Chapter 14: Feedback and Oscillator Circuits

Feedback has been mentioned previously, in particular, in op-amp circuits as described in Chapters 10 and 11. Depending on the relative polarity of the signal being fed back into a circuit, one may have negative or positive feedback. Negative feedback results in decreased voltage gain, for which a number of circuit features are improved, as summarized below. Positive feedback drives a circuit into oscillation as in various types of oscillator circuits.
A typical feedback connection is shown in Fig. 14.1. The input signal Vs is applied to a mixer network, where it is combined with a feedback signal Vf. The difference of these signals Vi is then the input voltage to the amplifier. A portion of the amplifier output Vo is connected to the feedback network () which provides a reduced portion of the output as feedback signal to the input mixer network.
If the feedback signal is of opposite polarity to the input signal, as shown in Fig. 14.1, negative feedback results. Although negative feedback results in reduced overall voltage gain, a number of improvements are obtained, among them being:

1. Higher input impedance.
2. Better stabilized voltage gain.
3. Improved frequency response.
4. Lower output impedance.
5. Reduced noise.
6. More linear operation.

Fig. 14.1
Simple block diagram of feedback amplifier.

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