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第1行： − 此词条暂由彩云小译翻译，翻译字数共1716，未经人工整理和审校，带来阅读不便，请见谅。+ 第29行：
第29行： − + − + + + + + − Mathematically, positive feedback is defined as a positive loop gain around a closed loop of cause and effect. 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.+ − 在数学上，正反馈被定义为一个围绕着一个闭合的因果循环的正循环增益。系统参数通常会加速到极值，这可能会损坏或破坏系统，或者结束时系统锁定到一个新的稳定状态。正反馈可以由系统中被滤波、阻尼或限制的信号来控制，也可以通过增加负反馈来抵消或减少正反馈。+ 第45行：
第49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 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.+ − 正反馈在数字电子学中被用来迫使中间电压进入“0”和“1”状态。另一方面，热失控是一种能够破坏半导体结的正反馈。化学反应中的正反馈可以提高反应速度，在某些情况下可以导致爆炸。机械设计中的正反馈会导致转折点，或者说“过中心” ，机构卡扣到位，例如在开关和大力钳中。失去控制，它可能导致桥梁倒塌。经济系统中的正反馈可以导致繁荣-然后衰退的循环。正反馈的一个熟悉的例子是公共广播系统中音频反馈产生的大声尖叫或啸叫声: 麦克风从自己的扬声器中收集声音，放大它，然后再通过扬声器发送出去。+ − |publisher = Jones & Bartlett Learning+ − [[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. ]]+ − 血小板凝固呈正反馈。受损的血管壁释放出化学物质，通过血小板聚集引发血凝块的形成。随着更多的血小板聚集，更多的化学物质被释放，从而加速了这一过程。这个过程变得越来越快，直到血管壁完全密封，正反馈循环结束。图表的指数形式说明了正反馈机制。]+ − |isbn = 9780867209662+ 第63行：
第67行： − 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 is increased. 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. 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 judgements as to the desirability of the outcomes or effects.+ − 正反馈通过对产生正反馈的过程产生影响而增强或放大效果。例如，当一个电子输出信号的一部分返回到输入端，并且与其同相时，系统增益增加。这样的系统可以给出丰富的定性行为，但是瞬时正反馈或负反馈对结果有极其重要的影响。积极的反馈加强和消极的反馈调节原来的过程。在这个意义上，积极和消极是指环路收益大于或小于零，并不意味着对结果或效果的可取性作出任何价值判断。+ + + + + + + + + 第79行：
第91行： − 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).]]+ − 老式的再生式收音机。由于控制使用正反馈，足够的放大可以从一个[[真空管或阀门(中心)]+ 第87行：
第99行： − Regenerative circuits were invented and patented in 1914+ − 再生电路于1914年发明并获得专利+ + + + + 第103行：
第119行： − 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.+ − 在大多数情况下，这种反馈回路最终导致反信号的释放，抑制或打破循环。当婴儿离开母亲的身体时，分娩就停止了。化学物质分解血凝块。当婴儿不再哺乳时，哺乳就停止了。+ − |page = 55+ + + + + − 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. 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. 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.+ − 正反馈是基因调控中一个被广泛研究的现象，它通常与双稳态联系在一起。当一个基因通过双重负反馈回路直接或间接地激活自身时，正反馈就发生了。基因工程师已经在细菌中构建和测试了简单的正反馈网络，以证明双稳性的概念。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重大的生理后果。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，比如从同一个亲本细胞中创造出表型不同的细胞群。这是因为正反馈会放大噪音。正反馈也可以发生在其他形式的细胞信号，如酶动力学或代谢途径。+ − |url-status = live+ 第125行：
第145行： − }}</ref>+ − 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. 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. Crespi (2004) discussed several other examples of positive feedback loops in evolution. The analogy of Evolutionary arms races provide further examples of positive feedback in biological systems.+ − 正反馈循环已经被用来描述生物进化中动态变化的各个方面。例如，从宏观层面开始，阿尔弗雷德 · j · 洛特卡(1945)认为物种的进化本质上是一个选择问题，这个选择反馈能量流以获取越来越多的能量供生命系统使用。在人类层面上，理查德 · 亚历山大(1989)提出，人类群体之间和群体内部的社会竞争反馈给智力的选择，从而不断产生越来越精细的人类智力。Crespi (2004)讨论了进化过程中正反馈循环的其他几个例子。进化军备竞赛的类比为生物系统中的正反馈提供了进一步的例子。+ − {{cite book+ − During the Phanerozoic the [[biodiversity shows a steady but not monotonic increase from near zero to several thousands of genera.]]+ − 在显生宙期间[生物多样性从接近零增加到数千属，呈现出稳定而非单调的增长]+ − 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 and 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. 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. 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. Vandervert also argued that this working memory/cerebellar positive feedback loop was responsible for language evolution in working memory.+ − 研究表明，显生宙生物多样性的变化与双曲线模型(广泛应用于人口学和宏观社会学)相关性远远好于指数模型和逻辑斯谛模型(传统上用于种群生物学，也广泛应用于化石生物多样性)。后一种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/或由于资源限制而产生的负反馈引导的。双曲模型是一个二阶正反馈模型。世界人口增长的双曲线模式已经证明(见下文)是由人口规模与技术增长率之间的二阶正反馈所产生的。生物多样性增长的双曲特征可以通过多样性与群落结构复杂性之间的正反馈来解释。生物多样性曲线与人类种群曲线之间的相似性可能是由于两者都是由双曲线趋势(正反馈产生)对周期性和随机性动力学的干扰而导致的。当这个核心过程补充了抑制剂和增强剂的 caspases 效应，这个过程呈现双稳态，从而模拟了一个细胞的存活和死亡状态。赢家把这种正反馈回路称为“掌控狂怒”Vandervert (2009a，2009b)提出，神童可以用工作记忆中思维/表演输出之间的正反馈回路来解释，然后这个回路被输送到小脑，在小脑进行流线型处理，然后反馈回工作记忆，从而稳步增加工作记忆的数量和质量输出。范德沃特还认为，这种工作记忆/小脑正反馈回路负责工作记忆中的语言进化。+ 第150行：
