Positive feedback

Positive feedback is a process that occurs in a feedback system where the effects of a small disturbance on a system 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 in which the results of a change act to reduce or counteract it has negative feedback. Both concepts play an important role in science and engineering, including biology, chemistry, and cybernetics.

Mechanisms

Positive feedback enhances or amplifies an effect by it having an influence on the process which gave rise to it. For example, when part of an electronic output signal is returned to the input, and is in phase with it, the system gain is increased. The feedback from the output to the input can be either direct, or be fed back via another route.

Examples

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 type of positive feedback. The more heat there is, the more heat is generated, and the more heat there is, the more heat is generated.

In biological systems

Positive feedback is often a normal way of producing rapid change. For example, in neurons, positive feedback plays a critical role in the release of neurotransmitter vesicles, which must occur rapidly in order to propagate nerve impulses.

See also

• Negative feedback
• Feedback
• Homeostasis

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