Positive Feedback

Author: N.H. Crowhurst

Using the original amplifier as a starting point, with a gain of 1000 (a 10-millivolt input produces 10-volt output), we could take the sample from the output the other way round, so it provided, say, 4/5 of the required input. This would mean 8 millivolts would be taken from the 10-volt output and fed back to the input, hence the actual input only needs to provide the remaining 1/5, or 2 millivolts. The addition of positive feedback thus increases the gain of the amplifier, instead of reducing it as with negative feedback. When positive feedback is used, the formula for gain with feedback is rewritten as Af = A/(l - Ap).

Formula for positive feedback

A represents the gain of the amplifier and p (a Greek letter called "beta"), represents the fraction fed back. The quantity A X p or AfJ is called the loop gain. It is the net gain of the combined arrangement, measured from the input to the amplifier, through to the output, and back through the feedback to the input again. The denominator of the fraction (1 -f A(3 or 1 - AP) is called the feedback factor, or sometimes just feedback. It represents the amount by which the gain is divided by connecting the feedback.

For positive feedback, the feedback factor is (1 - Afi). Thus, it is always a fraction. Hence, the gain will be greater than it was originally, because any number divided by a fraction becomes greater than the original number.