Engineering Principles V11

Clive W. Humphris

OSCILLATORS: Positive Feedback.  

Positive feedback occurs when a portion of an output signal is fed back to the input in the same phase. (Feedback is normally expressed as a ratio, however using the term Feedback % makes the calculations easier to follow).

The input signal is reinforced by the positive feedback and results in the output increasing still further. Under most circumstances, i.e. audio amplifiers this is undesirable. An example is the familiar 'howl around' which occurs when a microphone is placed too close to the output speaker in a P.A. system. However, the consequence is exploited to achieve oscillation.

To examine the effect modify the default values slightly and monitor the input and output amplitudes. Feedback amounts are normally only a few per-cent, just sufficient to maintain oscillation. The overall stage gain is then set to just compensate for circuit losses.

Note that this is exactly the opposite to the calculation for negative feedback, with only a minor modification required to the formula to achieve the opposite effect. The feedback signal must be exactly in phase with the input or some cancellation will occur.

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