稳定的开关状态。
=== Hysteresis ===
磁滞现象
{{main|Hysteresis}}
[[File:Hysteresis sharp curve.svg|thumb|Hysteresis causes the output value to depend on the history of the input]]
Hysteresis causes the output value to depend on the history of the input
Hysteresis causes the output value to depend on the history of the input
迟滞会导致输出值取决于输入的历史记录
迟滞会导致输出值取决于输入的历史记录
[[File:Op-Amp Schmitt Trigger.svg|thumb|In a [[Schmitt trigger]] circuit, feedback to the non-inverting input of an amplifier pushes the output directly away from the applied voltage towards the maximum or minimum voltage the amplifier can generate.]]
In a Schmitt trigger circuit, feedback to the non-inverting input of an amplifier pushes the output directly away from the applied voltage towards the maximum or minimum voltage the amplifier can generate.
在施密特触发器电路中，对放大器的非反相输入的反馈直接将输出从外加电压推向放大器所能产生的最大或最小电压。
In the real world, positive feedback loops typically do not cause ever-increasing growth, but are modified by limiting effects of some sort. According to [[Donella Meadows]]:
在现实世界中，正反馈循环通常不会引起不断增长，而是通过某种限制效应来改变。根据Donella Meadows的说法：
::"Positive feedback loops are sources of growth, explosion, erosion, and collapse in systems. A system with an unchecked positive loop ultimately will destroy itself. That’s why there are so few of them. Usually a negative loop will kick in sooner or later."<ref name=meadows>
"Positive feedback loops are sources of growth, explosion, erosion, and collapse in systems. A system with an unchecked positive loop ultimately will destroy itself. That’s why there are so few of them. Usually a negative loop will kick in sooner or later."
“正反馈循环是系统增长、爆炸、侵蚀和崩溃的源头。如果一个系统的正循环不受控制，最终将会自我毁灭。这就是为什么很少有这样的系统。通常情况下，负面循环迟早会发生。”
“正反馈循环是系统增长、爆炸、侵蚀和崩溃的源头。如果一个系统的正循环不受控制，最终将会自我毁灭。这就是为什么很少有这样的系统。通常情况下，负面循环迟早会发生。”
Donella Meadows, [http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf ''Leverage Points: Places to Intervene in a System''] {{webarchive|url=https://web.archive.org/web/20131008160618/http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf |date=2013-10-08 }}, 1999</ref>
Hysteresis, in which the starting point affects where the system ends up, can be generated by positive feedback. When the gain of the feedback loop is above 1, then the output moves away from the input: if it is above the input, then it moves towards the nearest positive limit, while if it is below the input then it moves towards the nearest negative limit.
Hysteresis, in which the starting point affects where the system ends up, can be generated by positive feedback. When the gain of the feedback loop is above 1, then the output moves away from the input: if it is above the input, then it moves towards the nearest positive limit, while if it is below the input then it moves towards the nearest negative limit.
磁滞现象，即起点影响系统的终点，可以通过正反馈产生。当反馈循环的增益高于1时，那么输出就会远离输入：如果高于输入，则向最近的正极限移动，而如果低于输入，则向最近的负极限移动。
磁滞现象，即起点影响系统的终点，可以通过正反馈产生。当反馈循环的增益高于1时，那么输出就会远离输入：如果高于输入，则向最近的正极限移动，而如果低于输入，则向最近的负极限移动。
Once it reaches the limit, it will be stable. However, if the input goes past the limit,{{clarify|date=June 2012}} then the feedback will change sign{{dubious|date=June 2012}} and the output will move in the opposite direction until it hits the opposite limit. The system therefore shows [[bistability|bistable]] behaviour.
Once it reaches the limit, it will be stable. However, if the input goes past the limit, then the feedback will change sign and the output will move in the opposite direction until it hits the opposite limit. The system therefore shows bistable behaviour.
一旦达到极限，它就会稳定下来。但是，如果输入超过极限，那么反馈将改变符号，输出将向相反的方向移动，直到达到相反的极限。因此，该系统表现出双稳态行为。
一旦达到极限，它就会稳定下来。但是，如果输入超过极限，那么反馈将改变符号，输出将向相反的方向移动，直到达到相反的极限。因此，该系统表现出双稳态行为。
== Terminology ==
术语学
The terms positive and negative were first applied to feedback before World War II. The idea of positive feedback was already current in the 1920s with the introduction of the regenerative circuit.
The terms positive and negative were first applied to feedback before World War II. The idea of positive feedback was already current in the 1920s with the introduction of the regenerative circuit.
|year = 2002
|year = 2002
described regeneration in a set of electronic amplifiers as a case where the "feed-back" action is positive in contrast to negative feed-back action, which they mention only in passing. Harold Stephen Black's classic 1934 paper first details the use of negative feedback in electronic amplifiers. According to Black:
Friis & Jensen (1924) described regeneration in a set of electronic amplifiers as a case where the "feed-back" action is positive in contrast to negative feed-back action, which they mention only in passing. Harold Stephen Black's classic 1934 paper first details the use of negative feedback in electronic amplifiers. According to Black:
将一组电子放大器中的再生描述为 "反馈 "作用是正的情况，与负反馈作用相反，只是顺便提到了负反馈。哈罗德·史蒂芬·布莱克 HaroldStephen Black 在1934年的经典论文首次详细阐述了负反馈在电子放大器中的应用。根据Black的说法:
Friis & Jensen(1924)将一组电子放大器中的再生描述为 "回馈 "作用是正的情况，与负回馈作用相反，他们只是顺便提到了负回馈。哈罗德•斯蒂芬•布莱克（Harold Stephen Black）在1934年的经典论文中首次详细介绍了负反馈在电子放大器中的应用。根据Black的说法：
|isbn = 9780750656429
|isbn = 9780750656429
|page = 247
|page = 247
According to confusion in the terms arose shortly after this:
According to Mindell (2002) confusion in the terms arose shortly after this:
据Mindell(2002年)说，在这之后不久就出现了术语上的混乱：
|url = https://books.google.com/books?id=lROa-MpIrucC&pg=PA247
|url = https://books.google.com/books?id=lROa-MpIrucC&pg=PA247
"...Friis and Jensen had made the same distinction Black used between 'positive feed-back' and 'negative feed-back', based not on the sign of the feedback itself but rather on its effect on the amplifier’s gain. In contrast, Nyquist and Bode, when they built on Black’s work, referred to negative feedback as that with the sign reversed. Black had trouble convincing others of the utility of his invention in part because confusion existed over basic matters of definition." for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single transistor amplifier can multiply its gain by 1,000 or more. Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the superheterodyne design, with many more amplification stages, but much more stable operation and no positive feedback.
"...Friis and Jensen had made the same distinction Black used between 'positive feed-back' and 'negative feed-back', based not on the sign of the feedback itself but rather on its effect on the amplifier’s gain. In contrast, Nyquist and Bode, when they built on Black’s work, referred to negative feedback as that with the sign reversed. Black had trouble convincing others of the utility of his invention in part because confusion existed over basic matters of definition."
