1,068
个编辑
更改
跳到导航
跳到搜索
第21行：
第21行： − Mathematically, positive feedback is defined as a positive [[loop gain]] around a closed loop of cause and effect.<ref name="zuckerman" /><ref name="theorymodelling"> − [[wikipedia:Positive_feedback#cite_note-zuckerman-1|Ben Zuckerman & David Jefferson (1996). ''Human Population and the Environmental Crisis''. Jones & Bartlett Learning. p. 42. ISBN <bdi>9780867209662</bdi>. Archived from the original on 2018-01-06.]] − </ref><ref name="zuckerman" /> 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. − <nowiki></ref></nowiki> <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 name=":0">{{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. − + − + − − − 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. − 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: 第46行：
第38行： − When AB > 1, the system is unstable, so does not have a well-defined gain; the gain may be called infinite. − 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. − − 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. − Positive feedback does not necessarily imply instability of an equilibrium, for example stable ''on'' and ''off'' states may exist in positive-feedback architectures.<ref name="ReferenceA">{{cite journal|last1=Lopez-Caamal|first1=Fernando|last2=Middleton|first2=Richard H.|last3=Huber|first3=Heinrich|title=Equilibria and stability of a class of positive feedback loops|journal=Journal of Mathematical Biology|date=February 2014|pages=609–645|doi = 10.1007/s00285-013-0644-z|pmid=23358701|volume=68|issue=3}}</ref> − 正反馈并不一定意味着平衡的不稳定性，例如，在正反馈结构中可能存在稳定的开关状态。<ref name="ReferenceA" />+ − + − {{main|Hysteresis}} 第77行：
第63行： − − 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]]: − "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." − − 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]] behavior. − + − − 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. − Friis & Jensen (1924) described regeneration in a set of electronic amplifiers as a case where the "feed-back" action is positive in contrast to negative feed-back action, which they mention only in passing. Harold Stephen Black's classic 1934 paper first details the use of negative feedback in electronic amplifiers. According to Black: − − "Positive feed-back increases the gain of the amplifier, negative feed-back reduces it." − According to 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." − + − + 第127行：
第100行： − − [[Regenerative circuit]]s were invented and patented in 1914<ref>{{cite patent |inventor-last=Armstrong |inventor-first=E. H. |country-code=US |patent-number=1113149 |title=Wireless receiving system |date=1914}}</ref> for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single [[transistor]] amplifier can multiply its [[Gain (electronics)|gain]] by 1,000 or more.<ref>{{cite web|last=Kitchin|first=Charles|title=A Short Wave Regenerative Receiver Project|url=http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|accessdate=23 September 2010|url-status=live|archiveurl=https://web.archive.org/web/20100710100031/http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|archivedate=10 July 2010}}</ref> Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the [[superheterodyne]] design, with many more amplification stages, but much more stable operation and no positive feedback. − − Regenerative circuits were invented and patented in 1914 for the amplification and reception of very weak radio signals. Carefully controlled positive feedback around a single transistor amplifier can multiply its gain by 1,000 or more. Therefore, a signal can be amplified 20,000 or even 100,000 times in one stage, that would normally have a gain of only 20 to 50. The problem with regenerative amplifiers working at these very high gains is that they easily become unstable and start to oscillate. The radio operator has to be prepared to tweak the amount of feedback fairly continuously for good reception. Modern radio receivers use the superheterodyne design, with many more amplification stages, but much more stable operation and no positive feedback. − − The oscillation that can break out in a regenerative radio circuit is used in [[electronic oscillator]]s. By the use of [[tuned circuit]]s or a [[piezoelectricity|piezoelectric]] [[crystal]] (commonly [[quartz]]), the signal that is amplified by the positive feedback remains linear and [[Sine wave|sinusoidal]]. There are several designs for such [[harmonic oscillator]]s, including the [[Armstrong oscillator]], [[Hartley oscillator]], [[Colpitts oscillator]], and the [[Wien bridge oscillator]]. They all use positive feedback to create oscillations.<ref>{{cite web|title=Sinewave oscillators|url=http://www.educypedia.be/electronics/analogosciltypes.htm|work=EDUCYPEDIA - electronics|accessdate=23 September 2010|url-status=dead|archiveurl=https://web.archive.org/web/20100927094330/http://www.educypedia.be/electronics/analogosciltypes.htm|archivedate=27 September 2010}}</ref> − − The oscillation that can break out in a regenerative radio circuit is used in electronic oscillators. By the use of tuned circuits or a piezoelectric crystal (commonly quartz), the signal that is amplified by the positive feedback remains linear and sinusoidal. There are several designs for such harmonic oscillators, including the Armstrong oscillator, Hartley oscillator, Colpitts oscillator, and the Wien bridge oscillator. They all use positive feedback to create oscillations. − − − 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. − 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. − − − 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. 第165行：
