Positive Feedback

Positive feedback (or regenerative feedback) occurs in a feedback loop when the mathematical sign of the net gain around the feedback loop (sometimes called the loop gain) is positive. That is, positive feedback is in phase with the input, in the sense that it adds to make the input larger. Positive feedback is a process in which the effects of a small disturbance on a system can include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that has negative gain around the loop has negative feedback.

Positive feedback tends to cause system instability. When the loop gain is positive and above 1, there will typically be exponential growth of any oscillations or divergences from equilibrium. System parameters will typically accelerate towards extreme values, which may damage or destroy the system, or may end with the system 'latched' into a new stable state. Positive feedback may be controlled by signals in the system being filtered, damped, or limited, or it can be cancelled or reduced by adding negative feedback.

Positive feedback is used in digital electronics to force voltages away from intermediate voltages into '0' and '1' states. On the other hand, thermal runaway is a positive feedback that can destroy semiconductor junctions. Positive feedback in chemical reactions can increase the rate of reactions, and in some cases can lead to explosions. Positive feedback in mechanical design causes tipping-point, or 'over-centre', mechanisms to snap into position, for example in switches and locking pliers. Out of control, it can cause bridges to collapse. Positive feedback in economic systems can cause boom-then-bust cycles. If a PA system's microphone picks up sounds from its own loudspeakers, and these sounds are re-amplified enough, the effect of this feedback can be loud squealing or howling noises from the loudspeakers.