feedback

electronics Part of a system output presented at its input.

Feedback may be unintended. When used as a design feature,

the output is usually transformed by passive components which

attenuate it in some manner; the result is then presented at

the system input.

Feedback is positive or negative, depending on the sign with

which a positive change in the original input reappears after

transformation. Negative feedback was invented by Black to

stabilise vacuum tube amplifiers. The behaviour becomes

largely a function of the feedback transformation and only

minimally a function of factors such as transistor gain which

are imperfectly known.

Positive feedback can lead to instability; it finds wide

application in the construction of oscillators.

Feedback can be used to control a system, as in feedbackcontrol.

(1996-01-02)