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The centrifugal governor as a classic example of feedback: the faster the machine turns, the further the balls are thrown outwards, which means that the throttle valve closes more with the help of the linkage, which slows down the machine: a state of equilibrium levels off.

A feedback , also feedback , feedback or Feedback [ fiːdˌbæk ] (Engl.), Is a mechanism in the signal amplifying or information processing systems, in which a part of the output is fed back directly or in modified form, to the input of the system.


Feedback occurs in many technical, biological, geological, economic and social systems . Depending on the type and direction of the returned variable, the process caused by the system either self-reinforces or weakens or self-limits. In the first case one speaks of positive feedback or positive feedback , in the latter case of negative feedback or negative feedback.

In technical systems, the aim is often not only to limit feedback processes by the capacity of the energy storage devices involved, but also to create a structure that prevents overloads through passive and active negative feedback. These are often control processes .

In nature , feedback can occur in complex structures in which elements react back on themselves via other, sometimes remote systems.

In psychologically determined behavior, the direction of the feedback is not determined in advance. For example, in the learning system at school, depending on the motivation, a bad grade rating can lead to increased diligence (negative feedback) or, in return, to resignation (positive feedback or reinforcing effect).

Positive feedback

In a loose snow avalanche , the
snow that has set in motion pulls even more snow with it in a positive feedback process, which in turn causes the phenomenon to swell further

With positive feedback, a return of the output variable with the correct sign or phase comes into play in interaction with reinforcing elements of the system. This can be useful, for example to compensate for friction losses, but it can also represent a danger, because the variables involved can increase dangerously as long as energy is provided for this purpose, and, if not additional variables that have a dampening effect on the process, can be effective, come to a destruction. Here the process is only limited by the limitation of (energy) resources .

In technical systems one speaks of an undamped periodic oscillation in resonance or an aperiodic oscillation . Depending on whether an avalanche-like swell can occur in the system or not, a distinction is made between subcritical, critical or supercritical positive feedback.

Positive feedback can often be found in growth processes . In non-technical systems one speaks of a virtuoso circle , or, if the result as such is not desired, also of a vicious circle or vicious circle .

Examples: debt trap , chain reaction , auto-catalysis , acoustic feedback , stock market crash , electronic circuits such as B. Schmitt triggers or oscillators ( Meißner circuit for realizing an undamped oscillating circuit), Benjamin Franklin effect in social psychology

Negative feedback

Example of negative feedback:
a heater and a PTC thermistor

With negative feedback, the output signal is fed back with a negative sign or polarity in antiphase. This negative feedback counteracts the external stimulation and leads to a decreasing change in state. Such systems either have a strong tendency to assume a stable position or to more or less decaying vibration behavior around a stable mean value.

The negative feedback is a fundamental principle in all of living nature and is common in technical systems. The aim of control technology in particular is to set up the negative feedback in such a way that stable system behavior is achieved. Automatic controllers are used for this in technical systems.

Undesired tendency to oscillate in a control loop can be avoided, for example, by variable dimensioning of the negative feedback by reducing the gain with increasing frequency .

The distinguishing feature in natural systems are feedback loops with self-regulating properties. In biological systems of organisms , this principle is crucial for homeostasis . Negative coupling plays a role in all ( self ) maintenance processes.

Self-excitation (i.e. the initial generation of vibrations) occurs when a positive coupling first occurs and then, after a delay, a negative coupling. The system parameters then oscillate between calm and typical maximum values. This case can occur especially in electronic control loops at high frequencies when the phase condition for negative feedback is no longer reliably given due to random phase shifts of all components.

Examples: centrifugal governor , filling a bucket with a hole in the floor, self-regulating processes in ecosystems , negative feedback in controllers and amplifiers , exposure of beneficial insects , three-phase motors , boilers with thermostats , refrigerators , body heat regulation in mammals, price formation through supply and demand

Examples with variable vibration behavior: predator-prey relationship , rhythm of cortisone secretion, circadian biological rhythms , menstrual cycle , business cycles , control oscillations (instability) of regulators , self-excitation of amplifiers (due to unsuitable negative feedback, inadequate shielding between input and output or unsuitable loads )

Stability conditions

The conditions for the stability of a technical system can be treated formally. The goal is clearly that the loop gain falls below one before a critical phase shift.

Details can be found under

Importance in different areas


Electrical circuit technology

In 1912 Siegmund Strauss (* 1875, † 1942) patented a feedback circuit with a Lieben tube . In 1913, Alexander Meißner (* 1883 in Vienna, † 1958 in Berlin) at Telefunken in Berlin and Edwin Howard Armstrong (* 1890, † 1954) in New York developed the system of a transmitter and receiver circuit using a tube amplifier with feedback . In 1914 Lee De Forest (* 1873, † 1961) patented his feedback circuit. De Forest was the last to patent, but was probably the first to discover the feedback in the lab.

