Distortion in Feedback Amplifiers

Abstract

Negative feedback applied to an amplifier linearizes the transfer characteristic of the amplifier and reduces the distortion of the input signal that is generated by the nonlinearity. The gain of the amplifier at an operating point is also reduced accordingly. These aspects are illustrated using the amplifier block diagram of Figure 5.1a. The transfer characteristic of the amplifier without feedback is taken to be that of Curve A in Figure 5.1b. Notice that the output variable of the plot is −vo. Initially, we are interested in negative feedback. Either the amplifier or the feedback must provide a net phase reversal. For plotting convenience here, −vo is used. For Curve A there is a significant nonlinearity. When feedback is applied, the slope at each point along the transfer characteristic is reduced by the corresponding amount of the ‘loop gain,’ i.e., the feedback, at that point. The slope of Curve B, at any input value in the figure, is the closed-loop gain at this point and tends to remain constant, even though the amount of feedback may be reduced at this operating point of interest when there is a falloff of gain of the original amplifier.