第123行： − − 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. − − }}</ref> − − 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. 第178行：
第130行： − 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. − + − − 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. − 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." − − 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. 第203行：
第149行： − + − − − − Similarly, if a [[video camera]] is pointed at a [[Video monitor|monitor]] screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to the [[Doctor Who (season 1)|first]] [[Doctor Who (season 10)|ten]] series of the television program ''[[Doctor Who]]''. − − Similarly, if a video camera is pointed at a monitor screen that is displaying the camera's own signal, then repeating patterns can be formed on the screen by positive feedback. This video feedback effect was used in the opening sequences to the first ten series of the television program Doctor Who. 第215行：
第155行： − + 第222行：
第162行： − + − + − 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. − + − − A number of examples of positive feedback systems may be found in [[physiology]]. − + − − + − − * Another example is the process of [[Coagulation|blood clotting]]. The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot.<ref name=Guyton1991/>{{rp|pages=392–394}} − − 另一个例子是血液凝固的过程。当受伤的组织释放出信号化学物质，激活血液中的血小板时，这个循环就启动了。被激活的血小板释放化学物质，激活更多的血小板，引起快速的级联反应，形成血栓。 − − * [[Lactation]] also involves positive feedback in that as the baby suckles on the nipple there is a nerve response into the spinal cord and up into the hypothalamus of the brain, which then stimulates the [[pituitary]] gland to produce more [[prolactin]] to produce more milk.<ref name=Guyton1991/>{{rp|page=926}} − − 哺乳也涉及正反馈，当婴儿吸吮乳头时，会有神经反应进入脊髓，并上传到大脑的下丘脑，然后刺激垂体产生更多的催乳素以产生更多的乳汁。 − − * A spike in [[estrogen]] during the [[follicular phase]] of the menstrual cycle causes [[ovulation]].<ref name=Guyton1991/>{{rp|page=907}} − 在月经周期的卵泡期期间，雌激素的飙升会导致排卵。+ + − + + − 神经信号的产生是另一个例子，神经纤维的膜使钠离子通过钠通道轻微渗漏，导致膜电位的变化，进而引起更多通道的开放（Hodgkin循环）。所以，最初的轻微渗漏会导致钠渗漏的爆发，从而产生神经动作电位。+ + − + + + + − In most cases, such feedback loops culminate in counter-signals being released that suppress or break the loop. Childbirth contractions stop when the baby is out of the mother's body. Chemicals break down the blood clot. Lactation stops when the baby no longer nurses.<ref name=Guyton1991/> − 在大多数情况下，这种反馈循环最终会释放出反信号，从而抑制或破坏循环。分娩宫缩在宝宝离开母体时停止。化学物质分解血凝块。当婴儿不再需要被哺乳时，泌乳停止。+ − − − − − − − − 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在进化生物学中 ==== − − − − − Positive feedback loops have been used to describe aspects of the dynamics of change in biological [[evolution]]. For example, beginning at the macro level, [[Alfred J. Lotka]] (1945) argued that the evolution of the species was most essentially a matter of selection that fed back energy flows to capture more and more energy for use by living systems.<ref name=Lotka1945/> At the human level, [[Richard D. Alexander]] (1989) proposed that social competition between and within human groups fed back to the selection of intelligence thus constantly producing more and more refined human intelligence. <ref name=Alexander1989/> [[Bernard Crespi|Crespi]] (2004) discussed several other examples of positive feedback loops in evolution.<ref name=Crespi2004/> The analogy of [[Evolutionary arms race]]s provide further examples of positive feedback in biological systems.<ref name=Blindwatchmaker/> − − 在进化生物学中正反馈循环被用来描述生物进化中变化动态的各个方面。 例如，在宏观层面，Alfred J. Lotka（1945）认为，物种的演变最重要的是选择反馈能量流动以捕获越来越多的能源系统的能量。<ref name=Lotka1945/>在人类层面，Richard D. Alexander(1989)提出，人类群体之间和群体内部的社会竞争会影响智力的选择，从而时不时地会产生更多、更完善的人类智力。 <ref name=Alexander1989/> Bernard Crespi（2004）讨论了进化中正反馈循环的其他几个例子。通过与军备竞赛进行类比，给生物系统中的正反馈提供了进一步的例子。<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.显生宙[[生物多样性]]呈现稳定而非单调的增长，从接近于零一直增长到有几千个属。|链接=Special:FilePath/Phanerozoic_Biodiversity.svg]]+ − It has been shown that changes in [[biodiversity]] through the [[Phanerozoic]] correlate much better with hyperbolic model (widely used in [[demography]] and [[macrosociology]]) than with [[Exponential growth|exponential]] and [[Logistic function|logistic]] models (traditionally used in [[population biology]] and extensively applied to [[fossil]] [[biodiversity]] as well). The latter models imply that changes in diversity are guided by a first-order positive feedback (more ancestors, more descendants) and/or a [[negative feedback]] arising from resource limitation. Hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of the [[world population growth]] has been demonstrated (see below) to arise from a second-order positive feedback between the population size and the rate of [[technological growth]]. The hyperbolic character of biodiversity growth can be similarly accounted for by a positive feedback between the diversity and community structure complexity. It has been suggested that the similarity between the curves of [[biodiversity]] and human population probably comes from the fact that both are derived from the interference of the hyperbolic trend (produced by the positive feedback) with cyclical and stochastic dynamics.