Chapter: Electronic Circuits : Feed Back Amplifiers
Effects of Negative Current Feedback
Effects of Negative Current Feedback
The negative current feedback has the following
effects on the performance of amplifiers :
(i)
Decreases the input impedance. The negative current feedback decreases the input impedance of most amplifiers.
Let
Zin = Input impedance of the
amplifier without feedback
Z ′in = Input impedance of the
amplifier with negative current feedback
Thus the input impedance of the amplifier is
decreased by the factor (1 + mi Ai). Note the primary
difference between negative current feedback and negative voltage feedback.
Negative current feedback decreases the input impedance of the amplifier while
negative voltage feedback increases the input impedance of the amplifier.
Increases
the output impedance. It can
be proved that with negative current feedback, the output impedance of the amplifier is increased by a factor (1 + mi
Ai).
Z ′out = Zout (1 + mi
Ai)
where
Zout = output impedance of the
amplifier without feedback
Z ′out = output impedance of the
amplifier with negative current feedback
The reader may recall that with negative
voltage feedback, the output impedance of the amplifier is decreased.
Increases
bandwidth. It can
be shown that with negative current feedback, the bandwidth of the amplifier is increased by the factor
(1 + mi Ai).
BW′ = BW (1 + mi Ai)
where
BW = Bandwidth of the amplifier without
feedback
BW′ = Bandwidth of the amplifier with negative
current feedback
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