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第179行： − 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.+ − 无论是音乐、电影、书籍、科技产品还是其他类型的产品，产品推荐信息和过去购买行为的信息都会显著影响消费者的选择。社会影响常常引发“富有致富”现象(马太效应) ，在这种现象中，流行产品往往变得更加流行。+ + + + + + + + + + + + + − Gunnar Myrdal described a vicious circle of increasing inequalities, and poverty, which is known as "circular cumulative causation".+ − 贡纳尔 · 缪尔达尔描述了不平等和贫困加剧的恶性循环，这被称为”循环累积因果关系”。+ − |archiveurl = https://web.archive.org/web/20170329045057/https://books.google.com/books?id=lROa-MpIrucC&pg=PA247+ + + + + − 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.+ − 干旱通过积极的反馈加剧。缺少雨水会减少土壤的水分，从而杀死植物和/或导致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散---- 水蒸气从表面进入大气的过程---- 以及向大气中添加干燥的尘埃，吸收水分。水蒸气减少意味着露点温度降低，白天加热效率提高，从而降低了大气中的湿度导致云形成的机会。最后，没有云，就不会有雨，这个循环就是完整的。+ − |title = Modern Dictionary of Electronics+ + + + + + + + + − 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“强迫”可能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导致地表变暖。强迫是气候系统外部的，反馈是系统的内部过程。一些反馈机制相对孤立于气候系统的其他部分，而另一些则是紧密耦合的。强迫、反馈和气候系统的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趋势增加了大气中的水汽量，进而导致进一步变暖。主要的负反馈来自斯蒂芬-波尔兹曼定律，从地球辐射到太空的热量与地球表面和大气温度的四次方成正比。+ − |page = 276+ − 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:+ − |url-status = live+ + + + + 第217行：
第269行： + + − 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政府间气候变化专门委员会第四次评估报告指出: “人为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。” + + + + 第229行：
第285行： + + − 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，他们的行为可以让它变成真的，对他们行为的观察可能反过来增加信念。一个典型的例子是银行挤兑。 第239行：
第295行： − 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范围，因此网络扩展得更快。病毒式视频是网络效应的一个例子，其中一个流行的视频链接被分享和重新分发，确保更多的人看到视频，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，种群大小是反馈效应的限制因素。+ + + + + 第251行：
第311行： − 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.+ − 如果一个化学反应导致了热量的释放，并且反应本身在更高的温度下发生得更快，那么正反馈的可能性就很大。如果产生的热量没有足够快地从反应物中移除，就会发生热失控，并很快导致化学爆炸。+ + + + + 第263行：
第327行： − 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.+ − 许多野生动物因为它们的身体部位而遭到猎杀，这些身体部位非常珍贵。目标物种越接近灭绝，其部分的价格就越高。这是一个积极反馈的例子。+ − :<math>G_c = A/(1-AB)</math>+ 第274行：
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第475行： − + − + − According to {{harvtxt|Mindell|2002}} confusion in the terms arose shortly after this:+ − ::"...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}}+ 第378行：
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第713行： − 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413行：
第725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>+
Moved page from wikipedia:en:Positive feedback (history)
此词条暂由彩云小译翻译，翻译字数共3765，未经人工整理和审校，带来阅读不便，请见谅。
{{short description|Destabilising process that occurs in a feedback loop}}
{{short description|Destabilising process that occurs in a feedback loop}}
'''Positive feedback''' ('''exacerbating feedback''', '''self-reinforcing feedback''') is a process that occurs in a [[feedback loop]] which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the magnitude of the perturbation.<ref name=zuckerman>{{cite book
'''Positive feedback''' ('''exacerbating feedback''', '''self-reinforcing feedback''') is a process that occurs in a [[feedback loop]] which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the magnitude of the perturbation.<ref name=zuckerman>{{cite book
Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the magnitude of the perturbation. Both concepts play an important role in science and engineering, including biology, chemistry, and cybernetics.
Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system in which the results of a change act to reduce or counteract it has negative feedback.
正反馈(加剧的反馈，自我强化的反馈)是一个在反馈回路中发生的过程，它加剧了小干扰的影响。也就是说，扰动对系统的影响包括扰动幅度的增加。这两个概念在科学和工程学中都扮演着重要的角色，包括生物学、化学和控制论。
正反馈(加剧的反馈，自我强化的反馈)是一个在反馈回路中发生的过程，它加剧了小干扰的影响。也就是说，扰动对系统的影响包括扰动幅度的增加。也就是说，a 产生了更多的 b，而 b 又产生了更多的 a。相比之下，一个系统中的结果的变化行动，以减少或抵消它的负反馈。
|title = Human Population and the Environmental Crisis
|title = Human Population and the Environmental Crisis
That is, positive feedback is in phase with the input, in the sense that it adds to make the input larger.
也就是说，正反馈与输入是同步的，也就是说，正反馈增加了输入，使输入变得更大。
|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.
正反馈容易导致系统不稳定。当环路增益为正且大于1时，通常会出现指数增长、增加的振荡、混沌行为或其它偏离平衡的现象。从结果到原始流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会变大。当一个系统发生变化时，正反馈会引起同一方向的进一步变化。
|author2 = David Jefferson
|author2 = David Jefferson
|name-list-style = amp
|name-list-style = amp
|publisher = Jones & Bartlett Learning
causal functions.]]
[因果函数]
|year = 1996
|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.
图中显示了一个简单的反馈回路。如果环路增益 AB 为正，则存在正反馈或再生反馈的条件。
|page = 42
|page = 42
|url = https://books.google.com/books?id=a1gW4uV-q8EC&pg=PA42
|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，AB 小于1，那么从输入到输出的整个系统增益是有限的，但是当 AB 接近1时，增益可以非常大。在这种情况下，可以表明从输入到输出的总体或“闭环”增益是:
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|archiveurl = https://web.archive.org/web/20180106192002/https://books.google.com/books?id=a1gW4uV-q8EC&pg=PA42
|archiveurl = https://web.archive.org/web/20180106192002/https://books.google.com/books?id=a1gW4uV-q8EC&pg=PA42
<math>G_c = A/(1-AB)</math>
[ math > g _ c = a/(1-AB)]
|archivedate = 2018-01-06
|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]].
}}</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]].