<ref>Markov A., [[Andrey Korotayev|Korotayev A.]] [https://archive.today/20120630063924/http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B83WC-4N0HJMK-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=74a80d7c55ff987c9fc8d9c7963feab9 "Phanerozoic marine biodiversity follows a hyperbolic trend." [[Palaeoworld]]. Volume 16, Issue 4, December 2007, Pages 311-318]</ref><ref>{{cite journal | last1 = Markov | first1 = A. | last2 = Korotayev | first2 = A. | year = 2008 | title = Hyperbolic growth of marine and continental biodiversity through the Phanerozoic and community evolution | url = http://elementy.ru/genbio/abstracts?artid=177 | journal = Journal of General Biology | volume = 69 | issue = 3 | pages = 175–194 | pmid = 18677962 | url-status = live | archiveurl = https://web.archive.org/web/20091225000305/http://elementy.ru/genbio/abstracts?artid=177 | archivedate = 2009-12-25 }}</ref>+ − 研究表明，在<font color="#32CD32"> 显生宙 </font>，生物多样性的变化与双曲模型（广泛用于人口学和宏观社会学）的相关性要比指数模型和逻辑斯特模型（传统上用于人口生物学，并广泛用于生物多样性化石）的相关性好得多。后者的模型意味着多样性的变化是由一阶正反馈（更多的祖先，更多的后代）和资源限制产生的负反馈所引导的。双曲模型意味着二阶正反馈。世界人口增长的双曲线模式已被证明源于人口数量与技术增长速度之间的二阶正反馈。生物多样性增长的双曲特征同样可以由多样性与群落结构复杂性之间的正反馈来解释。有人认为，生物多样性和人口曲线之间的相似性可能来自这样一个事实，即两者都是由双曲趋势(由正反馈产生)与周期性和随机性的动态干扰而产生的。 + − ==== Immune system 免疫系统==== + + − A [[cytokine storm]], or '''hypercytokinemia''' is a potentially fatal immune reaction consisting of a positive feedback loop between [[cytokine]]s and [[immune cell]]s, with highly elevated levels of various cytokines.<ref name="osterholm">{{cite journal | last = Osterholm | first = Michael T. | author-link = Michael Osterholm |title = Preparing for the Next Pandemic | journal = The New England Journal of Medicine | volume = 352 | issue = 18 | pages = 1839–1842 | date = 2005-05-05 | url = | doi = 10.1056/NEJMp058068 | pmid = 15872196 | citeseerx = 10.1.1.608.6200 }}</ref> In normal immune function, positive feedback loops can be utilized to enhance the action of B lymphocytes. When a B cell binds its antibodies to an antigen and becomes activated, it begins releasing antibodies and secreting a complement protein called C3. Both C3 and a B cell's antibodies can bind to a pathogen, and when a B cell has its antibodies bind to a pathogen with C3, it speeds up that B cell's secretion of more antibodies and more C3, thus creating a positive feedback loop.<ref>{{cite journal|last=Paul|first=William E.|title=Infectious Diseases and the Immune System|journal=Scientific American|volume=269|issue=3|date=September 1993|page=93|bibcode=1993SciAm.269c..90P|doi=10.1038/scientificamerican0993-90|pmid=8211095}}</ref> − + − − − [[Apoptosis]] is a [[caspase]]-mediated process of cellular death, whose aim is the removal of long-lived or damaged cells. A failure of this process has been implicated in prominent conditions such as [[cancer]] or [[Parkinson's disease]]. The very core of the apoptotic process is the auto-activation of caspases, which may be modeled via a positive-feedback loop. This positive feedback exerts an auto-activation of the [[effector caspase]] by means of intermediate caspases. When isolated from the rest of apoptotic pathway, this positive-feedback presents only one stable steady state, regardless of the number of intermediate activation steps of the effector caspase.<ref name="ReferenceA"/> When this core process is complemented with inhibitors and enhancers of caspases effects, this process presents bistability, thereby modeling the alive and dying states of a cell.<ref>{{cite journal|last=Eissing|first=Thomas |doi=10.1074/jbc.M404893200 |title=Bistability analyses of a caspase activation model for receptor-induced apoptosis|journal=Journal of Biological Chemistry|volume=279 |issue=35 |date=2014|pages=36892–36897|pmid=15208304 |doi-access=free}}</ref> − − 细胞凋亡是一种由酪蛋白酶介导的细胞死亡过程，其目的是清除长寿或受损的细胞。这一过程的失效与癌症或帕金森氏病等著名疾病有关。细胞凋亡过程的核心是半胱氨酸蛋白酶的自动激活，它可以通过一个正反馈循环来建模。这种正反馈通过中间胱天蛋白酶使效应子胱天蛋白酶自动活化。当从凋亡途径的其他部分分离出来时，无论效应子胱天蛋白酶的中间激活步骤数量有多少，这种正反馈仅呈现一种稳定的稳态。<ref name="ReferenceA"/> 当该核心过程与胱天蛋白酶作用的抑制剂和增强剂相辅相成时，该过程呈现双稳态，从而模拟细胞的存活和死亡状态。 − + + − 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. − + − − ====Markets with social influence 具有社会影响力的市场==== + − Product recommendations and information about past purchases have been shown to influence consumers choices significantly whether it is for music, movie, book, technological, and other type of products. Social influence often induces a rich-get-richer phenomenon ([[Matthew effect]]) where popular products tend to become even more popular.<ref name="altszyler2017">{{cite journal | title= Transient dynamics in trial-offer markets with social influence: Trade-offs between appeal and quality. | author1= Altszyler, E | author2= Berbeglia, F. | author3= Berbeglia, G. | author4= Van Hentenryck, P. | journal= PLOS ONE | year= 2017 | volume= 12 | issue= 7 | df= | doi=10.1371/journal.pone.0180040 |pmid = 28746334| pmc= 5528888 | page=e0180040| bibcode= 2017PLoSO..1280040A }}</ref> − + − − 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> − + − − 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". − 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. − The 2010 Flash Crash incident was blamed on the practice of high-frequency trading (HFT), although whether HFT really increases systemic risk remains controversial. − − |title= Human Population and the Environmental Crisis − 人口与环境危机 − − − Agriculture and human population can be considered to be in a positive feedback mode, which means that one drives the other with increasing intensity. It is suggested that this positive feedback system will end sometime with a catastrophe, as modern agriculture is using up all of the easily available phosphate and is resorting to highly efficient monocultures which are more susceptible to systemic risk. − − 可以认为农业和人口处于一种正反馈模式，这意味着双方越来越强烈地推动彼此。有人认为，这种正反馈系统终将在某一时刻以灾难结束，因为现代农业正在耗尽所有容易获得的磷酸盐，并且进行高效的单一栽培，使得现代农业更容易受到系统性风险影响。 + + − 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. − 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. − + − − − Gunnar Myrdal described a vicious circle of increasing inequalities, and poverty, which is known as "circular cumulative causation". − + − + − − − 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. − + − − − 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 中的"诱因 "可能会将气候系统推向变暖或变冷的方向，例如，大气中温室气体浓度的增加会导致地表变暖。