Editing Negative feedback (section)

== General description ==
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[[Image:105 Negative Feedback Loops.jpg|thumb|right|Feedback loops in the human body]]

In many physical and biological systems, qualitatively different influences can oppose each other. For example, in biochemistry, one set of chemicals drives the system in a given direction, whereas another set of chemicals drives it in an opposing direction. If one or both of these opposing influences are non-linear, equilibrium point(s) result.

In [[biology]], this process (in general, [[biochemistry|biochemical]]) is often referred to as [[homeostasis]]; whereas in [[mechanics]], the more common term is [[Mechanical equilibrium|equilibrium]].

In [[engineering]], [[mathematics]] and the physical, and biological sciences, common terms for the points around which the system gravitates include: attractors, [[stability theory|stable]] states, eigenstates/eigenfunctions, equilibrium points, and [[setpoint (control system)|setpoint]]s.

In [[control theory]], ''negative'' refers to the sign of the multiplier in mathematical models for feedback. In delta notation, −Δoutput is added to or mixed into the input. In multivariate systems, vectors help to illustrate how several influences can both partially complement and partially oppose each other.<ref name=mindell/>

Some authors, in particular with respect to modelling [[System dynamics|business systems]], use ''negative'' to refer to the reduction in difference between the desired and actual behavior of a system.<ref name="Ramaprasad">{{cite journal |author=Arkalgud Ramaprasad |title=On The Definition of Feedback |journal=Behavioral Science |volume=28 |issue=1 |pages=4–13 |year=1983 |doi=10.1002/bs.3830280103}}</ref><ref name=Sterman>
John D.Sterman, ''Business Dynamics: Systems Thinking and Modeling for a Complex World'' McGraw Hill/Irwin, 2000. {{ISBN|9780072389159}}
</ref> In a psychology context, on the other hand, ''negative'' refers to the [[Valence (psychology)|valence]] of the feedback – attractive versus aversive, or praise versus criticism.<ref name=herold1977>{{cite journal | last1 = Herold | first1 = David M. | last2 = Greller | first2 = Martin M. | year = 1977 | title = Research Notes. Feedback: The Definition of a Construct | journal = Academy of Management Journal | volume = 20 | issue = 1| pages = 142–147 | jstor = 255468 }}</ref>

In contrast, [[positive feedback]] is feedback in which the system responds so as to increase the magnitude of any particular perturbation, resulting in amplification of the original signal instead of stabilization. Any system in which there is positive feedback together with a gain greater than one will result in a runaway situation. Both positive and negative feedback require a feedback loop to operate.

However, negative feedback systems can still be subject to [[oscillation]]s. This is caused by a phase shift around any loop. Due to these phase shifts the feedback signal of some frequencies can ultimately become in phase with the input signal and thus turn into positive feedback, creating a runaway condition. Even before the point where the phase shift becomes 180 degrees, stability of the negative feedback loop will become compromised, leading to increasing under- and overshoot following a disturbance. This problem is often dealt with by attenuating or changing the phase of the problematic frequencies in a design step called compensation. Unless the system naturally has sufficient damping, many negative feedback systems have [[low pass filter]]s or [[dashpot|dampers]] fitted.