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，所以没有明确的增益，增益可以称为无穷大。
Mathematically, positive feedback is defined as a positive [[loop gain]] around a closed loop of cause and effect.<ref name=zuckerman/><ref name=theorymodelling>
Mathematically, positive feedback is defined as a positive [[loop gain]] around a closed loop of cause and effect.<ref name=zuckerman/><ref name=theorymodelling>
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。正反馈的结果是增加变化，因此小的扰动可能导致大的变化。
{{cite book
{{cite book
|title = Theory of Modeling and Simulation: Integrating Discrete Event and Continuous Complex Dynamic Systems
|title = Theory of Modeling and Simulation: Integrating Discrete Event and Continuous Complex Dynamic Systems
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，这个系统可能是不稳定的，在这种情况下，这个系统被称为处于不稳定的平衡状态。使这样一个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”的增加函数。
|author1 = Bernard P. Zeigler
|author1 = Bernard P. Zeigler
|author2 = Herbert Praehofer
|author2 = Herbert Praehofer
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不稳定，例如，在正反馈结构中可能存在稳定的开关状态。
|author3 = Tag Gon Kim Section
|author3 = Tag Gon Kim Section
|isbn = 9780127784557
|isbn = 9780127784557
|page = 55
Hysteresis causes the output value to depend on the history of the input
滞后导致输出值依赖于输入的历史
|section = 3.3.2 Feedback in continuous systems
|section = 3.3.2 Feedback in continuous systems
|url = https://books.google.com/books?id=REzmYOQmHuQC&pg=PA55
|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，对放大器非反相输入的反馈将输出直接从外加电压推向放大器可以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>.
|quote = A positive feedback loop is one with an even number of negative influences <nowiki>[around the loop]</nowiki>.
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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，正反馈回路通常不会导致不断增长的增长，而是通过某种形式的限制效应进行修正。根据多内拉 · 梅多斯的说法:
|archiveurl = https://web.archive.org/web/20170103061121/https://books.google.com/books?id=REzmYOQmHuQC&pg=PA55
|archiveurl = https://web.archive.org/web/20170103061121/https://books.google.com/books?id=REzmYOQmHuQC&pg=PA55
|archivedate = 2017-01-03
|archivedate = 2017-01-03
"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."
“正反馈回路是系统增长、爆炸、腐蚀和崩溃的根源。一个系统如果有一个不受控制的正循环，最终将会自我毁灭。这就是为什么它们如此稀少的原因。通常情况下，负面循环迟早会发生。”
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That is, positive feedback is [[Phase (waves)|in phase with]] the input, in the sense that it adds to make the input larger.<ref>
That is, positive feedback is [[Phase (waves)|in phase with]] the input, in the sense that it adds to make the input larger.<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.
起始点影响系统的最终位置的滞后现象，可以通过正反馈产生。当反馈回路的增益大于1时，输出就远离输入: 如果输出高于输入，那么输出就向最近的正极限移动; 如果输出低于输入，那么输出就向最近的负极限移动。
{{cite book
|title = Newnes Dictionary of Electronics
|title = Newnes Dictionary of Electronics
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。然而，如果输入超过极限，那么反馈将改变符号，输出将朝相反的方向移动，直到达到相反的极限。因此，该系统表现出双稳态行为。
|edition = 4th
|edition = 4th
|author2 = R W Amos
|author2 = R W Amos
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.
第二次世界大战前，正面和负面这两个词首次用于反馈。正反馈的想法在20世纪20年代随着再生回路的引入已经很流行了。
|publisher = Newnes
|publisher = Newnes
|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:
将一组电子放大器中的再生描述为这样一种情况，即“反馈”作用是积极的，而消极反馈作用只是顺便提到。哈罗德·史蒂芬·布莱克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。布莱克说:
|isbn = 9780750656429
|isbn = 9780750656429
"Positive feed-back increases the gain of the amplifier, negative feed-back reduces it."
正反馈增加放大器的增益，负反馈减小放大器的增益
|page = 247
|page = 247
According to confusion in the terms arose shortly after this:
根据这之后不久出现的术语混淆:
|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 和 Jensen 对 Black 所用的‘正反馈’和‘负反馈’做了同样的区分，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。相比之下，当 Nyquist 和 Bode 基于 Black 的研究成果时，他们将负面反馈称为符号相反的负面反馈。布莱克很难说服其他人相信他的发明是有用的，部分原因是在基本的定义问题上存在混淆。”用来放大和接收非常微弱的无线电信号。在一个晶体管放大器周围精心控制的正反馈可以使其增益乘以1000倍或更多。因此，一个信号在一个阶段可以被放大20,000倍甚至100,000倍，这通常只有20到50倍的增益。再生放大器工作在这些非常高的增益的问题是，他们很容易变得不稳定，并开始振荡。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外差设计，放大级数多，但工作稳定，无正反馈。
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|archiveurl = https://web.archive.org/web/20170329045057/https://books.google.com/books?id=lROa-MpIrucC&pg=PA247
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.