诱因是气候系统的外部因素，而反馈是系统的内部过程。一些反馈机制与气候系统的其他部分相对孤立地发挥作用，而另一些则是紧密耦合的。气候系统的作用力、反馈和动态决定了气候变化的程度和速度。全球变暖中的主要正反馈是变暖使大气中的水汽量增加，进而导致进一步变暖。主要的负反馈来自Stefan-Boltzmann定律，从地球辐射到空间的热量与地球表面和大气温度的四次方成正比。+ − Other examples of positive feedback subsystems in climatology include: − − A warmer atmosphere will melt ice and this changes the albedo which further warms the atmosphere. − Methane hydrates can be unstable so that a warming ocean could release more methane, which is also a greenhouse gas. − Peat, occurring naturally in peat bogs, contains carbon. When peat dries it decomposes, and may additionally burn. Peat also releases nitrous oxide. − Global warming affects the cloud distribution. Clouds at higher altitudes enhance the greenhouse effects, while low clouds mainly reflect back sunlight, having opposite effects on temperature. − − 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." − + − − 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. − 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. − + − − 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. − + − 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. − + 第471行：
第341行： − + 第588行：
第458行： − ====拓展阅读 ====+
无编辑摘要
正反馈(加剧反馈，自我强化反馈) 是一种在反馈循环中加剧微小扰动影响的过程。也就是说，一个扰动对系统的影响包括它自身扰动幅度的增大。或者说，A会产生更多的B，而B又会产生更多的A，与之相反的是，[[负反馈]]指的是一个系统中，变化的结果会减少或抵消它自己的影响。<ref name="theorymodelling" />这两个概念在科学和工程等领域中发挥着重要作用，包括生物学、化学和控制论。
正反馈(加剧反馈，自我强化反馈) 是一种在反馈循环中加剧微小扰动影响的过程。也就是说，一个扰动对系统的影响包括它自身扰动幅度的增大。或者说，A会产生更多的B，而B又会产生更多的A，与之相反的是，[[负反馈]]指的是一个系统中，变化的结果会减少或抵消它自己的影响。<ref name="theorymodelling" />这两个概念在科学和工程等领域中发挥着重要作用，包括生物学、化学和控制论。
在数学上，正反馈被定义为一个环绕在闭合因果循环下的正循环增益。<ref name="zuckerman" /><ref>Bernard P. Zeigler; Herbert Praehofer; Tag Gon Kim Section (2000). "3.3.2 Feedback in continuous systems". ''Theory of Modeling and Simulation: Integrating Discrete Event and Continuous Complex Dynamic Systems''. Academic Press. p. 55. ISBN <bdi>9780127784557</bdi>. Archived from the original on 2017-01-03. <q>A positive feedback loop is one with an even number of negative influences [around the loop].</q></ref><ref name="theorymodelling" /><ref name="zuckerman" /> 从结果到始发过程的反馈可以是直接的，也可以通过其他状态变量。这样的系统可以给出丰富的定性行为，但反馈的瞬时信号是正向还是负向，对结果有极其重要的影响。<ref name="theorymodelling" /> 正反馈强化原过程，而负反馈调节原过程。在这个含义下，''正''和''负''指的是大于或小于零的循环收益，并不代表着最终结果或效果的正负性。<ref name=":0" />因此，正反馈的一个重要特点是小扰动变大。当系统发生变化时，正反馈会引起进一步的同方向变化。
在数学上，正反馈被定义为一个环绕在闭合因果循环下的正循环增益。<ref name="zuckerman" /><ref>Bernard P. Zeigler; Herbert Praehofer; Tag Gon Kim Section (2000). "3.3.2 Feedback in continuous systems". ''Theory of Modeling and Simulation: Integrating Discrete Event and Continuous Complex Dynamic Systems''. Academic Press. p. 55. ISBN <bdi>9780127784557</bdi>. Archived from the original on 2017-01-03. <q>A positive feedback loop is one with an even number of negative influences [around the loop].</q></ref><ref name="zuckerman" /> 从结果到始发过程的反馈可以是直接的，也可以通过其他状态变量<ref name="theorymodelling" />。这样的系统可以给出丰富的定性行为，但反馈的瞬时信号是正向还是负向，对结果有极其重要的影响。<ref name="theorymodelling" /> 正反馈强化原过程，而负反馈调节原过程。在这个含义下，''正''和''负''指的是大于或小于零的循环收益，并不代表着最终结果或效果的正负性。<ref name=":0">{{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>因此，正反馈的一个重要特点是小扰动变大。当系统发生变化时，正反馈会引起进一步的同方向变化。
=== Basic 基础===
===基础===
[[File:Ideal feedback model.svg|thumb|【图4：A basic feedback system can be represented by this block diagram. In the diagram the + symbol is an adder and A and B are arbitrary causal functions. 一个基本的反馈系统可以用这个框图来表示。在图中，+号是加法器，A和B是任意因果函数。】|链接=Special:FilePath/Ideal_feedback_model.svg]]
[[File:Ideal feedback model.svg|thumb|【图4：A basic feedback system can be represented by this block diagram. In the diagram the + symbol is an adder and A and B are arbitrary causal functions. 一个基本的反馈系统可以用这个框图来表示。在图中，+号是加法器，A和B是任意因果函数。】|链接=Special:FilePath/Ideal_feedback_model.svg]]
图中显示了一个简单的反馈回路。 如果环增益AB为正值，则存在'正'或'再生'反馈的条件。
图中显示了一个简单的反馈回路。 如果环增益AB为正值，则存在'正'或'再生'反馈的条件。
如果函数A和B是线性的，且AB小于1，那么系统从输入到输出的整体增益是有限的，但当AB接近1时，系统的增益可以非常大。<ref name="smith"> Electronics circuits and devices second edition. Ralph J. Smith</ref> 在这种情况下，可以表明从输入到输出的整体或 "闭环 "增益为：
如果函数A和B是线性的，且AB小于1，那么系统从输入到输出的整体增益是有限的，但当AB接近1时，系统的增益可以非常大。<ref name="smith"> Electronics circuits and devices second edition. Ralph J. Smith</ref> 在这种情况下，可以表明从输入到输出的整体或 "闭环 "增益为：
:<math>G_c = A/(1-AB)</math>
:<math>G_c = A/(1-AB)</math>
当AB>1时，系统是不稳定的，因此不具有明确的增益；增益可称为无限。
当AB>1时，系统是不稳定的，因此不具有明确的增益；增益可称为无限。
所以系统状态的变化根据反馈可以是收敛的，也可以是发散的。 而正反馈的结果是增强变化，因此小的扰动就可能导致大的变化。
所以系统状态的变化根据反馈可以是收敛的，也可以是发散的。 而正反馈的结果是增强变化，因此小的扰动就可能导致大的变化。
对于一个当前处于平衡状态的系统，如果其状态的任何变化都发生了正反馈，从而造成了状态的不稳定，那么这个系统就是一个不稳定平衡的系统。使这种系统远离其平衡状态的力的大小是状态与平衡状态之间的距离的递增函数。
对于一个当前处于平衡状态的系统，如果其状态的任何变化都发生了正反馈，从而造成了状态的不稳定，那么这个系统就是一个不稳定平衡的系统。使这种系统远离其平衡状态的力的大小是状态与平衡状态之间的距离的递增函数。
正反馈并不一定意味着平衡的不稳定性，例如，在正反馈结构中可能存在稳定的开关状态<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>。
=== Hysteresis迟滞 ===
=== 迟滞 ===
[[File:Hysteresis sharp curve.svg|thumb|【图5 Hysteresis causes the output value to depend on the history of the input 迟滞现象会导致输出值取决于输入的历史记录。】|链接=Special:FilePath/Hysteresis_sharp_curve.svg]]
[[File:Hysteresis sharp curve.svg|thumb|【图5 Hysteresis causes the output value to depend on the history of the input 迟滞现象会导致输出值取决于输入的历史记录。】|链接=Special:FilePath/Hysteresis_sharp_curve.svg]]
在现实世界中，正反馈循环通常不会引起不断增长，而是通过某种限制效应来改变。根据Donella Meadows的说法：
在现实世界中，正反馈循环通常不会引起不断增长，而是通过某种限制效应来改变。根据Donella Meadows的说法：
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反馈，最终将会自我毁灭。这就是为什么正反馈如此稀少的原因。通常情况下，负反馈迟早会发生。<ref>Donella Meadows, ''[http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf Leverage Points: Places to Intervene in a System]''2013-10-08 at the Wayback Machine, 1999</ref>
