Positive feedback

Positive feedback or positive feedback occurs when a signal or a size reinforcing effect on themselves. The output signal is always limited in real systems, it is. Due to limited energy resources, nonlinear characteristics or the destruction of the system With negative feedback, also known as negative feedback, the output signal acts against the input signal.

General

Positive feedback can occur in all systems where output variables again have influence on the input variables, possibly in modified form. Such systems are known, among other things in technology and business. In the social sciences positive feedback is studied as a trigger for path dependence. In biology, positive feedbacks exist only briefly, usually breaks a mitgekoppeltes undamped system together quickly.

If in a loop an effect of the output signal in the same phase relationship to the input, the controller becomes unstable and may lead to self-excitation. Theoretically rise to oscillations beyond all limits, but in practice the vibrations rise only to the depletion of energy buffer, so long as the loop gain ( the total gain in the feedback loop) is greater than one. This is the case, inter alia, desirable and the oscillator and the laser. Undesirable it is for example in ( negative feedback ) amplifiers and must be avoided here by a frequency compensation.

Examples

• The recorded sound is amplified by a microphone from the speaker. Meeting the sound waves too hard on the microphone, this process is repeated and there is a self-excitation, as the increasingly loud humming or whistling (feedback) can be heard. If no signal clipping occurs, it leads to the destruction of the speakers. Even before the onset of self-excitation occur by the positive feedback on nonlinear distortion of the frequency response.
• Loose snow avalanche: the larger the rolling snowball gets, the more snow clings to him in one revolution. Also avalanches of type " snow board " increase in size, starting from a small event, rapidly.
• An oscillator a feedback amplifier with a resonant circuit. The vibrations are created by itself ( self-excitation ).
• The mechanical Froude pendulum.
• Laser start by itself to emit as a single spontaneously emitted photon avalanche photon triggers more (stimulated emission).
• Unemployment: Due to the structural crisis of an industry production capacity is closed and there is unemployment. This leads to a changed consumer behavior and demand in other sectors of the economy. The structural crisis and unemployment is spreading to other sectors of the economy. The result is even more unemployment, the crisis strengthened to mass unemployment and the collapse of the economic system.
• Interest Rates: The greater the interest-bearing assets, the faster it grows. Since the interest is compounded ( compound interest ), is an exponential growth.
• The feedback Audion the feedback is used to dedamp the resonant circuit.
• Psychology: When Phobophobie ( fear of fear ), man increases further and further into his fears.
• Benjamin Franklin Effect: When we help someone, the beneficiary will give us more sympathetic. If we do not help someone or even harm, the victim is devalued by us (cf. dehumanization ). A kind act makes our setting -friendly, which makes more friendly actions likely; for unfriendly acts, the same applies.

• A change in the average global temperature leads to a change in the average water vapor content of the atmosphere. Water vapor is the strongest greenhouse gas. An applied temperature change is amplified by the resulting change of the water vapor content on. The effect is called water vapor feedback.
• A change in snow and ice cover of soil and water surfaces where it leads to a significantly different absorption of incident solar radiation. This so-called ice albedo feedback is the strongest acting in the climate feedback.