“ ... ... Friis和Jensen对Black在 "正反馈 "和 "负反馈 "之间的区分是一样的，不是基于反馈本身的符号，而是基于它对放大器增益的影响。相反，Nyquist和Bode在Black的工作基础上，将负反馈称为符号相反的反馈。Black难以说服其他人相信他的发明的实用性，部分原因是在基本的定义问题上存在混乱。"
== Examples and applications ==
实例与应用
=== In electronics ===
在电子领域
[[File:Regenerartive Receiver-S7300056.JPG|thumb|right|A vintage style regenerative radio receiver. Due to the controlled use of positive feedback, sufficient amplification can be derived from a single [[vacuum tube]] or valve (centre).]]
一个老式的再生无线电接收器。由于使用正反馈的控制，真空管或阀门（中心）就可以产生足够的放大效果。
[[Regenerative circuit]]s were invented and patented in 1914<ref>{{cite patent |inventor-last=Armstrong |inventor-first=E. H. |country-code=US |patent-number=1113149 |title=Wireless receiving system |date=1914}}</ref> for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single [[transistor]] amplifier can multiply its [[Gain (electronics)|gain]] by 1,000 or more.<ref>{{cite web|last=Kitchin|first=Charles|title=A Short Wave Regenerative Receiver Project|url=http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|accessdate=23 September 2010|url-status=live|archiveurl=https://web.archive.org/web/20100710100031/http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|archivedate=10 July 2010}}</ref> Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the [[superheterodyne]] design, with many more amplification stages, but much more stable operation and no positive feedback.
Regenerative circuits were invented and patented in 1914 for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single transistor amplifier can multiply its gain by 1,000 or more. Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the superheterodyne design, with many more amplification stages, but much more stable operation and no positive feedback.
再生电路于1914年被发明并获得专利，用于放大和接收非常微弱的无线电信号。仔细控制单晶体管放大器周围的正反馈，可以使其增益增加1000倍或更多。因此，一个信号可以在一个阶段被放大20000甚至100000倍，通常只有20到50的增益。再生放大器在这些非常高的增益下工作的问题是，它们很容易变得不稳定，开始振荡。无线电操作员必须准备相当连续地调整反馈量，以获得良好的接收效果。现代无线电接收机采用超异构设计，多了许多放大级，但工作更稳定，没有正反馈。
The oscillation that can break out in a regenerative radio circuit is used in [[electronic oscillator]]s. By the use of [[tuned circuit]]s or a [[piezoelectricity|piezoelectric]] [[crystal]] (commonly [[quartz]]), the signal that is amplified by the positive feedback remains linear and [[Sine wave|sinusoidal]]. There are several designs for such [[harmonic oscillator]]s, including the [[Armstrong oscillator]], [[Hartley oscillator]], [[Colpitts oscillator]], and the [[Wien bridge oscillator]]. They all use positive feedback to create oscillations.<ref>{{cite web|title=Sinewave oscillators|url=http://www.educypedia.be/electronics/analogosciltypes.htm|work=EDUCYPEDIA - electronics|accessdate=23 September 2010|url-status=dead|archiveurl=https://web.archive.org/web/20100927094330/http://www.educypedia.be/electronics/analogosciltypes.htm|archivedate=27 September 2010}}</ref>
The oscillation that can break out in a regenerative radio circuit is used in electronic oscillators. By the use of tuned circuits or a piezoelectric crystal (commonly quartz), the signal that is amplified by the positive feedback remains linear and sinusoidal. There are several designs for such harmonic oscillators, including the Armstrong oscillator, Hartley oscillator, Colpitts oscillator, and the Wien bridge oscillator. They all use positive feedback to create oscillations.
The oscillation that can break out in a regenerative radio circuit is used in electronic oscillators. By the use of tuned circuits or a piezoelectric crystal (commonly quartz), the signal that is amplified by the positive feedback remains linear and sinusoidal. There are several designs for such harmonic oscillators, including the Armstrong oscillator, Hartley oscillator, Colpitts oscillator, and the Wien bridge oscillator. They all use positive feedback to create oscillations.
在再生无线电电路中能爆发出的振荡被用于电子振荡器中。通过使用调谐电路或压电晶体(常见的是石英)，经正反馈放大后的信号仍然是线性的、正弦的。这种谐波振荡器有几种设计，包括阿姆斯特朗振荡器、哈特利振荡器、科尔皮茨振荡器和维恩桥振荡器。它们都是利用正反馈来产生振荡。
在再生无线电电路中能爆发出的振荡被用于电子振荡器中。通过使用调谐电路或压电晶体(常见的是石英)，经正反馈放大后的信号仍然是线性的、正弦的。这种谐波振荡器有几种设计，包括阿姆斯特朗振荡器、哈特利振荡器、科尔皮茨振荡器和维恩桥振荡器。它们都是利用正反馈来产生振荡。
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许多电子电路，特别是放大器，都采用了负反馈。这降低了它们的增益，但改善了它们的线性度、输入阻抗、输出阻抗和带宽，并稳定了所有这些参数，包括闭环增益。这些参数也变得不那么依赖于放大器件本身的细节，而更多地依赖于反馈元件，因为反馈元件不太可能随着制造公差、年龄和温度而变化。交流信号的正反馈和负反馈的区别在于相位问题：如果信号反馈失相，则反馈为负，如果相位一致，则反馈为正。对于使用负反馈的放大器设计者来说，有一个问题是，电路中的一些元件会在反馈路径中引入相移。如果有一个频率(通常是高频)的相移达到180°，那么设计者必须确保该频率的放大器增益非常低(通常通过低通滤波)。如果任何频率下的环增益（放大器增益与正反馈程度的乘积）大于1，那么放大器将在该频率下发生振荡（巴克豪森稳定性准则）。这种振荡有时被称为寄生振荡。在一组条件下稳定的放大器在另一组条件下可能会发生寄生振荡。这可能是由于温度、电源电压的变化，前面板控制的调整，甚至是人或其他导电物品的接近。
许多电子电路，特别是放大器，都采用了负反馈。这降低了它们的增益，但改善了它们的线性度、输入阻抗、输出阻抗和带宽，并稳定了所有这些参数，包括闭环增益。这些参数也变得不那么依赖于放大器件本身的细节，而更多地依赖于反馈元件，因为反馈元件不太可能随着制造公差、年龄和温度而变化。交流信号的正反馈和负反馈的区别在于相位问题：如果信号反馈失相，则反馈为负，如果相位一致，则反馈为正。对于使用负反馈的放大器设计者来说，有一个问题是，电路中的一些元件会在反馈路径中引入相移。如果有一个频率(通常是高频)的相移达到180°，那么设计者必须确保该频率的放大器增益非常低(通常通过低通滤波)。如果任何频率下的环增益（放大器增益与正反馈程度的乘积）大于1，那么放大器将在该频率下发生振荡（巴克豪森稳定性准则）。这种振荡有时被称为寄生振荡。在一组条件下稳定的放大器在另一组条件下可能会发生寄生振荡。这可能是由于温度、电源电压的变化，前面板控制的调整，甚至是人或其他导电物品的接近。
Amplifiers may oscillate gently in ways that are hard to detect without an oscilloscope, or the oscillations may be so extensive that only a very distorted or no required signal at all gets through, or that damage occurs. Low frequency parasitic oscillations have been called 'motorboating' due to the similarity to the sound of a low-revving exhaust note.
Amplifiers may oscillate gently in ways that are hard to detect without an oscilloscope, or the oscillations may be so extensive that only a very distorted or no required signal at all gets through, or that damage occurs. Low frequency parasitic oscillations have been called 'motorboating' due to the similarity to the sound of a low-revving exhaust note.