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反馈，最终将会自我毁灭。这就是为什么正反馈如此稀少的原因。通常情况下，负反馈迟早会发生。<ref>Donella Meadows, ''[http://www.sustainabilityinstitute.org/pubs/Leverage_Points.pdf Leverage Points: Places to Intervene in a System]''2013-10-08 at the Wayback Machine, 1999</ref>
[[迟滞]]，即起点影响系统的终点的现象，可以通过正反馈产生。当反馈循环的增益高于1时，那么输出就会远离输入：如果大于输入，则向最近的正极限移动，而如果小于输入，则向最近的负极限移动。
[[迟滞]]，即起点影响系统的终点的现象，可以通过正反馈产生。当反馈循环的增益高于1时，那么输出就会远离输入：如果大于输入，则向最近的正极限移动，而如果小于输入，则向最近的负极限移动。
一旦达到极限，它就会稳定下来。但是，如果输入超过极限，那么反馈将改变符号，输出将向相反的方向移动，直到达到相反的极限。因此，该系统表现出<font color="#ff8000"> 双稳态行为bistable behavior</font>。
一旦达到极限，它就会稳定下来。但是，如果输入超过极限，那么反馈将改变符号，输出将向相反的方向移动，直到达到相反的极限。因此，该系统表现出<font color="#ff8000"> 双稳态行为bistable behavior</font>。
== Terminology 术语的由来==
== 术语的由来==
正反馈和负反馈这两个名词最早应用于反馈是在二战前。正反馈的概念随着再生电路的问世，在20世纪20年代已经出现。
正反馈和负反馈这两个名词最早应用于反馈是在二战前。正反馈的概念随着再生电路的问世，在20世纪20年代已经出现。
Friis 和 Jensen在1924年描述了一种在电子放大器中发生的"回馈 "是正的情况，这一情况与他们顺便提到的负回馈作用相反。到了1934年，Harold Stephen Black在他的经典论文中首次详细介绍了负反馈在电子放大器中的应用。根据Black的说法：
Friis 和 Jensen在1924年描述了一种在电子放大器中发生的"回馈 "是正的情况，这一情况与他们顺便提到的负回馈作用相反。到了1934年，Harold Stephen Black在他的经典论文中首次详细介绍了负反馈在电子放大器中的应用。根据Black的说法：
正反馈会增加放大器的增益，负反馈会降低增益
正反馈会增加放大器的增益，负反馈会降低增益
据Mindell(2002年)说，术语上的混乱是在这之后不久产生的：
据Mindell(2002年)说，术语上的混乱是在这之后不久产生的：
“ ... ... Friis 和 Jensen 对 Black 在对"正反馈 "和 "负反馈 "的区分方法是一样的，都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。与之相反的是，当Nyquist和Bode基于Black的工作基础时，将负反馈称为符号相反的反馈。Black难以说服其他人相信他的发明的实用性，有一部分原因是在基本的定义问题上存在混乱。"
“ ... ... Friis 和 Jensen 对 Black 在对"正反馈 "和 "负反馈 "的区分方法是一样的，都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。与之相反的是，当Nyquist和Bode基于Black的工作基础时，将负反馈称为符号相反的反馈。Black难以说服其他人相信他的发明的实用性，有一部分原因是在基本的定义问题上存在混乱。"
== Examples and applications 实例与应用==
==实例与应用==
=== In electronics 在电子领域===
=== 电子电路===
<font color="#ff8000"> 再生电路Regenerative circuit</font>于1914年被发明并获得专利<ref>{{cite patent |inventor-last=Armstrong |inventor-first=E. H. |country-code=US |patent-number=1113149 |title=Wireless receiving system |date=1914}}</ref>，用于放大和接收非常微弱的无线电信号。通过仔细控制单晶体管放大器周围的正反馈，可以使其增益增加1000倍或更多<ref>{{cite web|last=Kitchin|first=Charles|title=A Short Wave Regenerative Receiver Project|url=http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|accessdate=23 September 2010|url-status=live|archiveurl=https://web.archive.org/web/20100710100031/http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|archivedate=10 July 2010}}</ref> 。因此，一个信号可以在一个阶段被放大20000甚至100000倍，而在通常只有20到50的增益。在如此高的增益下工作带来的问题则是信号很容易变得不稳定，开始振荡。无线电操作员必须不断地调整反馈量，以获得良好的接收效果。而现代无线电接收机采用超异构设计，多了许多放大级，去掉了正反馈并使其工作更稳定。
<font color="#ff8000"> 再生电路Regenerative circuit</font>于1914年被发明并获得专利<ref>{{cite patent |inventor-last=Armstrong |inventor-first=E. H. |country-code=US |patent-number=1113149 |title=Wireless receiving system |date=1914}}</ref>，用于放大和接收非常微弱的无线电信号。通过仔细控制单晶体管放大器周围的正反馈，可以使其增益增加1000倍或更多<ref>{{cite web|last=Kitchin|first=Charles|title=A Short Wave Regenerative Receiver Project|url=http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|accessdate=23 September 2010|url-status=live|archiveurl=https://web.archive.org/web/20100710100031/http://www.electronics-tutorials.com/receivers/regen-radio-receiver.htm|archivedate=10 July 2010}}</ref> 。因此，一个信号可以在一个阶段被放大20000甚至100000倍，而在通常只有20到50的增益。在如此高的增益下工作带来的问题则是信号很容易变得不稳定，开始振荡。无线电操作员必须不断地调整反馈量，以获得良好的接收效果。而现代无线电接收机采用超异构设计，多了许多放大级，去掉了正反馈并使其工作更稳定。
在再生无线电电路中产生的振荡还可以被用于电子振荡器中。通过使用调谐电路或压电晶体(常见的是石英)，经正反馈放大后的信号仍然是线性的、正弦的。这种谐波振荡器有几种设计，包括阿姆斯特朗振荡器、哈特利振荡器、科尔皮茨振荡器和维恩桥振荡器。它们都是利用正反馈来产生振荡。<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>
在再生无线电电路中产生的振荡还可以被用于电子振荡器中。通过使用调谐电路或压电晶体(常见的是石英)，经正反馈放大后的信号仍然是线性的、正弦的。这种谐波振荡器有几种设计，包括阿姆斯特朗振荡器、哈特利振荡器、科尔皮茨振荡器和维恩桥振荡器。它们都是利用正反馈来产生振荡。<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>
许多电子电路，特别是放大器，都采用了负反馈。这降低了放大器的信号增益，但改善了它的线性度、输入阻抗、输出阻抗和带宽，并稳定了包括闭环增益等参数。同时，这些参数也变得不那么依赖于放大器件本身的细节，而更多地依赖于反馈元件，因为反馈元件一般不随着制造公差、使用年限和温度而变化。交流信号的正反馈和负反馈的区别在于相位：如果信号反馈失相，则反馈为负，如果相位一致，则反馈为正。对于需要使用负反馈放大器的设计者来说，引入负反馈放大器的问题是，电路中的一些元件会在反馈路径中引入相移。如果有一个频率(通常是高频)的相移达到180°，那么设计者必须确保该频率的放大器增益非常低（通常通过低通滤波来做到这一点）。如果任何频率下的环增益（放大器增益与正反馈程度的乘积）大于1，那么放大器将在该频率下发生振荡（巴克豪森稳定性准则）。这种振荡有时被称为寄生振荡：在一组条件下稳定的放大器在另一组条件下可能会发生寄生振荡。这可能是由于温度、电源电压的变化，前板（用户界面）的变化，甚至是由于人或其他导电物品的接近。
许多电子电路，特别是放大器，都采用了负反馈。这降低了放大器的信号增益，但改善了它的线性度、输入阻抗、输出阻抗和带宽，并稳定了包括闭环增益等参数。同时，这些参数也变得不那么依赖于放大器件本身的细节，而更多地依赖于反馈元件，因为反馈元件一般不随着制造公差、使用年限和温度而变化。交流信号的正反馈和负反馈的区别在于相位：如果信号反馈失相，则反馈为负，如果相位一致，则反馈为正。对于需要使用负反馈放大器的设计者来说，引入负反馈放大器的问题是，电路中的一些元件会在反馈路径中引入相移。如果有一个频率(通常是高频)的相移达到180°，那么设计者必须确保该频率的放大器增益非常低（通常通过低通滤波来做到这一点）。如果任何频率下的环增益（放大器增益与正反馈程度的乘积）大于1，那么放大器将在该频率下发生振荡（巴克豪森稳定性准则）。这种振荡有时被称为寄生振荡：在一组条件下稳定的放大器在另一组条件下可能会发生寄生振荡。这可能是由于温度、电源电压的变化，前板（用户界面）的变化，甚至是由于人或其他导电物品的接近。
放大器可能会以示波器很难检测到的方式轻轻振荡，有时的振荡也可能非常大，只有非常扭曲或根本没有真正的信号，甚至振荡也会引起损坏发生。由于低频寄生振荡与低转速排气音符的声音相似，因此低频寄生振荡也被称为 "汽艇"。
放大器可能会以示波器很难检测到的方式轻轻振荡，有时的振荡也可能非常大，只有非常扭曲或根本没有真正的信号，甚至振荡也会引起损坏发生。由于低频寄生振荡与低转速排气音符的声音相似，因此低频寄生振荡也被称为 "汽艇"。
【图8 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】
【图8 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】
许多常见的数字电路都采用正反馈。一般简单的布尔逻辑门通常只是依靠增益将数字信号电压从中间值推到代表布尔值0和1的值上，但许多更复杂的门都采用了反馈。当输入的模拟电压发生变化，但后期数字处理需要尖锐阈值时，施密特触发电路通过正反馈机制确保当输入电压轻微超过阈值时，输出电压可以巧妙而迅速地从一个逻辑状态转移到另一个逻辑状态。施密特触发器使用正反馈的一个必然结果是，如果输入电压再次缓慢下降，超过了相同的阈值，由于正反馈的机制，输出电压将保持在相同的逻辑状态而不改变。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，并将其重置为原始数字。通过减小正反馈的程度，可以减小滞后宽度，但宽度不能被完全消除。施密特触发器在某种程度上是一个闭锁电路。
许多常见的数字电路都采用正反馈。一般简单的布尔逻辑门通常只是依靠增益将数字信号电压从中间值推到代表布尔值0和1的值上，但许多更复杂的门都采用了反馈。当输入的模拟电压发生变化，但后期数字处理需要尖锐阈值时，施密特触发电路通过正反馈机制确保当输入电压轻微超过阈值时，输出电压可以巧妙而迅速地从一个逻辑状态转移到另一个逻辑状态。施密特触发器使用正反馈的一个必然结果是，如果输入电压再次缓慢下降，超过了相同的阈值，由于正反馈的机制，输出电压将保持在相同的逻辑状态而不改变。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，并将其重置为原始数字。通过减小正反馈的程度，可以减小滞后宽度，但宽度不能被完全消除。施密特触发器在某种程度上是一个闭锁电路。
