Negative feedback
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2008-06-19T13:59:06Z
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/* Overview */
'''Negative [[feedback]]''' feeds part of a [[system]]'s output, inverted, into the system's input; generally with the result that fluctuations are attenuated. Many real-world systems have one or several points around which the system gravitates. In response to a [[Perturbation (biology)|perturbation]], a negative feedback system with such point(s) will tend to re-establish [[Mechanical equilibrium|equilibrium]].
== Overview ==
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 (generally [[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|stable]] states, eigenstates/eigenfunctions, equilibrium points, and [[setpoint]]s.
'<i>Negative</i>' refers to the sign of the multiplier in mathematical models for feedback. In delta notation, -<math>\Delta</math>output is added to or mixed into the input. In multivariate systems, vectors help to illustrate how several influences can both partially compliment and partially oppose each other.
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'''Negative feedback''' is a type of [[feedback]] in which the [[system]] responds in an opposite direction to the [[Perturbation (biology)|perturbation]]. It is a process of feeding back to the input a part of a system's output, so as to reverse the direction of [[change]] of the output. This tends to keep the output from changing, so it is [[stability|stabilizing]] and attempts to maintain constant conditions. This often results in [[Mechanical equilibrium|equilibrium]] (in physical science) or [[homeostasis]] (in [[biology]]) such that the system will return to its original [[setpoint]]. -->
In contrast, [[positive feedback]] is a feedback in which the system responds in the same direction as the perturbation, resulting in amplification of the original signal instead of stabilizing the signal. Both positive and negative feedback require a feedback loop to operate, as opposed to [[feedforward]], which does not rely on a feedback loop for its control of the system.
== Examples ==
Examples of the use of negative feedback to control its system are: [[thermostat]] control, [[phase-locked loop]], [[hormonal]] regulation, and temperature regulation in animals.
A simple and practical example is a [[thermostat]]. When the [[temperature]] in a heated room reaches a certain upper limit the room heating is switched off so that the temperature begins to fall. When the temperature drops to a lower limit, the heating is switched on again. Provided the limits are close to each other, a steady room temperature is maintained. The same applies to a cooling system, such as an [[air conditioner]], a [[refrigerator]], or a [[freezer]].
Some biological systems exhibit negative feedback such as the [[baroreflex]] in [[blood pressure]] regulation and [[erythropoiesis]]. Many biological process (e.g., in the [[human anatomy]]) use negative feedback. Examples of this are numerous, from the regulating of body temperature, to the regulating of blood [[glucose]] levels. The disruption of negative feedback can lead to undesirable results: in the case of blood glucose levels, if negative feedback fails, the glucose levels in the blood may begin to rise dramatically, thus resulting in [[Diabetes mellitus|diabetes]].
== Electronic amplifiers ==
{{main|negative feedback amplifier}}
The negative feedback amplifier was invented by [[Harold Stephen Black]] at [[Bell Laboratories]] in 1927. Fundamentally, all electronic devices (e.g. [[vacuum tube]]s, [[BJT|bipolar transistors]], [[MOSFET|MOS transistors]]) are [[nonlinear]] devices. Negative feedback corrects this by trading unused gain for higher linearity (lower distortion). Though much more accurate, amplifiers with negative feedback can become unstable if not designed correctly, causing them to [[oscillate]]. [[Harry Nyquist]] of [[Bell Laboratories]] managed to work out a theory regarding how to make it stable.
Negative feedback is used in this way in many types of amplification systems to ''stabilize'' and improve their operating characteristics (see e.g., [[operational amplifiers]]).
==See also==
*[[Stability criterion]]
*[[Nyquist stability criterion]]
*[[Control theory]]
*[[Cybernetics]]
*[[Harold Stephen Black]]
*[[Asymptotic gain model]]
*[[Step response]]
== References ==
{{unreferenced|date=May 2007}}
== External links==
*http://www.biology-online.org/4/1_physiological_homeostasis.htm
[[Category:Control theory]]
[[Category:Cybernetics]]
[[Category:Electronic feedback]]
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