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nor gates with positive feedback. Red and black mean logical '1' and '0', respectively.]]
Illustration of an R-S ('reset-set') flip-flop made from two digital nor gates with positive feedback. Red and black mean logical '1' and '0', respectively.
R-S("复位-设置")触发器的说明，由两个带正反馈的数字诺尔门组成。红色和黑色分别表示逻辑上的 "1 "和 "0"。
|archiveurl = https://web.archive.org/web/20170329122152/https://books.google.com/books?id=uah1PkxWeKYC&pg=PA276
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Thermal runaway occurs in electronic systems because some aspect of a circuit is allowed to pass more current when it gets hotter, then the hotter it gets, the more current it passes, which heats it some more and so it passes yet more current. The effects are usually catastrophic for the device in question. If devices have to be used near to their maximum power-handling capacity, and thermal runaway is possible or likely under certain conditions, improvements can usually be achieved by careful design.
Thermal runaway occurs in electronic systems because some aspect of a circuit is allowed to pass more current when it gets hotter, then the hotter it gets, the more current it passes, which heats it some more and so it passes yet more current. The effects are usually catastrophic for the device in question. If devices have to be used near to their maximum power-handling capacity, and thermal runaway is possible or likely under certain conditions, improvements can usually be achieved by careful design.
在电子系统中发生热失控的原因是，当电路的某些方面变得更热时，允许它通过更多的电流，然后它越热，通过的电流就越多，这就使它更热一些，因此它又通过更多的电流。这种影响对有关器件来说通常是灾难性的。如果器件必须在接近其最大功率处理能力的情况下使用，并且在某些条件下可能或可能出现热失控，通常可以通过精心设计来改进。
在电子系统中发生热失控的原因是，当电路的某些方面变得更热时，允许它通过更多的电流，然后它越热，通过的电流就越多，这就使它更热一些，因此它又通过更多的电流。这种影响对有关器件来说通常是灾难性的。如果器件必须在接近其最大功率处理能力的情况下使用，并且在某些条件下可能或可能出现热失控，通常可以通过精心设计来改进。
[[File:Technics SL-1210MK2.jpg|thumb|left|A phonograph turntable is prone to acoustic feedback.]]
A phonograph turntable is prone to acoustic feedback.
留声机转盘容易受到声反馈的影响。
Audio and video systems can demonstrate positive feedback. If a microphone picks up the amplified sound output of loudspeakers in the same circuit, then howling and screeching sounds of audio feedback (at up to the maximum power capacity of the amplifier) will be heard, as random noise is re-amplified by positive feedback and filtered by the characteristics of the audio system and the room.
音频和视频系统可以表现出正反馈。如果麦克风拾取同一电路中扬声器的放大声音输出，那么就会听到音频反馈的嚎叫和尖叫声（在放大器的最大功率容量下），因为随机噪声被正反馈重新放大，并被音频系统和房间的特性所过滤。
===Audio and live music===
音频和现场音乐
Audio feedback (also known as acoustic feedback, simply as feedback, or the Larsen effect) is a special kind of positive feedback which occurs when a sound loop exists between an audio input (for example, a microphone or guitar pickup) and an audio output (for example, a loudly-amplified loudspeaker). In this example, a signal received by the microphone is amplified and passed out of the loudspeaker. The sound from the loudspeaker can then be received by the microphone again, amplified further, and then passed out through the loudspeaker again. The frequency of the resulting sound is determined by resonance frequencies in the microphone, amplifier, and loudspeaker, the acoustics of the room, the directional pick-up and emission patterns of the microphone and loudspeaker, and the distance between them. For small PA systems the sound is readily recognized as a loud squeal or screech.
音频反馈（也称为声反馈，简称反馈，或拉森效应）是一种特殊的正反馈，当音频输入（例如，麦克风或吉他拾音器）和音频输出（例如，大声放大的扬声器）之间存在声音回路时，就会出现这种反馈。在这个例子中，麦克风接收到的信号被放大并从扬声器传出。然后，来自扬声器的声音可以再次被麦克风接收，进一步放大，然后再次通过扬声器传递出去。 所产生的声音的频率由传声器、放大器和扬声器的共振频率、房间的声学特性、传声器和扬声器的定向拾音和发射模式以及它们之间的距离决定。对于小型的扩声系统来说，这种声音很容易被识别为响亮的吱吱声或尖叫声。
Feedback is almost always considered undesirable when it occurs with a singer's or public speaker's microphone at an event using a sound reinforcement system or PA system. Audio engineers use various electronic devices, such as equalizers and, since the 1990s, automatic feedback detection devices to prevent these unwanted squeals or screeching sounds, which detract from the audience's enjoyment of the event. On the other hand, since the 1960s, electric guitar players in rock music bands using loud guitar amplifiers and distortion effects have intentionally created guitar feedback to create a desirable musical effect. "I Feel Fine" by the Beatles marks one of the earliest examples of the use of feedback as a recording effect in popular music. It starts with a single, percussive feedback note produced by plucking the A string on Lennon's guitar. Artists such as the Kinks and the Who had already used feedback live, but Lennon remained proud of the fact that the Beatles were perhaps the first group to deliberately put it on vinyl. In one of his last interviews, he said, "I defy anybody to find a record—unless it's some old blues record in 1922—that uses feedback that way."
在使用扩声系统或扩音系统的活动中，当歌手或公众演讲者的麦克风发生反馈时，几乎总是被认为是不受欢迎的。音频工程师使用各种电子设备，如均衡器，自20世纪90年代以来，自动反馈检测设备，以防止这些不需要的尖叫声或尖叫声，这些声音影响了观众对活动的享受。另一方面，自20世纪60年代以来，摇滚乐队中的电吉他手使用大音量的吉他放大器和失真效果，有意制造吉他反馈，以创造理想的音乐效果。 披头士乐队的 "I Feel Fine "标志着流行音乐中最早使用反馈作为录音效果的例子之一。它的开头是由Lennon拨动吉他上的A弦产生的一个单一的、有冲击力的反馈音。像 Kinks 和 Who 等艺术家已经在现场使用了反馈，但是Lennon仍然为披头士乐队可能是第一个特意把它放在黑胶唱片上的乐队而感到骄傲。在他最后的一次采访中，他说，“我认为任何人都不可能找到一张唱片——除非它是1922年的一张旧布鲁斯唱片——那样使用反馈。”
The principles of audio feedback were first discovered by Danish scientist Søren Absalon Larsen. Microphones are not the only transducers subject to this effect. Record deck pickup cartridges can do the same, usually in the low frequency range below about 100 Hz, manifesting as a low rumble. Jimi Hendrix was an innovator in the intentional use of guitar feedback in his guitar solos to create unique sound effects. He helped develop the controlled and musical use of audio feedback in electric guitar playing, and later Brian May was a famous proponent of the technique.
音频反馈的原理是由丹麦科学家Søren Absalon Larsen首先发现的。麦克风并不是唯一受此影响的换能器。录音板拾音器也可以做到这一点，通常在100赫兹以下的低频范围内，表现为低沉的轰鸣声。Jimi Hendrix是一个创新者，在他的吉他独奏中有意使用吉他反馈来创造独特的声音效果。他帮助开发了电吉他演奏中音频反馈的可控性和音乐性，后来Brian May也是这种技术的著名支持者。
[[File:Adam Savage HOPE.jpg|thumb|right|220px|[[Video feedback]].]]
