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Phase Shift Due to Feedback

Author: N.H. Crowhurst

Thus far we have talked about feedback as being either positive or negative, as if the fed-back voltage (or current) either adds to or subtracts from the original input. This has helped us to lay down the simple rules of feedback. Practical circuits, however, do not behave quite as simply as this. At some frequencies, the fed-back audio is neither precisely in phase nor precisely out of phase with the original input, but somewhere in between. The resulting voltage fed to the amplifier input is what is called the vector difference between the input and the fed-back audio. We can see by examining the waveforms that their relationship is very much like that between voltage and current in a series circuit containing resistance and capacitive reactance.

All amplifiers possess some reactance, such as coupling capacitors, which affect the low-frequency response, and stray capacitances, which affect the high-frequency response. The output voltage gets out of phase with the ideal arrangement of negative or positive feedback, as the case may be, at these frequencies.

Feedback waveforms

Components causing phase shift

Phase response of the amplifier

Take the effect of the coupling capacitors on the low-frequency response. If each of these capacitors produces a phase difference of 60° at the same frequency, the total phase shift adds up to 180°. This amounts to phase reversal. If the feedback starts out being negative at this particular frequency, it will convert into positive feedback.

The dangerous thing now is that, with positive feedback, the amplifier will oscillate because the output voltage will equal the input voltage; consequently, no external input voltage is needed - the amplifier will continue to amplify its own output at this particular frequency.

This critical point is a loop gain A(3 = 1. In the formula for positive feedback, the feedback factor (by which the gain is divided) is (1 - Ap). As we said before, this is always a fraction, so the gain is not reduced in fact, but increased. However, when Ap becomes equal to 1, then 1 - Ap = 0. Anything divided by zero is infinity. It means that the gain of the amplifier becomes infinite at this frequency, and will go on amplifying its own output indefinitely, without the need for any input.

Three stage amplifier

Last Update: 2010-11-03