【图10 Illustration of an R-S ('reset-set') flip-flop made from two digital nor gates with positive feedback. Red and black mean logical '1' and '0', respectively. R-S("复位-设置")触发器的说明，由两个带正反馈的数字诺尔门组成。红色和黑色分别表示逻辑上的 "1 "和 "0"。】
【图10 Illustration of an R-S ('reset-set') flip-flop made from two digital nor gates with positive feedback. Red and black mean logical '1' and '0', respectively. R-S("复位-设置")触发器的说明，由两个带正反馈的数字诺尔门组成。红色和黑色分别表示逻辑上的 "1 "和 "0"。】
电子触发器，或“锁存器” ，或“双稳态多谐振荡器” ，是一种由于高正反馈而不稳定于平衡或中间状态的电路。这样的双稳态电路是一位电子存储器的基础。 触发器使用一对放大器、晶体管或逻辑门相互连接，正反馈机制使得输入信号被去除后，电路可以维持在两种非平衡稳定状态中的一种，直到一个合适的替代信号重新作为输入，以改变电路状态。计算机随机存取存储器(RAM)可以用这种方式运作，每位存储器有一个锁存电路。
电子触发器，或“锁存器” ，或“双稳态多谐振荡器” ，是一种由于高正反馈而不稳定于平衡或中间状态的电路。这样的双稳态电路是一位电子存储器的基础。 触发器使用一对放大器、晶体管或逻辑门相互连接，正反馈机制使得输入信号被去除后，电路可以维持在两种非平衡稳定状态中的一种，直到一个合适的替代信号重新作为输入，以改变电路状态。计算机随机存取存储器(RAM)可以用这种方式运作，每位存储器有一个锁存电路。
电子系统中发生<font color="#ff8000"> 热失控Thermal runaway</font>的原因是，当电路的某些方面变得更热时，它被允许通过更多的电流，然后它越热，通过的电流就越多，这就使它更热一些，因此它又通过更多的电流。这种现象对有关器件来说通常是灾难性的。如果器件不得不在接近其最大功率处理能力的情况下工作，那么某些条件下就可能出现热失控，这通常可以通过精心设计来改进。
电子系统中发生<font color="#ff8000"> 热失控Thermal runaway</font>的原因是，当电路的某些方面变得更热时，它被允许通过更多的电流，然后它越热，通过的电流就越多，这就使它更热一些，因此它又通过更多的电流。这种现象对有关器件来说通常是灾难性的。如果器件不得不在接近其最大功率处理能力的情况下工作，那么某些条件下就可能出现热失控，这通常可以通过精心设计来改进。
[[File:Technics SL-1210MK2.jpg|thumb|left|【图11 A phonograph turntable is prone to acoustic feedback. 留声机转盘容易受到声反馈的影响。】|链接=Special:FilePath/Technics_SL-1210MK2.jpg]]
[[File:Technics SL-1210MK2.jpg|thumb|left|【图11 A phonograph turntable is prone to acoustic feedback. 留声机转盘容易受到声反馈的影响。】|链接=Special:FilePath/Technics_SL-1210MK2.jpg]]
音频和视频系统可以表现出正反馈。如果麦克风录入了同一电路中扬声器的放大声音输出，那么就会听到音频反馈的嚎叫和尖叫声（在放大器的最大功率容量下），因为随机噪声被音频系统和房间的特性所过滤后，通过正反馈重新放大。
音频和视频系统可以表现出正反馈。如果麦克风录入了同一电路中扬声器的放大声音输出，那么就会听到音频反馈的嚎叫和尖叫声（在放大器的最大功率容量下），因为随机噪声被音频系统和房间的特性所过滤后，通过正反馈重新放大。
===Audio and live music音频和现场音乐===
===音频和现场音乐领域===
音频反馈（也称为声反馈，简称反馈，或拉森效应）是一种特殊的正反馈，当音频输入（例如，麦克风或吉他拾音器）和音频输出（例如，大声放大的扬声器）之间存在声音回路时，就会出现这种反馈。在这个例子中，麦克风接收到的信号被放大并从扬声器传出。然后，来自扬声器的声音可以再次被麦克风接收，进一步放大，然后再次通过扬声器传递出去。 所产生的声音的频率由传声器、放大器和扬声器的共振频率、房间的声学特性、传声器和扬声器的定向拾音和发射模式以及它们之间的距离决定。对于小型的扩声系统来说，这种声音很容易的体现的响亮的吱吱声或尖叫声。
音频反馈（也称为声反馈，简称反馈，或拉森效应）是一种特殊的正反馈，当音频输入（例如，麦克风或吉他拾音器）和音频输出（例如，大声放大的扬声器）之间存在声音回路时，就会出现这种反馈。在这个例子中，麦克风接收到的信号被放大并从扬声器传出。然后，来自扬声器的声音可以再次被麦克风接收，进一步放大，然后再次通过扬声器传递出去。 所产生的声音的频率由传声器、放大器和扬声器的共振频率、房间的声学特性、传声器和扬声器的定向拾音和发射模式以及它们之间的距离决定。对于小型的扩声系统来说，这种声音很容易的体现的响亮的吱吱声或尖叫声。
在歌手或公众演讲者使用扩声系统或扩音系统的活动中，麦克风发生的正反馈几乎总是被认为是不受欢迎的。自20世纪90年代以来，音频工程师使用各种电子设备，如均衡器或者自动反馈检测设备，来防止这些不受欢迎的尖叫声或尖叫声，这些声音影响了观众对活动的享受。另一方面，自20世纪60年代以来，摇滚乐队中的电吉他手使用大音量的吉他放大器和失真效果，有意制造吉他中的正反馈，以创造理想的音乐效果。 披头士乐队的 "I Feel Fine "是流行音乐中最早使用反馈作为录音效果的例子之一。它的开头是由Lennon拨动吉他上的A弦产生的一个单一的、有冲击力的反馈音。虽然像 Kinks 和 Who 等艺术家已经在表演中使用了正反馈，但是Lennon仍然为披头士乐队可能是第一个特意把它放在黑胶唱片上的乐队而感到骄傲。在他最后的一次采访中，他说，“我敢说任何人都找不到这样的唱片，除非是1922年这张用这种方式录制的老蓝调唱片。”
在歌手或公众演讲者使用扩声系统或扩音系统的活动中，麦克风发生的正反馈几乎总是被认为是不受欢迎的。自20世纪90年代以来，音频工程师使用各种电子设备，如均衡器或者自动反馈检测设备，来防止这些不受欢迎的尖叫声或尖叫声，这些声音影响了观众对活动的享受。另一方面，自20世纪60年代以来，摇滚乐队中的电吉他手使用大音量的吉他放大器和失真效果，有意制造吉他中的正反馈，以创造理想的音乐效果。 披头士乐队的 "I Feel Fine "是流行音乐中最早使用反馈作为录音效果的例子之一。它的开头是由Lennon拨动吉他上的A弦产生的一个单一的、有冲击力的反馈音。虽然像 Kinks 和 Who 等艺术家已经在表演中使用了正反馈，但是Lennon仍然为披头士乐队可能是第一个特意把它放在黑胶唱片上的乐队而感到骄傲。在他最后的一次采访中，他说，“我敢说任何人都找不到这样的唱片，除非是1922年这张用这种方式录制的老蓝调唱片。”
音频反馈的原理是由丹麦科学家Søren Absalon Larsen首先发现的。麦克风并不是唯一受此影响的传感器。录音板拾音器也会产生正反馈，通常是在100赫兹以下的低频范围内表现出低沉的轰鸣声。Jimi Hendrix是一个创新者，在他的吉他独奏中有意使用吉他正反馈来创造独特的声音效果。他帮助发展了电吉他演奏中音频反馈的可控性和音乐性，后来Brian May也是这种技术的著名支持者。
音频反馈的原理是由丹麦科学家Søren Absalon Larsen首先发现的。麦克风并不是唯一受此影响的传感器。录音板拾音器也会产生正反馈，通常是在100赫兹以下的低频范围内表现出低沉的轰鸣声。Jimi Hendrix是一个创新者，在他的吉他独奏中有意使用吉他正反馈来创造独特的声音效果。他帮助发展了电吉他演奏中音频反馈的可控性和音乐性，后来Brian May也是这种技术的著名支持者。
===视频===
===Video视频===
同样，如果一台摄像机对准一个正在显示摄像机自身信号的监控屏幕，那么通过正反馈就可以在屏幕上形成重复的图案。这种视频反馈效果在电视剧《神秘博士》前十季的开场白中就被使用了。
同样，如果一台摄像机对准一个正在显示摄像机自身信号的监控屏幕，那么通过正反馈就可以在屏幕上形成重复的图案。这种视频反馈效果在电视剧《神秘博士》前十季的开场白中就被使用了。
=== Switches 开关===
===开关===
在电气开关中，包括双金属条型恒温器，开关通常在开关动作中具有滞后性。在这些情况下，滞后是通过一个临界点机构内的正反馈来实现的。正反馈作用可最大限度地减少开关过程中发生电弧的时间，并使触点保持在断开或闭合状态。
在电气开关中，包括双金属条型恒温器，开关通常在开关动作中具有滞后性。在这些情况下，滞后是通过一个临界点机构内的正反馈来实现的。正反馈作用可最大限度地减少开关过程中发生电弧的时间，并使触点保持在断开或闭合状态。
=== In biology在生物学中===
=== 生物学===
[[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.生物学中的正反馈是指身体对刺激的反应的放大。例如，在分娩过程中，当胎儿的头顶到子宫颈时(1)，会刺激神经冲动从子宫颈到大脑(2)。大脑接到通知后，会向脑垂体发出信号，释放一种叫做<font color="#ff8000"> 催产素oxytocin</font>的激素(3)。催产素随后通过血液流向子宫(4)，引起宫缩，将胎儿推向子宫颈，最终促使分娩。|链接=Special:FilePath/Positive_Feedback-_Childbirth_(1).svg]]
[[File:Positive Feedback- Childbirth (1).svg|thumb|生物学中的正反馈是指身体对刺激的反应的放大。例如，在分娩过程中，当胎儿的头顶到子宫颈时(1)，会刺激神经冲动从子宫颈到大脑(2)。大脑接到通知后，会向脑垂体发出信号，释放一种叫做<font color="#ff8000"> 催产素oxytocin</font>的激素(3)。催产素随后通过血液流向子宫(4)，引起宫缩，将胎儿推向子宫颈，最终促使分娩。|链接=Special:FilePath/Positive_Feedback-_Childbirth_(1).svg]]
生物学中的正反馈是指身体对刺激的反应的放大。例如，在分娩过程中，当胎儿的头顶到子宫颈时(1)，会刺激神经冲动从子宫颈到大脑(2)。大脑接到通知后，会向脑垂体发出信号，释放一种叫做<font color="#ff8000"> 催产素oxytocin</font>的激素(3)。催产素随后通过血液流向子宫(4)，引起宫缩，将胎儿推向子宫颈，最终促使分娩。
生物学中的正反馈是指身体对刺激的反应的放大。例如，在分娩过程中，当胎儿的头顶到子宫颈时(1)，会刺激神经冲动从子宫颈到大脑(2)。大脑接到通知后，会向脑垂体发出信号，释放一种叫做<font color="#ff8000"> 催产素oxytocin</font>的激素(3)。催产素随后通过血液流向子宫(4)，引起宫缩，将胎儿推向子宫颈，最终促使分娩。
==== In physiology在生理学中 ====
==== 生理学====
在生理学中可以找到一些正反馈系统的例子。
在生理学中可以找到一些正反馈系统的例子。
* One example is the onset of [[Contraction (childbirth)|contractions]] in [[childbirth]], known as the [[Ferguson reflex]]. When a contraction occurs, the hormone [[oxytocin]] causes a nerve stimulus, which stimulates the [[hypothalamus]] to produce more oxytocin, which increases uterine contractions. This results in contractions increasing in [[amplitude]] and [[frequency]].<ref name=Guyton1991>Guyton, Arthur C. (1991) ''Textbook of Medical Physiology''. (8th ed). Philadelphia: W.B. Saunders. {{ISBN|0-7216-3994-1}}</ref>{{rp|pages=924–925}}
*