视频反馈
===Video===
视频
Similarly, if a [[video camera]] is pointed at a [[Video monitor|monitor]] screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to the [[Doctor Who (season 1)|first]] [[Doctor Who (season 10)|ten]] series of the television program ''[[Doctor Who]]''.Similarly, if a video camera is pointed at a monitor screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to the first ten series of the television program Doctor Who.
同样，如果一台摄像机对准一个正在显示摄像机自身信号的监控屏幕，那么通过正反馈就可以在屏幕上形成重复的图案。这种视频反馈效果在电视节目《神秘博士》前十季的开场白中就被使用了。
=== Switches ===
开关
In [[electrical switch]]es, including [[bimetallic strip]] based thermostats, the switch usually has hysteresis in the switching action. In these cases hysteresis is mechanically achieved via positive feedback within a tipping point mechanism. The positive feedback action minimises the length of time arcing occurs for during the switching and also holds the contacts in an open or closed state.
在电气开关中，包括双金属条型恒温器，开关通常在开关动作中具有滞后性。在这些情况下，滞后是通过一个临界点机构内的正反馈来实现的。正反馈作用可最大限度地减少开关过程中发生电弧的时间，并使触点保持在断开或闭合状态。
=== In biology ===
在生物学中
[[File:Positive Feedback- Childbirth (1).svg|thumb|Positive feedback is the amplification of a body's response to a stimulus. For example, in childbirth, when the head of the fetus pushes up against the cervix (1) it stimulates a nerve impulse from the cervix to the brain (2). When the brain is notified, it signals the pituitary gland to release a hormone called [[oxytocin]](3). Oxytocin is then carried via the bloodstream to the [[uterus]] (4) causing contractions, pushing the fetus towards the cervix eventually inducing childbirth.]]
正反馈是指身体对刺激的反应的放大。例如，在分娩过程中，当胎儿的头顶到子宫颈时(1)，会刺激神经冲动从子宫颈到大脑(2)。大脑接到通知后，会向脑垂体发出信号，释放一种叫做催产素的激素(3)。催产素随后通过血液流向子宫(4)，引起宫缩，将胎儿推向子宫颈，最终促使分娩。
==== In physiology ====
在生理学中
A number of examples of positive feedback systems may be found in [[physiology]].
在生理学中可以找到一些正反馈系统的例子。
* One example is the onset of [[Contraction (childbirth)|contractions]] in [[childbirth]], known as the [[Ferguson reflex]]. When a contraction occurs, the hormone [[oxytocin]] causes a nerve stimulus, which stimulates the [[hypothalamus]] to produce more oxytocin, which increases uterine contractions. This results in contractions increasing in [[amplitude]] and [[frequency]].<ref name=Guyton1991>Guyton, Arthur C. (1991) ''Textbook of Medical Physiology''. (8th ed). Philadelphia: W.B. Saunders. {{ISBN|0-7216-3994-1}}</ref>{{rp|pages=924–925}}
其中一个例子是分娩时宫缩的发生，称为弗格森反射。当宫缩发生时，激素催产素会引起神经刺激，刺激下丘脑产生更多的催产素，从而增加子宫收缩。这就导致宫缩的幅度和频率增加。
* Another example is the process of [[Coagulation|blood clotting]]. The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot.<ref name=Guyton1991/>{{rp|pages=392–394}}
另一个例子是血液凝固的过程。当受伤的组织释放出信号化学物质，激活血液中的血小板时，这个循环就启动了。被激活的血小板释放化学物质，激活更多的血小板，引起快速的级联，形成血栓。
* [[Lactation]] also involves positive feedback in that as the baby suckles on the nipple there is a nerve response into the spinal cord and up into the hypothalamus of the brain, which then stimulates the [[pituitary]] gland to produce more [[prolactin]] to produce more milk.<ref name=Guyton1991/>{{rp|page=926}}
哺乳也涉及正反馈，当婴儿吸吮乳头时，会有神经反应进入脊髓，并上传到大脑的下丘脑，然后刺激垂体产生更多的催乳素以产生更多的乳汁。
* A spike in [[estrogen]] during the [[follicular phase]] of the menstrual cycle causes [[ovulation]].<ref name=Guyton1991/>{{rp|page=907}}
在月经周期的卵泡期期间，雌激素的飙升会导致排卵。
{{reflist|30em|refs=
* The generation of [[nerve signal]]s is another example, in which the membrane of a nerve fibre causes slight leakage of sodium ions through sodium channels, resulting in a change in the membrane potential, which in turn causes more opening of channels, and so on ([[Hodgkin cycle]]). So a slight initial leakage results in an explosion of sodium leakage which creates the nerve [[action potential]].<ref name=Guyton1991/>{{rp|page=59}}
神经信号的产生是另一个例子，神经纤维的膜使钠离子通过钠通道轻微漏出，导致膜电位的变化，进而引起更多通道的开放，如此循环（Hodgkin循环）。所以，最初的轻微渗漏会导致钠渗漏的爆发，从而产生神经动作电位。
* In [[excitation–contraction coupling]] of the heart, an increase in intracellular calcium ions to the cardiac myocyte is detected by ryanodine receptors in the membrane of the sarcoplasmic reticulum which transport calcium out into the cytosol in a positive feedback physiological response.
在心脏的兴奋收缩耦合中，肌浆网膜中的兰尼碱受体检测到心肌细胞内钙离子的增加，该受体以正反馈生理反应将钙运出到细胞质中。
In most cases, such feedback loops culminate in counter-signals being released that suppress or break the loop. Childbirth contractions stop when the baby is out of the mother's body. Chemicals break down the blood clot. Lactation stops when the baby no longer nurses.<ref name=Guyton1991/>
在大多数情况下，这种反馈循环最终会释放出反信号，从而抑制或破坏循环。分娩宫缩在宝宝离开母体时停止。化学物质分解血块。当婴儿不再需要被哺乳时，泌乳停止。
=== Hysteresis ===
==== In gene regulation ====
基因调控
Positive feedback is a well studied phenomenon in gene regulation, where it is most often associated with [[bistability]]. Positive feedback occurs when a gene activates itself directly or indirectly via a double negative feedback loop. Genetic engineers have constructed and tested simple positive feedback networks in bacteria to demonstrate the concept of bistability.<ref name=Hasty2002/> A classic example of positive feedback is the [[lac operon]] in ''E. coli''. Positive feedback plays an integral role in cellular differentiation, development, and cancer progression, and therefore, positive feedback in gene regulation can have significant physiological consequences. Random motions in [[molecular dynamics]] coupled with positive feedback can trigger interesting effects, such as create population of phenotypically different cells from the same parent cell.<ref name=Veening2008/> This happens because noise can become amplified by positive feedback. Positive feedback can also occur in other forms of [[cell signaling]], such as enzyme kinetics or metabolic pathways.<ref name=Christoph2001/>
正反馈是基因调控中研究得很好的一种现象，其中最常见的是与双稳态有关。当一个基因通过双负反馈循环直接或间接激活自身时，就会出现正反馈。遗传工程师已经在细菌中构建并测试了简单的正反馈网络，以证明双稳态的概念。
正反馈的一个典型例子是大肠杆菌中的乳糖操纵子。正反馈在细胞分化、发育和癌症进展中起着不可或缺的作用，因此，基因调控中的正反馈可以产生显著的生理后果。分子动力学中的随机运动加上正反馈可以引发有趣的效应，例如从同一母细胞中产生表型不同的细胞群。
这种情况的发生是因为噪声会被正反馈放大。正反馈也可以发生在细胞信号的其他形式中，如酶动力学或代谢途径。
==== In evolutionary biology ====
在进化生物学中
[[File:Op-Amp Schmitt Trigger.svg|thumb|In a [[Schmitt trigger]] circuit, feedback to the non-inverting input of an amplifier pushes the output directly away from the applied voltage towards the maximum or minimum voltage the amplifier can generate.]]