In 1927, telephone engineer Harold Stephen Black discovered that the quality of a signal amplifier can be significantly improved by subtracting part of the output signal from the input signal, which is basically a negative feedback.

Further electrotechnical examples:

  • Supercritical positive feedback , including positive feedback, from electrical systems for generating high or low frequency vibrations. It is used in oscillator circuits such as astable trigger circuits and function generators .
  • negative feedback , including negative feedback, from electrical systems to linearize the frequency response as well as non-linear distortions such as reducing the distortion factor . If the phase delay in the amplifier is too great, the negative feedback can be converted into positive feedback and undesired oscillations arise.
  • Each voltage regulator compares the output voltage with a reference voltage and regulates the current permeability of a power transistor with the amplified differential voltage. This means that the output voltage is largely independent of fluctuations in load.
  • Subcritical positive feedback from resonant circuits for undamping and thus increasing the resonance resistance . This feedback was mainly used in audio switching . In this case, the use of feedback has increased the selectivity and sensitivity of the receiver without having to increase the number of the usually expensive electron tubes in the receiver.

Sound engineering

In sound engineering or electroacoustics, feedback (coupling or “coupling” or “feedback” for short) is a generally undesirable self-excitation of the sound sensor - amplifier - loudspeaker system that is perceived as an unpleasant whistling . It mainly arises between the loudspeakers fed by the amplifier and the microphones . Any other electroacoustic sound pick-up, such as electric guitars or record players, can also be affected. Feedback can develop when a loudspeaker reproduces the signal from a microphone and at the same time the microphone picks up this signal again if it is too close to the loudspeaker, as is typically the case with hearing aids . The signal is amplified again, played back via the loudspeaker and an electroacoustic loop is created, which rocks until it self-excites. The frequency of self-excitation depends on the frequency-selective properties and the phase shift of the transmission path (air path, equalizer, loudspeaker, microphone properties, reflective room walls).

In the extreme case of feedback, in addition to annoying the listener, the loudspeakers can also be destroyed, in particular the tweeters can be damaged as a result. Woofer and subwoofer are less sensitive to feedback and can cope with stronger feedback than tweeters. Crossovers located in loudspeakers can be damaged.

The following measures can help to avoid coupling during stage operation:

  • Equalizer with which the PA system is "whistled" during the sound check. This intentionally generates feedback that a trained technician can by ear using the equalizer to minimize or eliminate by lowering those frequencies that tend to couple.
  • So-called feedback destroyers (feedback suppressors) recognize and automatically suppress building up feedback. The problem with this: You cannot distinguish between unwanted and wanted feedback and suppress both equally.
  • Microphones and loudspeakers can be set up so that little direct sound is possible between them;
  • The use of microphones with a suitable directional characteristic ("cardioid")
  • A special arrangement and interconnection of several microphones can cancel out ambient noise and thus reduce the risk of feedback (see for example Grateful Dead );
  • The elimination of microphones through the use of vibration sensors or electrical instruments.
  • If the microphone is close to the sound source, the level of the useful signal increases, but not that of the sound from the loudspeaker.
  • With Optogates , unused microphones can be switched off automatically.

Coupling often occurs sooner when the auditorium is empty than when it is full, as the audience dampens the sound and its reflection in the room.

The feedback is deliberately used in various modern music styles, but above all in rock music , especially in heavy metal , as sound design for sound processing.

There are also a few DJs who incorporate feedback into their performance. For this purpose, the signal at the headphone output of the mixer is applied to an input. With extremely careful use of the various controls (highs, mids, lows, etc.), sounds ranging from distorted to whistling to rhythmic beating and other variations can be generated. Every mixer reacts differently to this manipulation. Even with minor changes to the controls, suddenly changing noises can arise which, if handled carelessly, as mentioned above, can destroy the speakers.

In computer technology

In computer technology, feedback can be used to give the user information about the current status of an application. This can be done with optical means, for example by highlighting details in a graphical user interface . Or acoustic signals are used by playing a sound through a loudspeaker. Some input devices have the ability to give the user feedback by mechanical means. This force feedback is used, among other things, to make the impression of a simulation more realistic.

The hardware of some computer games is able to give the player feedback about events in the game by mechanical means. This is often used for a direct reaction to the player's actions. An example of this is vibrations from a steering wheel.

A closed control loop with the user as a system block is used in the User in the loop process to e.g. B. to achieve a demand regulation in mobile communications or smart grids .