在再生式无线电电路中可能爆发的振荡用于电子振荡器。通过使用调谐电路或压电晶体(通常是石英) ，正反馈放大的信号保持线性和正弦。这种谐振子有几种设计方案，包括阿姆斯特朗振荡器、哈特莱振荡器、 Colpitts振盪器和文氏电桥。它们都利用正反馈来产生振荡。
|archivedate = 2017-03-29
|archivedate = 2017-03-29
}}</ref><ref>
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Many electronic circuits, especially amplifiers, incorporate negative feedback. This reduces their gain, but improves their linearity, input impedance, output impedance, and 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 AC signals is one of 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 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 filtering). 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 oscillations. 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.
许多电子电路，特别是放大器，都包含负反馈。这降低了它们的增益，但是提高了它们的线性度、输入阻抗、输出阻抗和带宽，并且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设备本身的细节，而更多地依赖于反馈元件，这些元件不太可能随着制造公差、年龄和温度而改变。正反馈和负反馈对于交流信号的区别是相位的区别: 如果信号反馈出相位，反馈是负的，如果反馈是同相的，反馈是正的。对于使用负反馈的放大器设计者来说，一个问题是电路中的一些元件会在反馈通路中引入相移。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放大器在该频率的增益非常低(通常通过低通滤波)。如果环路增益(放大器增益的乘积和正反馈的范围)在任何频率大于1，那么放大器将振荡在该频率(巴克豪森稳定性准则)。这种振荡有时称为寄生振荡。一个在一组条件下稳定的放大器可以在另一组条件下变成寄生振荡。这可能是由于温度的变化，电源电压，调整前面板控制，甚至接近一个人或其他传导项目。
{{cite book
{{cite book
|title = Modern Dictionary of Electronics
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.
放大器可能会以一种没有示波器就很难检测到的方式轻微振荡，或者振荡范围太广，以至于只有一个非常失真或根本不需要的信号才能通过，或者发生损坏。低频寄生振荡被称为“汽艇” ，由于类似的声音低转速排气说明。
|edition = 7th
|edition = 7th
|author = Rudolf F. Graf
|author = Rudolf F. Graf
The effect of using a Schmitt trigger (B) instead of a comparator (A)
使用施密特触发器(b)代替比较器(a)的效果
|publisher = Newnes
|publisher = Newnes
|year = 1999
|year = 1999
Many common digital electronic circuits employ positive feedback. While normal simple boolean logic gates usually rely simply on gain to push digital signal voltages away from intermediate values to the values that are meant to represent boolean '0' and '1', but many more complex gates use feedback. When an input voltage is expected to vary in an 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 latching circuit.
许多常用的数字电路都采用正反馈。一般简单的布尔逻辑门通常仅仅依靠增益将数字信号的电压从中间值推到代表布尔值0和1的值，但是许多更复杂的门使用反馈。当输入电压以模拟方式发生变化，但是需要明确的阈值以便后续的数字处理时，施密特触发电路使用正反馈，以确保当输入电压轻微超过阈值时，输出被巧妙而迅速地从一个逻辑状态转移到另一个。施密特触发器使用正反馈的一个推论是，如果输入电压再次缓慢下降，超过相同的阈值，正反馈将使输出保持在相同的状态而不改变。这种效应被称为滞后: 输入电压必须降到一个不同的、较低的阈值，才能“解锁”输出，并将其复位到原来的数字值。通过减小正反馈的范围，可以减小滞后宽度，但不能完全消除。施密特触发器在某种程度上是一个闭锁电路。
|isbn = 9780750698665
|isbn = 9780750698665
|page = 276
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]
正反馈是一种增强输出的机制，例如蛋白质水平。然而，为了避免蛋白质水平的任何波动，该机制随机被抑制(i) ，因此当激活蛋白质(a)的浓度超过阈值([ i ])时，环路机制被激活，当 d [ a ] = k [ a ]时，a 的浓度呈指数增加
|url = https://books.google.com/books?id=uah1PkxWeKYC&pg=PA276
|url = https://books.google.com/books?id=uah1PkxWeKYC&pg=PA276
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nor gates with positive feedback. Red and black mean logical '1' and '0', respectively.]]
