Positive feedback

This Article This article covers the importance of positive feedback in control engineering. For the meaning in psychology, see feedback (group dynamics) .

Positive feedback or positive feedback occurs when a signal or a variable has a reinforcing effect on itself. In real systems, the output signal is always limited, be it due to limited energy resources, non-linear properties or the destruction of the system. With negative feedback , also called negative feedback , the output signal counteracts the input signal.

Principle of positive feedback with amplifier A and positive feedback B.

General

Circuit diagram of a Schmitt trigger with U _e as input voltage, U _r as reference value, feedback to the input that supports positive feedback

Positive feedback can occur in all systems in which output variables again influence the input variables, possibly in a modified form. Such systems are known in technology and business, among others. In the social sciences, positive feedback is studied as a trigger for path dependence . In biology, positive feedback only exists for a short time, as a rule, a coupled, undamped system breaks down quickly.

If the output signal has the same phase effect on the input in a control loop , the controller becomes unstable and self-excitation can occur. Theoretically, the amplitude of the oscillations increases beyond all limits, but in practice the amplitude only increases until the energy buffer is exhausted, so as long as the loop gain (the total gain in the feedback loop) is greater than one. This is the case and desirable for the oscillator and the laser , among others . It is undesirable, for example, with (negative feedback) amplifiers and must be avoided here by means of frequency compensation .

(Limited) positive feedback formed the basis for self-organization processes or emergence in many systems .

Examples

• The sound picked up by a microphone is amplified through loudspeakers. If the sound waves hit the microphone too strongly, this process is repeated and self-excitation occurs, which can be heard as an ever louder hum or whistle ( feedback ). If there is no signal limitation, the loudspeakers will be destroyed. Even before self-excitation begins, the positive feedback causes linear distortions in the frequency response.
• Loose snow avalanche : The larger the rolling snowball becomes, the more snow will stick to it during one turn. Also, avalanches of the "slab" type grow rapidly, starting from a small event.
• An oscillator is a resonant circuit that is fed back to an amplifier . The vibrations arise by themselves (self-excitation).
• The mechanical Froude pendulum .
• Lasers begin to emit by themselves, as a single, spontaneously emitted photon releases further photons like an avalanche ( stimulated emission ).
• Unemployment : As a result of the structural crisis in an economic sector, production capacities are shut down and unemployment occurs. This leads to changed consumer behavior and a decline in demand in other sectors of the economy. The structural crisis and unemployment spread to other branches of the economy. There is even more unemployment, which further exacerbates the crisis, leading to mass unemployment and the collapse of the economic system.
• Overturning of an object: if the center of gravity of an unsupported object is perpendicular to its base, the object will tip over. The further out the center of gravity is, the faster the object tilts and shifts its center of gravity further outwards.
• Interest: The larger the interest-bearing assets, the faster it grows. Since interest is also paid (compound interest), growth is exponential .
• With the feedback audion , the feedback is used to de-dampen the resonant circuit.
• Psychology:
  • With phobophobia (fear of fear), the person increases more and more into his fears.
  • Benjamin Franklin Effect : When we help someone, the recipient becomes more sympathetic to us. If we do not help or even harm someone, the sacrifice is devalued by us (cf. dehumanization ). Kind action makes our attitude friendlier, which makes further kind action more likely; the same applies to unfriendly actions.
• Climatology:
  • Globally, the water vapor content of the atmosphere rises with the temperature and thus the greenhouse effect. The effect called water vapor feedback also has a reinforcing effect in phases of global cooling.
  • A change in the snow and ice cover of the ground and water surfaces leads to a greatly changed absorption of incident solar radiation. This so-called ice-albedo feedback is the strongest feedback that affects the climate.
  • In the Pacific, trade winds drive warm surface water to the west and bare cold deep water in the east, which causes the wind-intensifying air pressure differences, see El Niño-Southern Oscillation .

See also

• Matthew Effect
• resonance

literature

• Wolfgang Bauer, Hans Herbert Wagener: Components and basic circuits of electronics. Volume 2: Basic Circuits. Carl Hanser, Munich 1981, ISBN 3-446-12319-9 .
• Gregor Häberle, Heinz Häberle, Thomas Kleiber: Expertise in radio, television and radio electronics. 3rd, revised and expanded edition. Verlag Europa-Lehrmittel, Haan-Gruiten 1996, ISBN 3-8085-3263-7 .
• 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. ^ Elliot Aronson , Robin M. Akert, Timothy D. Wilson: Social Psychology. 6th, updated edition. Pearson studies, Munich a. a. 2008, ISBN 978-3-8273-7359-5 , p. 181 ff.