Feedback Amplifiers part2

SOME PROPERTIES OF NEGATIVE FEEDBACK

1. Gain Desensitivity

The effect of negative feedback on desensitizing the closed-loop gain is such that, a higher reduction in the gain of the basic amplifier will cause a lesser reduction in the gain of the closed-loop amplifier. This sensitivity-reduction property can be analytically established as follows:

Assume that β is constant. Ta er both sides of Eq. (5.4) results in

which says that the percentage change in variations in some circuit parameter) is smaller than the percentage ch by the amount of feedback.

For this reason the amount of feedback, (1 + so known as the desensitivity factor.

2. Bandwidth Extension

Consider an amplifier whose uency response is characterized by a single pole. Its gain at mid and high freq ressed as

where denotes the midband gain and is the upper 3-dB frequency. Application of negative feedback, with a frequency-independent factor β, around this amplifier results in a closed-loop gain ( ) given by

Thus the feedback amplifier will have a midband gain of ⁄(1 + ) and an upper 3-dB frequency given by

It follows that the upper 3-dB frequency is increased by a factor equal to the amount of feedback. Similarly, it can be shown that if the open loop gain is characterized by a dominant low frequency pole giving rise to a lower 3-dB frequency , then the feedback amplifier will have a lower 3-dB frequency ,

Note that the amplifier bandwidth is increased by the same factor by which its midband gain is decreased, maintaining tlie gain—bandwidth product at a constant value.

Noise Reduction

Negative feedback can be employed to reduce the noise or interference in an amplifier or, more precisely, to increase the ratio of signal to noise. However, this noisereduction process is possible only under certain conditions. Consider the situation illustrated in Fig. 5.2. Figure 5.2(a) shows an amplifier with gain , an input signal , and noise, or interference, . It is assumed that for some reason this amplifier suffers from noise and that the noise can be assumed to be introduced at the input of the amplifier. The signal-to-noise ratio for this amplifier is

Consider next the circuit in Fig. ). Here we assume that it is possible t d another amplifier stage with gain at does not suffer from the noise problem. s is the case, then we may precede ginal amplifier A; by the clean amplifier d apply negative feedback around the overall cascade of such an amount as to k e overall gain constant.

which is mes higher than in the original case.

The im ent in signal-to-noise ratio by the application of feedback is possible only if one can precede the noisy stage by a (relatively) noise-free stage. The best example is found in the output power-amplifier stage of an audio amplifier. Such a stage usually suffers from a problem known as power-supply hum. We may therefore precede the power-output stage by a small-signal amplifier that provides large voltage gain, and apply a large amount of negative feedback, thus restoring the voltage gain to its original value. Since the small-signal amplifier can be fed from another, less hefty (and hence better regulated) power supply, it will not suffer from the hum problem. The hum at the output will then be reduced by the amount of the voltage gain of this added preamplifier.