也没有盖茨的积极反馈。红色和黑色分别代表逻辑上的’1’和’0’。]
|archiveurl = https://web.archive.org/web/20170329122152/https://books.google.com/books?id=uah1PkxWeKYC&pg=PA276
|archiveurl = https://web.archive.org/web/20170329122152/https://books.google.com/books?id=uah1PkxWeKYC&pg=PA276
|archivedate = 2017-03-29
|archivedate = 2017-03-29
An electronic 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 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. Computer random access memory (RAM) can be made in this way, with one latching circuit for each bit of memory.
电子触发器，或“锁存器” ，或“双稳态多谐振荡器” ，是一个电路，由于高正反馈是不稳定的平衡或居间态。这种双稳态电路是一位电子存储器的基础。触发器使用一对相互连接的放大器、晶体管或逻辑门，以便在输入信号被移除后，正反馈将电路的状态维持在两个不平衡稳定状态之一，直到应用合适的替代信号来改变状态。计算机随机存取存储器(RAM)可以这样制作，每位存储器有一个锁存电路。
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}}</ref>
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.
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.
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.
热失控发生在电子系统中，因为电路的某些方面被允许通过更多的电流，当它变得更热时，它变得越热，它通过的电流就越多，这样它就会加热更多，从而通过更多的电流。这种影响通常对相关设备是灾难性的。如果设备必须使用在接近其最大功率处理能力的地方，并且在某些条件下有可能发生或可能发生热失控，那么通常可以通过仔细的设计来实现改进。
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.
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.
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. ]]
[[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. ]]
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证明正反馈。如果麦克风接收到同一电路中扬声器的放大声音输出，则会听到音频反馈的啸叫和尖叫声(在放大器的最大功率容量内) ，因为随机噪声通过正反馈再次放大，并根据音频系统和房间的特性进行滤波。
== Overview ==
== Overview ==
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}}</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.<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.
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}}</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.<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.
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(也称为声学反馈，简称为反馈，或拉森效应)是一种特殊的正反馈，当声音循环存在于音频输入(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)之间时发生。在这个例子中，麦克风接收到的信号被放大并从扬声器中传出。从扬声器发出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发出。由此产生的声音的频率由麦克风、放大器和扬声器的共振频率、房间的声学、麦克风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 PA 系统，这种声音很容易被识别为高声尖叫或尖叫。
=== Basic ===
=== Basic ===
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."
反馈几乎总是被认为是不受欢迎的，当它发生与歌手或公共演讲者的麦克风在一个事件使用公共广播系统或扩音系统。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，以防止这些不必要的尖叫或尖叫的声音，这减少了观众的享受活动。另一方面，自20世纪60年代以来，摇滚乐队中的电吉他演奏者使用响亮的吉他放大器和失真效果，有意创造出吉他反馈来创造令人满意的音乐效果。甲壳虫乐队的《我感觉很好》标志着在流行音乐中使用反馈作为录音效果的最早的例子之一。它开始于一个单一的，敲击性的反馈音调产生的拨动 a 弦列侬的吉他。像 Kinks 和 Who 这样的艺术家已经使用了现场反馈，但是列侬仍然为披头士乐队可能是第一个故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.]]
[[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.]]
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.
音频反馈的原理最早是由丹麦科学家 s ø ren Absalon Larsen 发现的。麦克风不是唯一受到这种影响的传感器。记录甲板拾音弹可以做同样的事情，通常在低于100赫兹的低频范围内，表现为低隆隆声。吉米亨德里克斯是一个创新者，在他的吉他独奏中有意识地使用吉他反馈来创造独特的声音效果。他帮助开发了音频反馈在电吉他演奏中的控制和音乐应用，后来布莱恩 · 梅成为这一技术的著名支持者。
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.
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.
[[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:
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:
:<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，如果摄像机指向显示摄像机自身信号的监视屏幕，那么通过正反馈可以在屏幕上形成重复的图案。这个视频反馈效果被用于电视节目《神秘博士》前十集的开头序列。
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.
在电子开关中，包括基于双金属片的恒温器，开关在开关动作中通常具有滞后现象。在这种情况下，滞后现象是通过一个临界点机制内的正反馈机械实现的。正反馈动作最大限度地减少了在切换过程中发生电弧的时间，并且使触点处于开启或闭合状态。
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.
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.
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.]]
