Environmental Engineering Reference
In-Depth Information
Chapter 3
FEEDBACK
Feedback, whether intentional or parasitic, is pervasive in all electronic
circuits and systems. Usually, a feedback network includes a subcircuit that
allows a fraction of the output signal to modify the effective input signal, in
such a way as to produce a circuit response that can differ substantially from
the response produced in the absence of such feedback. If the magnitude and
the relative phase angle of the fed back signal decreases the magnitude of the
signal applied to the input, the feedback is said to be
negative
or
degenerative.
Otherwise, the feedback is said to be
positive
or
regenerative.
Because negative feedback tends to produce stable
1
circuit responses, it is
used in most applications. Note, however, that the parasitic feedback
incurred by the energy storage elements associated with circuit layout,
circuit packaging, and second order high frequency device phenomena often
degrades an otherwise negative feedback circuit into a potentially
regenerative or severely underdumped network. In contrast, positive
feedback often enhances an inclination towards unstable behavior. This
property proves useful when designing oscillators. Moreover, small amounts
of positive feedback can be useful even in linear applications. For instance,
to reduce component spreads and/or Nevertheless,
positive feedback is always applied with some caution [WH95], [SPJT98],
[LG98], [PP994].
Feedback has been exploited in the design of amplifiers since the early
days of vacuum-tube electronics. Feedback theory was developed by
electronic engineers to satisfy the demand for amplifiers and repeaters
exhibiting stable performance for telephone applications. In particular,
Black, who was an electronic engineer at Bell Laboratories, is credited as the
revolutionary inventor of the feedback amplifier in 1927 [B34]. Since then,
1
The definition of stability and related issues are presented in the next chapter.
