Feedback and Distortion

Author: N.H. Crowhurst

Feedback - a means of reducing distortion. Distortion in the fed-back signal acts to reduce distortion in the output

As the development of audio amplifiers produced better and better performance, the struggle to reduce distortion became steadily greater. A point of diminishing returns was reached, beyond which it did not seem possible to go. Then came the idea of feedback.

Tube development has made it easily possible to get more gain from an amplifier. What proved difficult was getting the output to be a more exact replica of the input. Feedback uses some of the additional gain, which can easily be obtained, to achieve this objective, A portion of the output is fed back or returned to the input. The difference between the input and output waveforms, which represents the distortion component, acts to reduce the amount of the distortion.

Suppose that an amplifier originally gives an output of 10 volts for an input of 10 millivolts and that the 10 volts contain 5% distortion. This will be 0.5 volt of some frequency (the distortion) that was not included in the 10-millivolt input.

It is comparatively easy, by using an extra tube, to increase the gain of an amplifier so that an input of only 1 millivolt will produce the 10-volt output. Bearing this in mind, if we take a 9-millivolt sample of the output and subtract this from the original input of 10 millivolts, we shall have the required 1-millivolt input. This input consists of 10 millivolts original audio minus 9 millivolts fed back from the output, which is introduced into the signal by the amplifier. If there is 5%, or 0.5-volt distortion in the 10-volt output, the 9 millivolts fed back to the input will include 0.45-millivolt distortion.

The original audio almost cancels itself by the feedback - for the 1 millivolt actually fed into the amplifier, 9 millivolts are fed back to offset the 10 millivolts of audio fed into the complete arrangement. The distortion component, however, which originally was 0.5 volt in the output, has no original input to "offset/* hence the whole distortion component fed back from the output will get amplified again and thus come out as 9/10 of its original size in the opposite direction. Thus, the ultimate amount of distortion left will be 1/10 of the original 5%, 0.5%, or .05 volt.

Feedback reduces distortion from 0.5 volts to 0.05 volts