System earth and climate research

A lot of feedback can be observed in climatology . With ice albedo feedback (positive feedback), for example, more sunlight is reflected by icing, making it colder. This allows larger areas to ice up and it gets even colder. The process works the other way round as well.

Further feedbacks include cloud-water vapor couplings (“The water vapor effect in the upper troposphere is the strongest known feedback process.”) Or the carbon dioxide absorption capacity of seas. Many of these processes have not yet been researched precisely enough and make accurate climate prediction with climate models difficult . The humidity at the top of the weather layer has increased by an average of around ten percent over the past 35 years .

Medicine and biology

In biology and medicine, negative feedback maintains the homeostasis ( state of equilibrium within permissible limits) of a system . Such regulatory processes run z. B. in maintaining the body temperature of warm-blooded animals or in the regulation of gene activity.

In contrast, biofeedback means that z. B. a signal tone or a lamp that a value exceeding a measured value (e.g. skin temperature, muscle tension / tone or EEG wave amplitude of a preselected frequency) is reported back to the examined person, which otherwise cannot be perceived or not sufficiently. It is made tangible in this way and can e.g. B. serve to learn a self-control.

The human body is a complex system that is steered and controlled by a very effective feedback control system - the central nervous system . The current sensory impressions are always combined with the appropriate information (experiences) from memory in order to provide the individual with a suitable prediction of the future as a suggestion for the current situation. This allows you to react appropriately to every situation immediately. The brain's suggestion is always adapted to the current situation. For example, when walking / running, the brain controls how one has to put the foot down on the basis of the visual information seen about the nature of the path. The feedback system can e.g. B. be disturbed by drugs (alcohol consumption); then you stagger or fall.


  • In general psychology , the unconscious perception of one's own expressive behavior (posture, gestures, facial expressions) and their effect on one's own well-being is referred to as feedback. In an experiment by Strack et al. a. (1988) describe that the evaluation of the subjectively perceived joke of cartoons is better if the test persons activate the facial muscles that are typical for joy.
  • In psychology / communication theory , Paul Watzlawick has addressed feedback processes by describing communication cycles that have no beginning and no end, that is , can be arbitrarily " punctuated ". He speaks of “symmetrical escalation ” or “ complementarity ”. What is meant is the mutual interaction of the behavioral expressions of communication partners, who in one case compete for the same position, in the other case reinforce or hold on to each other in complementary roles.
  • Walter Milowiz represents a similar concept in systems theory , which he primarily applies to systemic social work . Here feedback loops and escalating developments in the interaction between people and their environment are perceived as a description of social problem situations.

See also


  • Siegfried Wirsum: Practical sound technology, device concepts, installation, optimization. 1st edition. Franzis-Verlag, Munich 1991, ISBN 3-7723-5862-4 .
  • Helmut Röder, Heinz Ruckriegel, Heinz Häberle: Electronics 3rd part, communications electronics. 5th edition. Verlag Europa-Lehrmittel, Wuppertal 1980, ISBN 3-8085-3225-4 .
  • Gustav Büscher, A. Wiegemann: Little ABC of electroacoustics. 6th edition. Franzis Verlag, Munich 1972, ISBN 3-7723-0296-3 .
  • R. Beckmann: Manual of PA technology, basic component practice. 2nd Edition. Elektor-Verlag, Aachen 1990, ISBN 3-921608-66-X .
  • I. Jöns, W. Bungard (Ed.): Feedback instruments in companies - basics, design tips, experience reports . Gabler, Wiesbaden 2005, ISBN 3-409-12738-0 .
  • Frank Pieper: The PA manual: Practical introduction to professional sound reinforcement technology. 4th edition. GC Carstensen Verlag, Munich 2011, ISBN 978-3-910098-42-8 .
  • Frederic Vester : The art of networked thinking. 8th edition. dtv, Munich 2011, ISBN 978-3-423-33077-0 .
  • Jürgen Beetz : Feedback: How feedback determines our lives and controls nature, technology, society and the economy . Springer Spectrum, Heidelberg 2015, ISBN 978-3-662-47089-3 .

Individual evidence

  1. RADIO CORPORATION OF AMERICA et al. v. RADIO ENGINEERING LABORATORIES, Inc. ( English ) May 21, 1934. Retrieved October 14, 2019.
  2. Brian Soden. In: Volker Mrasek : Increase in water vapor in the atmosphere . Deutschlandfunk , Research News, July 29, 2014
  3. Volker Mrasek: Increase in water vapor in the atmosphere . Deutschlandfunk , Research News, July 29, 2014