Positive feedback loops have been used to describe aspects of the dynamics of change in biological [[evolution]]. For example, beginning at the macro level, [[Alfred J. Lotka]] (1945) argued that the evolution of the species was most essentially a matter of selection that fed back energy flows to capture more and more energy for use by living systems.<ref name=Lotka1945/> At the human level, [[Richard D. Alexander]] (1989) proposed that social competition between and within human groups fed back to the selection of intelligence thus constantly producing more and more refined human intelligence. <ref name=Alexander1989/> [[Bernard Crespi|Crespi]] (2004) discussed several other examples of positive feedback loops in evolution.<ref name=Crespi2004/> The analogy of [[Evolutionary arms race]]s provide further examples of positive feedback in biological systems.<ref name=Blindwatchmaker/>
正反馈循环被用来描述生物进化中变化动态的各个方面。 例如，在宏观层面，阿尔弗雷德•J•洛特卡Alfred J. Lotka(1945)认为，物种的进化最本质上是一个选择问题，它反馈了能量流，以获取越来越多的能量供生物系统使用。在人类层面，理查德•D•亚历山大 Richard D. Alexander(1989)提出，人类群体之间和群体内部的社会竞争反馈到智力的选择上，从而不断产生更多、更完善的人类智力。 Bernard Crespi(2004)讨论了进化中正反馈循环的其他几个例子。进化军备竞赛的类比给生物系统中的正反馈提供了进一步的例子。
[[File:Phanerozoic Biodiversity.svg|300px|right|thumb|During the Phanerozoic the [[biodiversity]] shows a steady but not monotonic increase from near zero to several thousands of genera.]]
显生宙，生物多样性显示出稳定但不单调的增加，从接近零到几千属。
It has been shown that changes in [[biodiversity]] through the [[Phanerozoic]] correlate much better with hyperbolic model (widely used in [[demography]] and [[macrosociology]]) than with [[Exponential growth|exponential]] and [[Logistic function|logistic]] models (traditionally used in [[population biology]] and extensively applied to [[fossil]] [[biodiversity]] as well). The latter models imply that changes in diversity are guided by a first-order positive feedback (more ancestors, more descendants) and/or a [[negative feedback]] arising from resource limitation. Hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of the [[world population growth]] has been demonstrated (see below) to arise from a second-order positive feedback between the population size and the rate of [[technological growth]]. The hyperbolic character of biodiversity growth can be similarly accounted for by a positive feedback between the diversity and community structure complexity. It has been suggested that the similarity between the curves of [[biodiversity]] and human population probably comes from the fact that both are derived from the interference of the hyperbolic trend (produced by the positive feedback) with cyclical and stochastic dynamics.<ref>Markov A., [[Andrey Korotayev|Korotayev A.]] [https://archive.today/20120630063924/http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B83WC-4N0HJMK-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=74a80d7c55ff987c9fc8d9c7963feab9 "Phanerozoic marine biodiversity follows a hyperbolic trend." [[Palaeoworld]]. Volume 16, Issue 4, December 2007, Pages 311-318]</ref><ref>{{cite journal | last1 = Markov | first1 = A. | last2 = Korotayev | first2 = A. | year = 2008 | title = Hyperbolic growth of marine and continental biodiversity through the Phanerozoic and community evolution | url = http://elementy.ru/genbio/abstracts?artid=177 | journal = Journal of General Biology | volume = 69 | issue = 3 | pages = 175–194 | pmid = 18677962 | url-status = live | archiveurl = https://web.archive.org/web/20091225000305/http://elementy.ru/genbio/abstracts?artid=177 | archivedate = 2009-12-25 }}</ref>
研究表明，在显生宙，生物多样性的变化与双曲模型（广泛用于人口学和宏观社会学）的相关性要比指数模型和逻辑模型（传统上用于人口生物学，并广泛用于生物多样性化石）的相关性好得多。后者的模型意味着多样性的变化是由一阶正反馈（更多的祖先，更多的后代）和/或资源限制产生的负反馈所引导的。双曲模型意味着二阶正反馈。世界人口增长的双曲线模式已被证明（见下文），源于人口数量与技术增长速度之间的二阶正反馈。生物多样性增长的双曲特征同样可以由多样性与群落结构复杂性之间的正反馈来解释。有人认为，生物多样性和人口曲线之间的相似性可能来自这样一个事实，即两者都是由双曲趋势(由正反馈产生)与周期性和随机性动态的干扰而产生的。
==== Immune system ====
免疫系统
A [[cytokine storm]], or '''hypercytokinemia''' is a potentially fatal immune reaction consisting of a positive feedback loop between [[cytokine]]s and [[immune cell]]s, with highly elevated levels of various cytokines.<ref name="osterholm">{{cite journal | last = Osterholm | first = Michael T. | author-link = Michael Osterholm |title = Preparing for the Next Pandemic | journal = The New England Journal of Medicine | volume = 352 | issue = 18 | pages = 1839–1842 | date = 2005-05-05 | url = | doi = 10.1056/NEJMp058068 | pmid = 15872196 | citeseerx = 10.1.1.608.6200 }}</ref> In normal immune function, positive feedback loops can be utilized to enhance the action of B lymphocytes. When a B cell binds its antibodies to an antigen and becomes activated, it begins releasing antibodies and secreting a complement protein called C3. Both C3 and a B cell's antibodies can bind to a pathogen, and when a B cell has its antibodies bind to a pathogen with C3, it speeds up that B cell's secretion of more antibodies and more C3, thus creating a positive feedback loop.<ref>{{cite journal|last=Paul|first=William E.|title=Infectious Diseases and the Immune System|journal=Scientific American|volume=269|issue=3|date=September 1993|page=93|bibcode=1993SciAm.269c..90P|doi=10.1038/scientificamerican0993-90|pmid=8211095}}</ref>
细胞因子风暴，或称高细胞因子血症，是一种潜在的致命性免疫反应，由细胞因子和免疫细胞之间的正反馈环组成，各种细胞因子水平高度升高[36]在正常的免疫功能中，可以利用正反馈环来增强B淋巴细胞的作用。当B细胞将其抗体与抗原结合并被激活后，就开始释放抗体并分泌一种称为C3的补体蛋白。C3和B细胞的抗体都可以与病原体结合，当B细胞的抗体与C3结合后，就会加快该B细胞分泌更多的抗体和更多的C3，从而形成一个正反馈循环。
==== Cell death ====
细胞死亡
Apoptosis is a caspase-mediated process of cellular death, whose aim is the removal of long-lived or damaged cells. A failure of this process has been implicated in prominent conditions such as cancer or Parkinson's disease. The very core of the apoptotic process is the auto-activation of caspases, which may be modeled via a positive-feedback loop. This positive feedback exerts an auto-activation of the effector caspase by means of intermediate caspases. When isolated from the rest of apoptotic pathway, this positive-feedback presents only one stable steady state, regardless of the number of intermediate activation steps of the effector caspase.