正反馈是身体对刺激的反应的放大。例如，在分娩时，当胎儿的头部顶向子宫颈时(1)它刺激了从子宫颈到大脑的神经冲动(2)。当大脑接到通知时，它向脑垂体发出信号，让它释放一种叫做[催产素(3)]的荷尔蒙。然后催产素通过血液进入子宫，引起宫缩，将胎儿推向子宫颈，最终导致分娩
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>
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>
{{main|Hysteresis}}
{{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。
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]]:
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]]:
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，或称高细胞分裂症是一种潜在的致命性免疫反应，包括细胞因子和免疫细胞之间的正反馈环，以及各种细胞因子的高水平。在正常的免疫功能中，正反馈环可以被用来增强 b 淋巴细胞的作用。当 b 细胞将抗体与抗原结合并被激活时，它开始释放抗体并分泌一种称为 C3的补体蛋白。3和 b 细胞的抗体都可以与病原体结合，当 b 细胞的抗体与病原体与 c 3结合时，它会加速 b 细胞分泌更多的抗体和 c 3，从而产生一个正反馈环。
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.
凋亡是一种半胱氨酸蛋白酶介导的细胞死亡过程，其目的是清除长寿命或受损的细胞。这一过程的失败与癌症或帕金森氏症等突出疾病有关。细胞凋亡的核心是半胱氨酸蛋白酶的自动激活，这可以通过一个正反馈回路来建模。这种正反馈通过中间半胱氨酸蛋白酶自动激活效应器半胱氨酸蛋白酶。当从凋亡途径中分离出来时，这种正反馈仅呈现一种稳定的稳定状态，与效应蛋白酶中间激活步骤的数目无关。
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.
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,{{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.
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.
Winner (1996)将天才儿童描述为由正反馈循环驱动的，包括设置他们自己的学习课程，这种反馈回来的满足感，从而进一步设置他们的学习目标到更高的水平等等。
== Terminology ==
== Terminology ==
{{Cite book
{{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)提出的反身性(reflexivity)理论，价格变动是由一个正反馈过程推动的，即投资者的预期受到价格变动的影响，因此他们的行为会加强这一方向的变动，直到它变得不可持续，从而反馈推动价格走向相反的方向。
|first = David A.
|first = David A.
|date = 2002
|date = 2002
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".
系统性风险是一个放大或杠杆或正反馈过程呈现给系统的风险。这通常是未知的，并在某些条件下，这个过程可以扩大指数和迅速导致破坏性或混乱的行为。庞氏骗局是正反馈系统的一个很好的例子: 新投资者的资金被用来支付异常高的回报，这反过来又吸引了更多的新投资者，导致快速增长走向崩溃。布莱恩 · 亚瑟也研究并撰写了经济中的积极反馈(例如:。布莱恩 · 阿瑟，1990)。海曼•明斯基(Hyman Minsky)提出了一个理论，认为某些信贷扩张行为可能会使市场经济变成一个“偏离放大系统” ，从而可能突然崩溃，有时被称为“明斯基时刻”。
|publisher = Johns Hopkins University Press
|publisher = Johns Hopkins University Press
|location = Baltimore, MD
|location = Baltimore, MD
Simple systems that clearly separate the inputs from the outputs are not prone to systemic risk. This risk is more likely as the complexity of the system increases, because it becomes more difficult to see or analyze all the possible combinations of variables in the system even under careful stress testing conditions. The more efficient a complex system is, the more likely it is to be prone to systemic risks, because it takes only a small amount of deviation to disrupt the system. Therefore, well-designed complex systems generally have built-in features to avoid this condition, such as a small amount of friction, or resistance, or inertia, or time delay to decouple the outputs from the inputs within the system. These factors amount to an inefficiency, but they are necessary to avoid instabilities.
将输入和输出明确分开的简单系统不易出现系统性风险。随着系统的复杂性增加，这种风险更有可能出现，因为即使在仔细的压力测试条件下，也更难看到或分析系统中所有可能的变量组合。一个复杂的系统越有效率，就越有可能出现系统性风险，因为只需要一点点偏差就能扰乱系统。因此，设计良好的复杂系统通常具有内置的特性，以避免这种情况，如少量的摩擦，或阻力，或惯性，或时间延迟，以解耦的输出从系统内的输入。这些因素相当于无效率，但它们是避免不稳定所必需的。
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The 2010 Flash Crash incident was blamed on the practice of high-frequency trading (HFT), although whether HFT really increases systemic risk remains controversial.
2010年的闪电崩盘事件被归咎于高频交易银行的做法，尽管高频交易是否真的增加了系统性风险仍有争议。
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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.
