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Chapter: Electronic Circuits - Feed Back Amplifiers

Effects of Negative Current Feedback

The negative current feedback has the following effects on the performance of amplifiers :

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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