Editing Positive feedback (section)

{{Short description|Feedback loop that increases an initial small effect}}
{{Use British English|date=September 2024}}

[[File:Stampede loop.png|thumb|[[Causal loop diagram]] that depicts the causes of a stampede as a positive feedback loop. Alarm or panic can sometimes be spread by positive feedback among a herd of animals to cause a [[stampede]].]]
[[File:Birmingham Northern Rock bank run 2007.jpg|thumb|right|In sociology a [[network effect]] can quickly create the positive feedback of a [[bank run]]. The above photo is of the [[Nationalisation of Northern Rock#Run on the bank|UK Northern Rock 2007 bank run]].]]

'''Positive feedback''' ('''exacerbating feedback''', '''self-reinforcing feedback''') is a process that occurs in a [[feedback loop]] where the outcome of a process reinforces the inciting process to build momentum. As such, these forces can exacerbate 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
 |title         = Human Population and the Environmental Crisis
 |author1       = Ben Zuckerman
 |author2       = David Jefferson
 |name-list-style = amp
 |publisher     = Jones & Bartlett Learning
 |year          = 1996
 |isbn          = 9780867209662
 |page          = 42
 |url           = https://books.google.com/books?id=a1gW4uV-q8EC&pg=PA42
 |url-status       = live
 |archive-url    = https://web.archive.org/web/20180106192002/https://books.google.com/books?id=a1gW4uV-q8EC&pg=PA42
 |archive-date   = 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]].

Mathematically, positive feedback is defined as a positive [[loop gain]] around a closed loop of cause and effect.<ref name=zuckerman/><ref name=theorymodelling>
{{cite book
 |title       = Theory of Modeling and Simulation: Integrating Discrete Event and Continuous Complex Dynamic Systems
 |author1     = Bernard P. Zeigler
 |author2     = Herbert Praehofer
 |author3     = Tag Gon Kim Section
 |publisher   = Academic Press
 |year        = 2000
 |isbn        = 9780127784557
 |page        = 55
 |section     = 3.3.2 Feedback in continuous systems
 |url         = https://books.google.com/books?id=REzmYOQmHuQC&pg=PA55
 |quote       = A positive feedback loop is one with an even number of negative influences <nowiki>[around the loop]</nowiki>.
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20170103061121/https://books.google.com/books?id=REzmYOQmHuQC&pg=PA55
 |archive-date = 2017-01-03
}}</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>
{{cite book
 |title       = Newnes Dictionary of Electronics
 |edition     = 4th
 |author1     = S W Amos
 |author2     = R W Amos
 |publisher   = Newnes
 |year        = 2002
 |isbn        = 9780750656429
 |page        = 247
 |url         = https://books.google.com/books?id=lROa-MpIrucC&pg=PA247
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20170329045057/https://books.google.com/books?id=lROa-MpIrucC&pg=PA247
 |archive-date = 2017-03-29
}}</ref><ref>
{{cite book
 |title       = Modern Dictionary of Electronics
 |edition     = 7th
 |author      = Rudolf F. Graf
 |publisher   = Newnes
 |year        = 1999
 |isbn        = 9780750698665
 |page        = 276
 |url         = https://books.google.com/books?id=uah1PkxWeKYC&pg=PA276
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20170329122152/https://books.google.com/books?id=uah1PkxWeKYC&pg=PA276
 |archive-date = 2017-03-29
}}</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, [[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]], [[damped]], or [[Maxima and minima|limited]], or it can be cancelled or reduced by adding negative feedback.

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 junction]]s. Positive feedback in [[chemical reaction]]s can increase the rate of reactions, and in some cases can lead to [[explosion]]s. 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 [[Business cycle|boom-then-bust cycles]].   A familiar example of positive feedback is the loud squealing or howling sound produced by [[audio feedback]] in [[public address system]]s: the microphone picks up sound from its own loudspeakers, amplifies it, and sends it through the speakers again.

[[File:Positive Feedback Diagram (2).svg|thumb|[[Platelet]] clotting demonstrates positive feedback. The damaged blood vessel wall releases chemicals that initiate the formation of a blood clot through platelet congregation. As more platelets gather, more chemicals are released that speed up the process. The process gets faster and faster until the blood vessel wall is completely sealed and the positive feedback loop has ended.  The exponential form of the graph illustrates the positive feedback mechanism. ]]