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Encyclopedia > Feedback loop
For other uses, see Feedback (disambiguation).

In cybernetics and control theory, feedback is a process whereby some proportion or in general, function, of the output signal of a system is passed (fed back) to the input. Often this is done intentionally, in order to control the dynamic behaviour of the system. Feedback is observed or used in various areas dealing with complex systems, such as engineering, economics, and biology.

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In diagrams that depict information flow in a system, an arrowed lines are usually drawn, directed from input through the system and to output. The feedback is drawn by another arrowed line, directed from output outside the system to an input, resulting in a loop on the diagram, called feedback loop. This notion is important; e.g., the feedback loop is a convenient place for a control device.

## Types of feedback

Main articles: Negative feedback, Positive feedback

Feedback may be negative, which tends to reduce output, or positive, which tends to increase output. Systems which include feedback are prone to hunting, which is oscillation of output resulting from improperly tuned inputs of first positive then negative feedback. This is audio feedback.

## Feedback in electronic engineering

Feedback is designed into many electronic and other technical devices.

The most common general-purpose controller is a proportional-integral-derivative controller. Each term of the PID controller copes with time. The proportional term handles the present state of the system, the integral term handles its past, and the derivative or slope term tries to predict and handle the future.

If the signal is inverted on its way round the control loop, the system is said to have negative feedback; otherwise, the feedback is said to be positive. Negative feedback is often deliberately introduced to increase the stability and accuracy of a system, as in the feedback amplifier invented by Harold Stephen Black. This scheme can fail if the input changes faster than the system can respond to it. When this happens, the negative feedback signal begins to act as positive feedback, causing the output to oscillate or hunt. Positive feedback is usually an unwanted consequence of system behaviour.

With mechanical devices, hunting can be severe enough to destroy the device.

### Feedback in audio systems

Main article: Audio feedback.

A well-known example of runaway positive feedback in electronic systems is called "howl" or "howl-round". This occurs in public address systems when sound from the loudspeakers reaches the microphone, is amplified by the system, and is then fed back into the system at even higher volume. All electrical systems contain capacitance and inductance and so act as band-pass filters responding better to certain frequencies. In a single loop through the system, this effect is negligible, but it becomes severe when the signal passes through the system repeatedly. This effect is the basis of the simplest kinds of analogue electronic oscillator.

## Feedback in economics and finance

A system prone to hunting (oscillating) is the stock market, which has both positive and negative feedback mechanisms. This is due to cognitive and emotional factors belonging to the field of behavioral finance. For example,

• When stocks are rising (a bull market), the belief that further rises are probable gives investors an incentive to buy (positive feedback); but the increased price of the shares, and the knowledge that there must be a peak after which the market will fall, ends up deterring buyers (negative feedback).
• Once the market begins to fall regularly (a bear market), some investors may expect further losing days and refrain from buying (positive feedback), but others may buy because stocks become more and more of a bargain (negative feedback).

## Feedback in nature

In biological systems such as organisms, ecosystems, or the biosphere, most parameters must stay under control within a narrow range around a certain optimal level under certain environmental conditions. The deviation of the optimal value of the controlled parameter can result from the changes in internal and external environments. A change of some of the environmental conditions may also require change of that range to change for the system to function. The value of the parameter to maintain is recorded by a reception system and conveyed to a regulation module via an information channel.

Biological systems contain many types of regulatory circuits, among which positive and negative feedbacks. Positive and negative don't imply consequences of the feedback have positive or negative final effect. The negative feedback loop tends to slow down a process, while the positive feedback loop tends to accelerate it.

Feedback and regulation are self related. The negative feedback helps to maintain stability in a system in spite of external changes. It is related to homeostasis. Positive feedback amplifies possibilities of divergences (evolution, change of goals); it is the condition to change, evolution, growth; it gives the system the ability to access new points of equilibrium.

For example, in an organism, most positive feedbacks provide for fast autoexcitation of elements of endocrine and nervous systems (in particular, in stress responses conditions) and play a key role in regulation of morphogenesis, growth, and development of organs, all processes which are in essence a rapid escape from the initial state. Homeostasis is especially visible in the nervous and endocrine systems when considered at organism level.

Feedback is also central to the operations of genes and gene regulatory networks. repressor (see Lac repressor) and activator proteins are used to create genetic operons, which were identified by Francois Jacob and Jacques Monod in 1961 as feedback loops.

Any self-regulating natural process involves feedback and is prone to hunting. A well known example in ecology, is the oscillation of the population of snowshoe hares due to predation from lynxes.

In zymology, feedback serves as regulation of activity of an enzyme by its direct product(s) or downstream metabolite(s) in the metabolic pathway.

Compare with: feed-forward

## Feedback in organizations

As an organization seeks to improve its performance, feedback helps it to make required adjustments.

Examples of feedback in organizations:

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