农业和人口可以被认为是在一个正反馈模式，这意味着一个驱动其他与增加的强度。有人认为，这种正反馈系统有时会以一场灾难而告终，因为现代农业正在耗尽所有容易获得的磷酸盐，而且正在采用更容易受到系统性风险影响的高效单一作物。
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Technological innovation and human population can be similarly considered, and this has been offered as an explanation for the apparent hyperbolic growth of the human population in the past, instead of a simpler exponential growth.
技术创新和人口也可以被类似地考虑，这已经被作为过去人口明显的双曲线增长的解释，而不是一个简单的指数增长。
{{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:
{{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:
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.
提出了人口与技术之间存在二阶正反馈的结论，认为人口增长速度正在加快。技术的发展增加了人们的土地承载能力，从而导致人口的增长，这反过来又推动了进一步的技术发展。
::"Positive feed-back increases the gain of the amplifier, negative feed-back reduces it."<ref name=black>
::"Positive feed-back increases the gain of the amplifier, negative feed-back reduces it."<ref name=black>
{{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>
{{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:
}}
::"...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}}
Gunnar Myrdal described a vicious circle of increasing inequalities, and poverty, which is known as "circular cumulative causation".
贡纳尔 · 缪尔达尔描述了不平等和贫困加剧的恶性循环，这被称为”循环累积因果关系”。
=== In electronics ===
=== 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.
干旱通过积极的反馈加剧。缺少雨水会减少土壤的水分，从而杀死植物和/或导致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散---- 水蒸气从表面进入大气的过程---- 以及向大气中添加干燥的尘埃，吸收水分。水蒸气减少意味着露点温度降低，白天加热效率提高，从而降低了大气中的湿度导致云形成的机会。最后，没有云，就不会有雨，这个循环就是完整的。
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 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>
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“强迫”可能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导致地表变暖。强迫是气候系统外部的，反馈是系统的内部过程。一些反馈机制相对孤立于气候系统的其他部分，而另一些则是紧密耦合的。强迫、反馈和气候系统的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趋势增加了大气中的水汽量，进而导致进一步变暖。主要的负反馈来自斯蒂芬-波尔兹曼定律，从地球辐射到太空的热量与地球表面和大气温度的四次方成正比。
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.
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:
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>
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政府间气候变化专门委员会第四次评估报告指出: “人为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。”
[[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.]]
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，他们的行为可以让它变成真的，对他们行为的观察可能反过来增加信念。一个典型的例子是银行挤兑。
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>
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范围，因此网络扩展得更快。病毒式视频是网络效应的一个例子，其中一个流行的视频链接被分享和重新分发，确保更多的人看到视频，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，种群大小是反馈效应的限制因素。
[[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>
[[File:Technics SL-1210MK2.jpg|thumb|left|A phonograph turntable is prone to acoustic feedback.]]
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.
如果一个化学反应导致了热量的释放，并且反应本身在更高的温度下发生得更快，那么正反馈的可能性就很大。如果产生的热量没有足够快地从反应物中移除，就会发生热失控，并很快导致化学爆炸。
[[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.
===Audio and live music===
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.
许多野生动物因为它们的身体部位而遭到猎杀，这些身体部位非常珍贵。目标物种越接近灭绝，其部分的价格就越高。这是一个积极反馈的例子。
[[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.
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>
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>
[[File:Adam Savage HOPE.jpg|thumb|right|220px|[[Video feedback]].]]
===Video===
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]]''.
=== Switches ===
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>
=== In biology ===
[[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.]]
==== In physiology ====
A number of examples of positive feedback systems may be found in [[physiology]].
* 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}}
* 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}}
* [[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}}
* A spike in [[estrogen]] during the [[follicular phase]] of the menstrual cycle causes [[ovulation]].<ref name=Guyton1991/>{{rp|page=907}}
{{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}}
* 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.
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/>
==== In gene regulation ====
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/>
==== In evolutionary biology ====
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/>
[[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.]]
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>
==== Immune system ====
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>
==== Cell death ====
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[[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>
=== In psychology ===
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.
=== In economics ===
====Markets with social influence====
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>
Category:Classical control theory
Category:Classical control theory
范畴: 经典控制理论
范畴: 经典控制理论
====Market dynamics====
Category:Cybernetics
Category:Cybernetics
类别: 控制论
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>
Category:Electronic feedback
Category:Electronic feedback
分类: 反馈
分类: 反馈
==== Systemic risk ====