feedback
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 {feedback
control}.
(1996-01-02)