[[Apoptosis]] is a [[caspase]]-mediated process of cellular death, whose aim is the removal of long-lived or damaged cells. A failure of this process has been implicated in prominent conditions such as [[cancer]] or [[Parkinson's disease]]. The very core of the apoptotic process is the auto-activation of caspases, which may be modeled via a positive-feedback loop. This positive feedback exerts an auto-activation of the [[effector caspase]] by means of intermediate caspases. When isolated from the rest of apoptotic pathway, this positive-feedback presents only one stable steady state, regardless of the number of intermediate activation steps of the effector caspase.<ref name="ReferenceA"/> When this core process is complemented with inhibitors and enhancers of caspases effects, this process presents bistability, thereby modeling the alive and dying states of a cell.<ref>{{cite journal|last=Eissing|first=Thomas |doi=10.1074/jbc.M404893200 |title=Bistability analyses of a caspase activation model for receptor-induced apoptosis|journal=Journal of Biological Chemistry|volume=279 |issue=35 |date=2014|pages=36892–36897|pmid=15208304 |doi-access=free}}</ref>
细胞凋亡是一种由酪蛋白酶介导的细胞死亡过程，其目的是清除长寿或受损的细胞。这一过程的失败与癌症或帕金森氏病等著名疾病有关。细胞凋亡过程的核心是半胱氨酸蛋白酶的自动激活，它可以通过一个正反馈循环来建模。这种正反馈通过中间胱天蛋白酶使效应子胱天蛋白酶自动活化。当从凋亡途径的其他部分分离出来时，无论效应子胱天蛋白酶的中间激活步骤的数量如何，这种正反馈仅呈现一种稳定的稳态。
细胞凋亡是一种由酪蛋白酶介导的细胞死亡过程，其目的是清除长寿或受损的细胞。这一过程的失败与癌症或帕金森氏病等著名疾病有关。细胞凋亡过程的核心是半胱氨酸蛋白酶的自动激活，它可以通过一个正反馈循环来建模。这种正反馈通过中间胱天蛋白酶使效应子胱天蛋白酶自动活化。当从凋亡途径的其他部分分离出来时，无论效应子胱天蛋白酶的中间激活步骤的数量如何，这种正反馈仅呈现一种稳定的稳态。
当该核心过程与胱天蛋白酶作用的抑制剂和增强剂相辅相成时，该过程呈现双稳态，从而模拟细胞的存活和死亡状态。
=== In psychology ===
在心理学上
Winner (1996) described gifted children as driven by positive feedback loops involving setting their own learning course, this feeding back satisfaction, thus further setting their learning goals to higher levels and so on.<ref name=Winner1996/> Winner termed this positive feedback loop as a "rage to master." Vandervert (2009a, 2009b) proposed that the [[child prodigy]] can be explained in terms of a positive feedback loop between the output of thinking/performing in [[working memory]], which then is fed to the [[cerebellum]] where it is streamlined, and then fed back to working memory thus steadily increasing the quantitative and qualitative output of working memory.<ref name=Vandervert2009a/><ref name=Vandervert2009b/> Vandervert also argued that this working memory/cerebellar positive feedback loop was responsible for [[language]] evolution in working memory.
Winner (1996) described gifted children as driven by positive feedback loops involving setting their own learning course, this feeding back satisfaction, thus further setting their learning goals to higher levels and so on.
温纳（Winner，1996）将有天赋的孩子描述为受到正反馈循环的驱动，这些反馈循环涉及设置自己的学习课程，反馈自己的满意程度，从而进一步将他们的学习目标提高到更高水平等等。
温纳（Winner，1996）将有天赋的孩子描述为受到正反馈循环的驱动，这些反馈循环涉及设置自己的学习课程，反馈自己的满意程度，从而进一步将他们的学习目标提高到更高水平等等。
Winner将这种正反馈循环称为 "愤怒的掌握"。 Vandervert(2009a，2009b)提出，神童可以用工作记忆中的思维/表现的输出之间的正反馈回路来解释，工作记忆中的思维/表现的输出被反馈到小脑，在那里被精简，然后再反馈到工作记忆中，从而稳定地增加工作记忆的数量和质量输出。
== Terminology ==
=== In economics ===
在经济学中
====Markets with social influence====
具有社会影响力的市场
Product recommendations and information about past purchases have been shown to influence consumers choices significantly whether it is for music, movie, book, technological, and other type of products. Social influence often induces a rich-get-richer phenomenon ([[Matthew effect]]) where popular products tend to become even more popular.<ref name="altszyler2017">{{cite journal | title= Transient dynamics in trial-offer markets with social influence: Trade-offs between appeal and quality. | author1= Altszyler, E | author2= Berbeglia, F. | author3= Berbeglia, G. | author4= Van Hentenryck, P. | journal= PLOS ONE | year= 2017 | volume= 12 | issue= 7 | df= | doi=10.1371/journal.pone.0180040 |pmid = 28746334| pmc= 5528888 | page=e0180040| bibcode= 2017PLoSO..1280040A }}</ref>
事实证明，产品推荐和过去购买的信息对消费者的选择影响很大，无论是音乐、电影、书籍、技术还是其他类型的产品。社会影响往往会诱发一种 "富者越富 "的现象（马太效应），即热门产品往往会变得更加受欢迎。
====Market dynamics====
市场动态
According to the theory of [[reflexivity (social theory)|reflexivity]] advanced by [[George Soros]], price changes are driven by a positive feedback process whereby investors' expectations are influenced by price movements so their behaviour acts to reinforce movement in that direction until it becomes unsustainable, whereupon the feedback drives prices in the opposite direction.<ref>{{citation |title=Behavioural Technical Analysis |first=Paul V. |last=Azzopardi |publisher=Harriman House Limited |year=2010 |page=116 |isbn=9780857190680 |url=https://books.google.com/books?id=04Ay8qviuwgC&pg=PA116&lpg=PA116 |url-status=live |archiveurl=https://web.archive.org/web/20170329103058/https://books.google.com/books?id=04Ay8qviuwgC&pg=PA116&lpg=PA116&source=bl&hl=en&sa=X&f=false |archivedate=2017-03-29 }}</ref>
根据乔治•索罗斯 George Soros提出的反射性理论，价格变化是由一个正反馈过程驱动的，即投资者的预期受到价格变动的影响，因此他们的行为会强化这个方向的运动，直到它变得不可持续，于是反馈推动价格向相反的方向发展。
==== Systemic risk ====
系统性风险
Systemic risk is the risk that an amplification or leverage or positive feedback process presents to a system. This is usually unknown, and under certain conditions this process can amplify exponentially and rapidly lead to destructive or chaotic behavior. A Ponzi scheme is a good example of a positive-feedback system: funds from new investors are used to pay out unusually high returns, which in turn attract more new investors, causing rapid growth toward collapse. W. Brian Arthur has also studied and written on positive feedback in the economy (e.g. W. Brian Arthur, 1990). Hyman Minsky proposed a theory that certain credit expansion practices could make a market economy into "a deviation amplifying system" that could suddenly collapse, sometimes called a "Minsky moment".