其中一个例子是分娩时宫缩的发生，称为弗格森反射。当宫缩发生时，激素催产素会引起神经刺激，刺激下丘脑产生更多的催产素，从而增加子宫收缩。这就导致宫缩的幅度和频率增加。
其中一个例子是分娩时宫缩的发生，称为弗格森反射。当宫缩发生时，激素催产素会引起神经刺激，刺激下丘脑产生更多的催产素，从而增加子宫收缩。这就导致宫缩的幅度和频率增加。
<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}}
<ref name=Guyton1991>Guyton, Arthur C. (1991) ''Textbook of Medical Physiology''. (8th ed). Philadelphia: W.B. Saunders. {{ISBN|0-7216-3994-1}}</ref>
*
另一个例子是血液凝固的过程。当受伤的组织释放出信号化学物质，激活血液中的血小板时，这个循环就启动了。被激活的血小板释放化学物质，激活更多的血小板，引起快速的级联反应，形成血栓。<ref name=Guyton1991/>
* 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}}
*
哺乳也涉及正反馈，当婴儿吸吮乳头时，会有神经反应进入脊髓，并上传到大脑的下丘脑，然后刺激垂体产生更多的催乳素以产生更多的乳汁<ref name=Guyton1991/>。
*
在月经周期的卵泡期期间，雌激素的飙升会导致排卵<ref name=Guyton1991/>。
* 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.
*
神经信号的产生是另一个例子，神经纤维的膜使钠离子通过钠通道轻微渗漏，导致膜电位的变化，进而引起更多通道的开放（Hodgkin循环）。所以，最初的轻微渗漏会导致钠渗漏的爆发，从而产生神经动作电位<ref name=Guyton1991/>。
*
在心脏的兴奋收缩耦合中，肌浆网膜中的兰尼碱受体检测到心肌细胞内钙离子的增加，该受体以正反馈生理反应将钙运出到细胞质中。
在心脏的兴奋收缩耦合中，肌浆网膜中的兰尼碱受体检测到心肌细胞内钙离子的增加，该受体以正反馈生理反应将钙运出到细胞质中。
在大多数情况下，这种反馈循环最终会释放出反信号，从而抑制或破坏循环。分娩宫缩在宝宝离开母体时停止。化学物质分解血凝块。当婴儿不再需要被哺乳时，泌乳停止<ref name=Guyton1991/>。
====基因调控====
==== In gene regulation 基因调控====
正反馈是基因调控中研究较好的一种现象，其中最常见的是与双稳态有关。当一个基因通过双负反馈循环直接或间接激活自身时，就会出现正反馈。遗传工程师已经在细菌中构建并测试了简单的正反馈网络，以证明双稳态的概念。<ref name=Hasty2002/>
正反馈是基因调控中研究较好的一种现象，其中最常见的是与双稳态有关。当一个基因通过双负反馈循环直接或间接激活自身时，就会出现正反馈。遗传工程师已经在细菌中构建并测试了简单的正反馈网络，以证明双稳态的概念。<ref name=Hasty2002/>
正反馈的一个典型例子是大肠杆菌中的乳糖操纵子。正反馈在细胞分化、发育和癌症进展中起着不可或缺的作用，因此，基因调控中的正反馈可以产生显著的生理结果。分子动力学中的随机运动加上正反馈可以引发有趣的效应，例如从同一母细胞中产生表型不同的细胞群。<ref name=Veening2008/> 这种情况的发生是因为噪声会被正反馈放大。正反馈也可以发生在细胞信号的其他形式中，如酶动力学或代谢途径。<ref name=Christoph2001/>
正反馈的一个典型例子是大肠杆菌中的乳糖操纵子。正反馈在细胞分化、发育和癌症进展中起着不可或缺的作用，因此，基因调控中的正反馈可以产生显著的生理结果。分子动力学中的随机运动加上正反馈可以引发有趣的效应，例如从同一母细胞中产生表型不同的细胞群。<ref name=Veening2008/> 这种情况的发生是因为噪声会被正反馈放大。正反馈也可以发生在细胞信号的其他形式中，如酶动力学或代谢途径。<ref name=Christoph2001/>
==== 进化生物学 ====
在进化生物学中正反馈循环被用来描述生物进化中变化动态的各个方面。 例如，在宏观层面，Alfred J. Lotka（1945）认为，物种的演变最重要的是选择反馈能量流动以捕获越来越多的能源系统的能量。<ref name=Lotka1945/>在人类层面，Richard D. Alexander(1989)提出，人类群体之间和群体内部的社会竞争会影响智力的选择，从而时不时地会产生更多、更完善的人类智力。 <ref name=Alexander1989/> Bernard Crespi（2004）讨论了进化中正反馈循环的其他几个例子<ref name=Crespi2004/>。通过与军备竞赛进行类比，给生物系统中的正反馈提供了进一步的例子。<ref name=Blindwatchmaker/>
[[File:Phanerozoic Biodiversity.svg|300px|right|thumb|显生宙[[生物多样性]]呈现稳定而非单调的增长，从接近于零一直增长到有几千个属。|链接=Special:FilePath/Phanerozoic_Biodiversity.svg]]
研究表明，在<font color="#32CD32"> 显生宙 </font>，生物多样性的变化与双曲模型（广泛用于人口学和宏观社会学）的相关性要比指数模型和逻辑斯特模型（传统上用于人口生物学，并广泛用于生物多样性化石）的相关性好得多。后者的模型意味着多样性的变化是由一阶正反馈（更多的祖先，更多的后代）和资源限制产生的负反馈所引导的。双曲模型意味着二阶正反馈。世界人口增长的双曲线模式已被证明源于人口数量与技术增长速度之间的二阶正反馈。生物多样性增长的双曲特征同样可以由多样性与群落结构复杂性之间的正反馈来解释。有人认为，生物多样性和人口曲线之间的相似性可能来自这样一个事实，即两者都是由双曲趋势(由正反馈产生)与周期性和随机性的动态干扰而产生的。<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>
==== 免疫系统====
细胞因子风暴，或称高细胞因子血症，是一种潜在的致命性免疫反应，表现为各种细胞因子水平高度升高，这是由细胞因子和免疫细胞之间的正反馈环组成。[36]在正常的免疫功能中，可以利用正反馈环来增强B淋巴细胞的作用。<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>当B细胞将其抗体与抗原结合并被激活后，就开始释放抗体并分泌一种称为C3的补体蛋白。C3和B细胞的抗体都可以与病原体结合，当B细胞的抗体与C3结合后，就会加快该B细胞分泌更多的抗体和更多的C3蛋白，从而形成一个正反馈循环。<ref>{{cite journal|last=Paul|first=William E.|title=Infectious Diseases and the Immune System|journal=Scientific American|volume=269|issue=3|date=September 1993|page=93|bibcode=1993SciAm.269c..90P|doi=10.1038/scientificamerican0993-90|pmid=8211095}}</ref>
细胞因子风暴，或称高细胞因子血症，是一种潜在的致命性免疫反应，表现为各种细胞因子水平高度升高，这是由细胞因子和免疫细胞之间的正反馈环组成。[36]在正常的免疫功能中，可以利用正反馈环来增强B淋巴细胞的作用。<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>当B细胞将其抗体与抗原结合并被激活后，就开始释放抗体并分泌一种称为C3的补体蛋白。C3和B细胞的抗体都可以与病原体结合，当B细胞的抗体与C3结合后，就会加快该B细胞分泌更多的抗体和更多的C3蛋白，从而形成一个正反馈循环。<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 细胞凋亡====
====细胞凋亡====
=== In psychology 在心理学上===
细胞凋亡是一种由酪蛋白酶介导的细胞死亡过程，其目的是清除长寿或受损的细胞。这一过程的失效与癌症或帕金森氏病等著名疾病有关。细胞凋亡过程的核心是半胱氨酸蛋白酶的自动激活，它可以通过一个正反馈循环来建模。这种正反馈通过中间胱天蛋白酶使效应子胱天蛋白酶自动活化。当从凋亡途径的其他部分分离出来时，无论效应子胱天蛋白酶的中间激活步骤数量有多少，这种正反馈仅呈现一种稳定的稳态。<ref name="ReferenceA"/> 当该核心过程与胱天蛋白酶作用的抑制剂和增强剂相辅相成时，该过程呈现双稳态，从而模拟细胞的存活和死亡状态。<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>
=== 心理学===
Winner（1996）将有天赋的孩子描述为受到正反馈循环的驱动，这些反馈循环体现在他们自己的课程学习上，通过反馈自己的满意程度，从而进一步将他们的学习目标提高到更高水平等。<ref name=Winner1996/>Winner将这种正反馈循环称为 "狂热的掌握"。 Vandervert(2009a，2009b)提出，神童可以用工作记忆中的思维/表现输出之间的正反馈回路来解释，工作记忆中的思维/表现输出被反馈到小脑，在那里被精简，然后再反馈到工作记忆中，从而稳定地增加工作记忆的数量和质量输出。<ref name=Vandervert2009a/><ref name=Vandervert2009b/>
Winner（1996）将有天赋的孩子描述为受到正反馈循环的驱动，这些反馈循环体现在他们自己的课程学习上，通过反馈自己的满意程度，从而进一步将他们的学习目标提高到更高水平等。<ref name=Winner1996/>Winner将这种正反馈循环称为 "狂热的掌握"。 Vandervert(2009a，2009b)提出，神童可以用工作记忆中的思维/表现输出之间的正反馈回路来解释，工作记忆中的思维/表现输出被反馈到小脑，在那里被精简，然后再反馈到工作记忆中，从而稳定地增加工作记忆的数量和质量输出。<ref name=Vandervert2009a/><ref name=Vandervert2009b/>
=== In economics在经济学中 ===
=== 经济学===
====市场上的社会影响====
事实证明，产品推荐和消费者先前的购买信息对消费者的选择影响很大，无论是音乐、电影、书籍、电子产品还是其他类型的产品。社会影响往往会诱发一种 "富者越富 "的现象（马太效应），即热门产品往往会变得更加受欢迎。<ref name="altszyler2017">{{cite journal | title= Transient dynamics in trial-offer markets with social influence: Trade-offs between appeal and quality. | author1= Altszyler, E | author2= Berbeglia, F. | author3= Berbeglia, G. | author4= Van Hentenryck, P. | journal= PLOS ONE | year= 2017 | volume= 12 | issue= 7 | df= | doi=10.1371/journal.pone.0180040 |pmid = 28746334| pmc= 5528888 | page=e0180040| bibcode= 2017PLoSO..1280040A }}</ref>
事实证明，产品推荐和消费者先前的购买信息对消费者的选择影响很大，无论是音乐、电影、书籍、电子产品还是其他类型的产品。社会影响往往会诱发一种 "富者越富 "的现象（马太效应），即热门产品往往会变得更加受欢迎。<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>
====Market dynamics市场动态====
====市场动向====
根据George Soros提出的反射性理论，价格变化是由一个正反馈过程驱动的，即投资者的预期受到价格变动的影响，因此他们的行为会强化这个方向的价格变动，直到价格的变化变得不可持续，于是反馈推动价格向相反的方向发展。<ref>{{citation |title=Behavioural Technical Analysis |first=Paul V. |last=Azzopardi |publisher=Harriman House Limited |year=2010 |page=116 |isbn=9780857190680 |url=https://books.google.com/books?id=04Ay8qviuwgC&pg=PA116&lpg=PA116 |url-status=live |archiveurl=https://web.archive.org/web/20170329103058/https://books.google.com/books?id=04Ay8qviuwgC&pg=PA116&lpg=PA116&source=bl&hl=en&sa=X&f=false |archivedate=2017-03-29 }}</ref>
