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Negative feedback
is used in amplifiers for a variety of reasons. The term 'feedback' means using a fraction of the output voltage of the
amplifier as input or as part of input. When the signals at input and output
are of opposite phase (i.e., they are mirror-imaged), then the feedback signal is said
to be
negative.
Negative feedback signals are subtracted from the amplifier's input
signal(s). In effect, they reduce the overall gain of
the amplifier. If G is the gain of the amplifier with no feedback
(also known as the 'open-loop gain'), and n is the feedback fraction
(or loop gain) such that Vout/n is fed back to the input of
the amplifier, then the gain of the amplifier when negative feedback
is applied (closed-loop gain) is as follows:
Closed-Loop Gain = G / (1 + G/n).
For
example, if the open-loop gain G = 100 and n = 10 (so that 1/10 of
the output voltage is fed back), then the closed-loop gain is
100/(1+100/10) = 100/11 = 9.09.
Note that
if the open-loop gain G is very much larger than the loop gain n,
then the closed loop gain becomes approximately G/(G/n), or simply
equal to n.
Negative feedback, aside from reducing gain, also reduces noise signals
generated by the components of the amplifier. Distortion that
does not result in loss of open-loop gain will also be reduced by
negative feedback.
The input
resistance of an amplifier may also be affected by feedback.
If the feedback signal is in shunt with the input signal (i.e., they
are applied to the same terminal, as shown in Figure 1a), then the input resistance of the
amplifier decreases. Using a feedback signal that is in series
with the input signal (as shown in Figure 1b), on the other hand, will increase the
amplifier's input resistance.
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Figure 1a. Feedback in Shunt with the Input Signal |
Figure 1b. Feedback in Series with Input (through the
emitter of first transistor) |
The
output resistance may also be affected by a feedback network,
although to a lesser extent than the input resistance.
Connecting the feedback circuit in series with the output load
increases the amplifier's output resistance while connecting it in
parallel with the output load will decrease the amplifier's output
resistance.
See Also:
What is an Amplifier?; More
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