Systemic risk is the risk that an amplification or leverage or positive feedback process presents to a system. This is usually unknown, and under certain conditions this process can amplify exponentially and rapidly lead to destructive or chaotic behavior. A Ponzi scheme is a good example of a positive-feedback system: funds from new investors are used to pay out unusually high returns, which in turn attract more new investors, causing rapid growth toward collapse. W. Brian Arthur has also studied and written on positive feedback in the economy (e.g. W. Brian Arthur, 1990). Hyman Minsky proposed a theory that certain credit expansion practices could make a market economy into "a deviation amplifying system" that could suddenly collapse, sometimes called a "Minsky moment".
2010年的闪崩事件被归咎于高频交易（HFT）的做法，不过HFT是否真的会增加系统性风险仍然存在争议。
2010年的闪崩事件被归咎于高频交易（HFT）的做法，不过HFT是否真的会增加系统性风险仍然存在争议。
|url-status = live
|title= Human Population and the Environmental Crisis
人口与环境危机
Agriculture and human population can be considered to be in a positive feedback mode, which means that one drives the other with increasing intensity. It is suggested that this positive feedback system will end sometime with a catastrophe, as modern agriculture is using up all of the easily available phosphate and is resorting to highly efficient monocultures which are more susceptible to systemic risk.
Agriculture and human population can be considered to be in a positive feedback mode, which means that one drives the other with increasing intensity. It is suggested that this positive feedback system will end sometime with a catastrophe, as modern agriculture is using up all of the easily available phosphate and is resorting to highly efficient monocultures which are more susceptible to systemic risk.
技术创新和人类人口也可以有类似的考虑，这也是过去人类人口明显的双曲线增长，而不是简单的指数增长的一个解释。
技术创新和人类人口也可以有类似的考虑，这也是过去人类人口明显的双曲线增长，而不是简单的指数增长的一个解释。
It is proposed that the growth rate is accelerating because of second-order positive feedback between population and technology. Technological growth increases the carrying capacity of land for people, which leads to a growing population, and this in turn drives further technological growth.
It is proposed that the growth rate is accelerating because of second-order positive feedback between population and technology. Technological growth increases the carrying capacity of land for people, which leads to a growing population, and this in turn drives further technological growth.
有人提出，由于人口和技术之间的二阶正反馈，增长速度正在加快。技术增长增加了土地对人的承载能力，从而导致人口增长，而这反过来又推动了技术的进一步增长。
有人提出，由于人口和技术之间的二阶正反馈，增长速度正在加快。技术增长增加了土地对人的承载能力，从而导致人口增长，而这反过来又推动了技术的进一步增长。
Prejudice, social institutions and poverty
Gunnar Myrdal described a vicious circle of increasing inequalities, and poverty, which is known as "circular cumulative causation".
Gunnar Myrdal described a vicious circle of increasing inequalities, and poverty, which is known as "circular cumulative causation".
==== In meteorology ====
== Examples and applications ==
=== In electronics ===
Drought intensifies through positive feedback. A lack of rain decreases soil moisture, which kills plants and/or causes them to release less water through transpiration. Both factors limit evapotranspiration, the process by which water vapor is added to the atmosphere from the surface, and add dry dust to the atmosphere, which absorbs water. Less water vapor means both low dew point temperatures and more efficient daytime heating, decreasing the chances of humidity in the atmosphere leading to cloud formation. Lastly, without clouds, there cannot be rain, and the loop is complete.
Drought intensifies through positive feedback. A lack of rain decreases soil moisture, which kills plants and/or causes them to release less water through transpiration. Both factors limit evapotranspiration, the process by which water vapor is added to the atmosphere from the surface, and add dry dust to the atmosphere, which absorbs water. Less water vapor means both low dew point temperatures and more efficient daytime heating, decreasing the chances of humidity in the atmosphere leading to cloud formation. Lastly, without clouds, there cannot be rain, and the loop is complete.
干旱通过正反馈加剧。缺雨会降低土壤湿度，从而杀死植物和/或使它们通过蒸腾作用释放更少的水分。这两个因素都限制了蒸发，即水蒸气从地表加到大气中的过程，并使吸收水分的干燥灰尘进入大气。水汽少了，既意味着露点温度低，白天的供暖效率也高，减少了大气中湿度导致云的形成的机会。最后，没有云，就不会有雨，这个循环就完成了。
干旱通过正反馈加剧。缺雨会降低土壤湿度，从而杀死植物和/或使它们通过蒸腾作用释放更少的水分。这两个因素都限制了蒸发，即水蒸气从地表加到大气中的过程，并使吸收水分的干燥灰尘进入大气。水汽少了，既意味着露点温度低，白天的供暖效率也高，减少了大气中湿度导致云的形成的机会。最后，没有云，就不会有雨，这个循环就完成了。
==== In climatology ====
Climate "forcings" may push a climate system in the direction of warming or cooling, for example, increased atmospheric concentrations of greenhouse gases cause warming at the surface. Forcings are external to the climate system and feedbacks are internal processes of the system. Some feedback mechanisms act in relative isolation to the rest of the climate system while others are tightly coupled. Forcings, feedbacks and the dynamics of the climate system determine how much and how fast the climate changes. The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, which in turn leads to further warming. The main negative feedback comes from the Stefan–Boltzmann law, the amount of heat radiated from the Earth into space is proportional to the fourth power of the temperature of Earth's surface and atmosphere.
Climate "forcings" may push a climate system in the direction of warming or cooling, for example, increased atmospheric concentrations of greenhouse gases cause warming at the surface. Forcings are external to the climate system and feedbacks are internal processes of the system. Some feedback mechanisms act in relative isolation to the rest of the climate system while others are tightly coupled. Forcings, feedbacks and the dynamics of the climate system determine how much and how fast the climate changes. The main positive feedback in global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, which in turn leads to further warming. The main negative feedback comes from the Stefan–Boltzmann law, the amount of heat radiated from the Earth into space is proportional to the fourth power of the temperature of Earth's surface and atmosphere.
气候 "诱因 "可能会将气候系统推向变暖或变冷的方向，例如，大气中温室气体浓度的增加会导致地表变暖。诱因是气候系统的外部因素，而反馈是系统的内部过程。一些反馈机制与气候系统的其他部分相对孤立地发挥作用，而其他机制则是紧密耦合的。气候系统的作用力、反馈和动态决定了气候变化的程度和速度。全球变暖的主要正反馈是变暖使大气中的水汽量增加的趋势，进而导致进一步变暖。主要的负反馈来自Stefan-Boltzmann定律，从地球辐射到空间的热量与地球表面和大气温度的四次方成正比。
气候 "诱因 "可能会将气候系统推向变暖或变冷的方向，例如，大气中温室气体浓度的增加会导致地表变暖。诱因是气候系统的外部因素，而反馈是系统的内部过程。一些反馈机制与气候系统的其他部分相对孤立地发挥作用，而其他机制则是紧密耦合的。气候系统的作用力、反馈和动态决定了气候变化的程度和速度。全球变暖的主要正反馈是变暖使大气中的水汽量增加的趋势，进而导致进一步变暖。主要的负反馈来自Stefan-Boltzmann定律，从地球辐射到空间的热量与地球表面和大气温度的四次方成正比。
Other examples of positive feedback subsystems in climatology include:
Other examples of positive feedback subsystems in climatology include:
气候学中正反馈子系统的其他例子包括:
气候学中正反馈子系统的其他例子包括:
A warmer atmosphere will melt ice and this changes the albedo which further warms the atmosphere.
大气变暖会使冰融化，从而改变反照率，从而使大气进一步变暖。
Methane hydrates can be unstable so that a warming ocean could release more methane, which is also a greenhouse gas.