根据George Soros提出的反射性理论，价格变化是由一个正反馈过程驱动的，即投资者的预期受到价格变动的影响，因此他们的行为会强化这个方向的价格变动，直到价格的变化变得不可持续，于是反馈推动价格向相反的方向发展。<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>
==== Systemic risk 系统性风险====
==== 系统风险====
系统性风险是指放大效应或杠杆或正反馈过程给系统带来的风险。这通常是未知的，在某些条件下，这个过程会成倍放大，并迅速导致破坏性或混乱的行为。 庞氏骗局就是正反馈系统的一个很好的例子：来自新投资者的资金被用来支付异常高的回报，反过来又吸引了更多的新投资者，导致快速增长进而走向崩溃。W. Brian Arthur 也对经济中的正反馈进行了研究和著述（如W. Brian Arthur，1990）。Hyman Minsky提出了一个理论，认为某些信用扩张行为会使市场经济变成一个 "偏差放大系统"，从而可能会突然崩溃，这有时被称为 "明斯基时刻"。
系统性风险是指放大效应或杠杆或正反馈过程给系统带来的风险。这通常是未知的，在某些条件下，这个过程会成倍放大，并迅速导致破坏性或混乱的行为。 庞氏骗局就是正反馈系统的一个很好的例子：来自新投资者的资金被用来支付异常高的回报，反过来又吸引了更多的新投资者，导致快速增长进而走向崩溃。W. Brian Arthur 也对经济中的正反馈进行了研究和著述（如W. Brian Arthur，1990）。Hyman Minsky提出了一个理论，认为某些信用扩张行为会使市场经济变成一个 "偏差放大系统"，从而可能会突然崩溃，这有时被称为 "明斯基时刻"。
输入和输出明确分开的简单系统不容易发生系统性风险。 随着系统复杂性的增加，这种风险更容易发生，因为即使在详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。 一个复杂系统的效率越高，就越容易发生系统性风险，因为只需要很小的偏差就可以破坏系统。 因此，设计良好的复杂系统一般都会有一些内在的功能来避免这种情况的发生，比如在系统内有少量的摩擦力，或阻力，或惯性，或时间延迟来使输出与输入脱钩。这些因素造成了低效率，但它们是避免不稳定的必要条件。
输入和输出明确分开的简单系统不容易发生系统性风险。 随着系统复杂性的增加，这种风险更容易发生，因为即使在详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。 一个复杂系统的效率越高，就越容易发生系统性风险，因为只需要很小的偏差就可以破坏系统。 因此，设计良好的复杂系统一般都会有一些内在的功能来避免这种情况的发生，比如在系统内有少量的摩擦力，或阻力，或惯性，或时间延迟来使输出与输入脱钩。这些因素造成了低效率，但它们是避免不稳定的必要条件。
2010年的闪崩事件被归咎于高频交易（HFT）的做法，不过HFT是否真的会增加系统性风险仍然存在争议。
2010年的闪崩事件被归咎于高频交易（HFT）的做法，不过HFT是否真的会增加系统性风险仍然存在争议。
====人口与环境危机====
可以认为农业和人口之间处于一种正反馈模式，这意味着双方越来越强烈地推动彼此。有人认为，这种正反馈系统终将在某一时刻以灾难结束，因为现代农业正在耗尽所有容易获得的磷酸盐，并且进行高效的单一栽培，使得现代农业更容易受到系统性风险影响。
技术创新和人类人口也可以有类似的考虑，这也是过去人类人口明显的双曲线增长，而不是简单的指数增长的一个解释。
技术创新和人类人口也可以有类似的考虑，这也是过去人类人口明显的双曲线增长，而不是简单的指数增长的一个解释。
有人提出，由于人口和技术之间的二阶正反馈，增长速度正在加快。技术增长增加了土地对人的承载能力，从而导致人口增长，而这反过来又推动了技术的进一步增长。
有人提出，由于人口和技术之间的二阶正反馈，增长速度正在加快。技术增长增加了土地对人的承载能力，从而导致人口增长，而这反过来又推动了技术的进一步增长。
==== Prejudice, social institutions and poverty 偏见、社会制度与贫困====
==== 偏见、社会制度与贫困====
Gunnar Myrdal描述了一个不平等和贫困加剧的恶性循环，这就是所谓的”循环累积因果关系”。
Gunnar Myrdal描述了一个不平等和贫困加剧的恶性循环，这就是所谓的”循环累积诱因”。
==== In meteorology 在气象学中====
==== 气象学====
干旱通过正反馈效应加剧。缺雨会降低土壤湿度，从而杀死植物，使它们通过蒸腾作用释放更少的水分。这两个因素都限制了水蒸气从地表加到大气中的过程，并使吸收水分的干燥灰尘进入大气。水汽少了，既意味着露点温度低，白天的供暖效率也高，减少了大气中湿度导致云的形成的机会。最后，没有云，就不会有雨，这个正反馈循环就形成了。
干旱通过正反馈效应加剧。缺雨会降低土壤湿度，从而杀死植物，使它们通过蒸腾作用释放更少的水分。这两个因素都限制了水蒸气从地表加到大气中的过程，并使吸收水分的干燥灰尘进入大气。水汽少了，既意味着露点温度低，白天的供暖效率也高，减少了大气中湿度导致云的形成的机会。最后，没有云，就不会有雨，这个正反馈循环就形成了。
==== In climatology 在气候学中====
==== 气候学====
气候中的"诱因 "可能会将气候系统推向变暖或变冷的方向，例如，大气中温室气体浓度的增加会导致地表变暖。诱因是气候系统的外部因素，而反馈是系统的内部过程。一些反馈机制与气候系统的其他部分相对孤立地发挥作用，而另一些则是紧密耦合的。气候系统的作用力、反馈和动态决定了气候变化的程度和速度。全球变暖中的主要正反馈是变暖使大气中的水汽量增加，进而导致进一步变暖。主要的负反馈来自Stefan-Boltzmann定律，从地球辐射到空间的热量与地球表面和大气温度的四次方成正比。
气候学中正反馈子系统的其他例子包括:
气候学中正反馈子系统的其他例子包括:
大气变暖会使冰融化，从而改变反照率，从而使大气进一步变暖。
大气变暖会使冰融化，从而改变反照率，从而使大气进一步变暖。
甲烷水合物可能是不稳定的，所以海洋变暖可能会释放更多的温室气体之一的甲烷。
甲烷水合物可能是不稳定的，所以海洋变暖可能会释放更多的温室气体之一的甲烷。
泥炭，天然存在于泥炭沼泽中，含有碳。当泥炭干燥时，它会分解，并可能额外燃烧。泥炭还会释放一氧化二氮。
泥炭，天然存在于泥炭沼泽中，含有碳。当泥炭干燥时，它会分解，并可能额外燃烧。泥炭还会释放一氧化二氮。
全球变暖会影响云的分布。高空的云层会增强温室效应，而低空的云层则主要反射太阳光，对温度产生相反的影响。
全球变暖会影响云的分布。高空的云层会增强温室效应，而低空的云层则主要反射太阳光，对温度产生相反的影响。
政府间气候变化专门委员会(气专委)第四次评估报告指出，"人类活动的变暖可能导致一些突然或不可逆转的影响，这取决于气候变化的速度和程度"。
政府间气候变化专门委员会(气专委)第四次评估报告指出，"人类活动的变暖可能导致一些突然或不可逆转的影响，这取决于气候变化的速度和程度"。
==== In sociology 在社会学中====
==== 社会学====
自我应验预言是信念和行为之间的一个社会正反馈循环: 如果有足够多的人相信某件事是真的，他们的行为就能让它变成真的，而对他们行为的观察又可能反过来增加信念。一个典型的例子是银行挤兑。
自我应验预言是信念和行为之间的一个社会正反馈循环: 如果有足够多的人相信某件事是真的，他们的行为就能让它变成真的，而对他们行为的观察又可能反过来增加信念。一个典型的例子是银行挤兑。
正反馈的另一个社会学例子是网络效应。当更多的人被鼓励加入一个网络时，这就增加了网络的覆盖面，因此网络扩张得越来越快。病毒视频就是网络效应的一个例子，在这个例子中，一个热门视频的链接被分享和再传播，确保更多的人看到这个视频，然后重新发布链接。这是许多社会现象的基础，包括庞氏骗局和连锁信。在许多情况下，人口量是反馈效应的限制因素。
正反馈的另一个社会学例子是网络效应。当更多的人被鼓励加入一个网络时，这就增加了网络的覆盖面，因此网络扩张得越来越快。病毒视频就是网络效应的一个例子，在这个例子中，一个热门视频的链接被分享和再传播，确保更多的人看到这个视频，然后重新发布链接。这是许多社会现象的基础，包括庞氏骗局和连锁信。在许多情况下，人口量是反馈效应的限制因素。
==== In chemistry 在化学中====
==== 化学====
如果化学反应引起热量的释放，而反应本身在较高的温度下发生得更快，那么就很有可能出现正反馈。如果产生的热量没有足够快地从反应物中排除，就会发生热失控，并很快导致化学爆炸。
如果化学反应引起热量的释放，而反应本身在较高的温度下发生得更快，那么就很有可能出现正反馈。如果产生的热量没有足够快地从反应物中排除，就会发生热失控，并很快导致化学爆炸。
==== In conservation 在自然保护中====
==== 自然保护====
许多野生动物被猎杀的原因是它们身体的某些部位可能相当有价值。目标物种越是接近灭绝，其部位的价格就越高。这就是一个正反馈的例子。
许多野生动物被猎杀的原因是它们身体的某些部位可能相当有价值。目标物种越是接近灭绝，其部位的价格就越高。这就是一个正反馈的例子。
====See also另请参阅====
==参见==
* Chain reaction – Sequence of reactions 链式反应 -- -- 反应的顺序
* Chain reaction – Sequence of reactions 链式反应 -- -- 反应的顺序
====References 参考资料====
==参考文献==
1. ^ a b c Ben Zuckerman & David Jefferson (1996). Human Population and the Environmental Crisis. Jones & Bartlett Learning. p. 42. ISBN 9780867209662. Archived from the original on 2018-01-06.
1. ^ a b c Ben Zuckerman & David Jefferson (1996). Human Population and the Environmental Crisis. Jones & Bartlett Learning. p. 42. ISBN 9780867209662. Archived from the original on 2018-01-06.
58. ^ Positive feedback occurs when one is told he has done something well or correctly. Tom Coens and Mary Jenkins, "Abolishing Performance Appraisals", p116.
58. ^ Positive feedback occurs when one is told he has done something well or correctly. Tom Coens and Mary Jenkins, "Abolishing Performance Appraisals", p116.
==拓展阅读==
* Norbert Wiener (1948), Cybernetics or Control and Communication in the Animal and the Machine, Paris, Hermann et Cie - MIT Press, Cambridge, MA.
* Norbert Wiener (1948), Cybernetics or Control and Communication in the Animal and the Machine, Paris, Hermann et Cie - MIT Press, Cambridge, MA.
* Katie Salen and Eric Zimmerman. Rules of Play. MIT Press. 2004. ISBN 0-262-24045-9. Chapter 18: Games as Cybernetic Systems.
* Katie Salen and Eric Zimmerman. Rules of Play. MIT Press. 2004. ISBN 0-262-24045-9. Chapter 18: Games as Cybernetic Systems.