甲烷水合物可能是不稳定的，所以海洋变暖可能会释放更多的甲烷，甲烷也是一种温室气体。
Peat, occurring naturally in peat bogs, contains carbon. When peat dries it decomposes, and may additionally burn. Peat also releases nitrous oxide.
泥炭，天然存在于泥炭沼泽中，含有碳。当泥炭干燥时，它会分解，并可能额外燃烧。泥炭还会释放一氧化二氮。
Global warming affects the cloud distribution. Clouds at higher altitudes enhance the greenhouse effects, while low clouds mainly reflect back sunlight, having opposite effects on temperature.
全球变暖会影响云的分布。高空的云层会增强温室效应，而低空的云层则主要反射太阳光，对温度产生相反的影响。
The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report states that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."
The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report states that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."
政府间气候变化专门委员会(气专委)第四次评估报告指出，"人类活动的变暖可能导致一些突然或不可逆转的影响，这取决于气候变化的速度和程度"。
政府间气候变化专门委员会(气专委)第四次评估报告指出，"人类活动的变暖可能导致一些突然或不可逆转的影响，这取决于气候变化的速度和程度"。
==== In sociology ====
A self-fulfilling prophecy is a social positive feedback loop between beliefs and behavior: if enough people believe that something is true, their behavior can make it true, and observations of their behavior may in turn increase belief. A classic example is a bank run.
A self-fulfilling prophecy is a social positive feedback loop between beliefs and behavior: if enough people believe that something is true, their behavior can make it true, and observations of their behavior may in turn increase belief. A classic example is a bank run.
自我应验预言是信念和行为之间的一个社会正反馈循环: 如果有足够多的人相信某件事是真的，他们的行为就能让它变成真的，而对他们行为的观察又可能反过来增加信念。一个典型的例子是银行挤兑。
自我应验预言是信念和行为之间的一个社会正反馈循环: 如果有足够多的人相信某件事是真的，他们的行为就能让它变成真的，而对他们行为的观察又可能反过来增加信念。一个典型的例子是银行挤兑。
Another sociological example of positive feedback is the network effect. When more people are encouraged to join a network this increases the reach of the network therefore the network expands ever more quickly. A viral video is an example of the network effect in which links to a popular video are shared and redistributed, ensuring that more people see the video and then re-publish the links. This is the basis for many social phenomena, including Ponzi schemes and chain letters. In many cases population size is the limiting factor to the feedback effect.
Another sociological example of positive feedback is the network effect. When more people are encouraged to join a network this increases the reach of the network therefore the network expands ever more quickly. A viral video is an example of the network effect in which links to a popular video are shared and redistributed, ensuring that more people see the video and then re-publish the links. This is the basis for many social phenomena, including Ponzi schemes and chain letters. In many cases population size is the limiting factor to the feedback effect.
正反馈的另一个社会学例子是网络效应。当更多的人被鼓励加入一个网络时，这就增加了网络的覆盖面，因此网络扩张得越来越快。病毒视频就是网络效应的一个例子，在这个例子中，一个热门视频的链接被分享和再传播，确保更多的人看到这个视频，然后重新发布链接。这是许多社会现象的基础，包括庞氏骗局和连锁信。在许多情况下，人口数量是反馈效应的限制因素。
正反馈的另一个社会学例子是网络效应。当更多的人被鼓励加入一个网络时，这就增加了网络的覆盖面，因此网络扩张得越来越快。病毒视频就是网络效应的一个例子，在这个例子中，一个热门视频的链接被分享和再传播，确保更多的人看到这个视频，然后重新发布链接。这是许多社会现象的基础，包括庞氏骗局和连锁信。在许多情况下，人口数量是反馈效应的限制因素。
==== In chemistry ====
If a chemical reaction causes the release of heat, and the reaction itself happens faster at higher temperatures, then there is a high likelihood of positive feedback. If the heat produced is not removed from the reactants fast enough, thermal runaway can occur and very quickly lead to a chemical explosion.
If a chemical reaction causes the release of heat, and the reaction itself happens faster at higher temperatures, then there is a high likelihood of positive feedback. If the heat produced is not removed from the reactants fast enough, thermal runaway can occur and very quickly lead to a chemical explosion.
如果化学反应引起热量的释放，而反应本身在较高的温度下发生得更快，那么就很有可能出现正反馈。如果产生的热量没有足够快地从反应物中排除，就会发生热失控，并很快导致化学爆炸。
如果化学反应引起热量的释放，而反应本身在较高的温度下发生得更快，那么就很有可能出现正反馈。如果产生的热量没有足够快地从反应物中排除，就会发生热失控，并很快导致化学爆炸。
==== In conservation ====
Many wildlife are hunted for their parts which can be quite valuable. The closer to extinction that targeted species become, the higher the price there is on their parts. This is an example of positive feedback.
Many wildlife are hunted for their parts which can be quite valuable. The closer to extinction that targeted species become, the higher the price there is on their parts. This is an example of positive feedback.
许多野生动物被猎杀的原因是它们身体的某些部位可能相当有价值。目标物种越是接近灭绝，其部位的价格就越高。这就是一个正反馈的例子。
====See also====
另请参阅
* Chain reaction – Sequence of reactions 链式反应 -- -- 反应的顺序
* Donella Meadows' twelve leverage points to intervene in a system Donella Meadows干预系统的十二个杠杆点
* Hyperbolic growth 双曲增长
* Reflexivity (social theory)反射性（社会理论）
* Stability criterion稳定性标准
* Strategic complements战略补充
* System dynamics 系统动力学
* Technological singularity – Hypothetical point in time at which technological growth becomes uncontrollable and irreversible 技术奇点 -- -- 假设技术增长变得不可控制和不可逆转的时间点
* Thermal runaway 热失控
* Vicious/virtuous circle: in social and financial systems, a complex of events that reinforces itself through a feedback loop. 恶性/恶性循环：在社会和金融系统中，是通过反馈循环自我强化的事件综合体。
* Positive reinforcement: a situation in operant conditioning where a consequence increases the frequency of a behaviour. 正强化：在操作性条件下，结果会增加行为的频率。
* Praise of performance: a term often applied in the context of performance appraisal,although this usage is disputed 绩效表扬：这个词经常被应用于绩效评估中，尽管这种用法有争议。
* Self-reinforcing feedback: a term used in systems dynamics to avoid confusion with the "praise" usage 自我强化反馈：系统动力学中使用的术语，以避免与 "表扬 "的用法相混淆。
* Matthew effect – Effect originally observed by Robert K. Merton 马太效应 -- -- 罗伯特-K-默顿最初观察到的效应。
* Self-fulfilling prophecy – Prediction that causes itself to become true 自证预言 -- -- 导致自己成为现实的预言
* Virtuous circle and vicious circle 良性循环和恶性循环
* Autocatalysis 自催化
* Meander – Sinuous bend in a series in the channel of a river 蜿蜒曲折 -- -- 河道中一系列蜿蜒曲折的弯道。
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====Further reading ====
* Norbert Wiener (1948), Cybernetics or Control and Communication in the Animal and the Machine, Paris, Hermann et Cie - MIT Press, Cambridge, MA.
* Katie Salen and Eric Zimmerman. Rules of Play. MIT Press. 2004. ISBN 0-262-24045-9. Chapter 18: Games as Cybernetic Systems.