Radio Principles

No7. Feedback

Feedback is a difficult concept to juggle with, and so let us start with the assumption that the more power you put into an amplifier the stronger the signal you will get at the output. This can only be considered true up to a point. An amplifier can only go as high as the parameters allow. If you attempt to overload it with too strong an input, you will find that it tries to exceed its supply voltage causing distortion of the signal, and worse still, can start to oscillate. Armstrong used De Forest’s Audion Amplifier in this way to make a transmitter. Obviously we have to have some way of attenuating it.
Feedback can be either negative or positive, and in positive feedback we increase the gain, in negative we decrease it. In our model there may be a shortcoming in the modulator or the linear amplifier, and so from the linear amplifier we take a sample of the output, rectify it to recover the audio modulation and feed it back to the audio stages to oppose the signal there. Such a device as an operational amplifier may be used
for the purpose.
Let us say that a harmonic of an audio signal is accentuated, then we can feed the recovered audio signal back to the audio stages and it would contain the accentuated harmonic. Negative feedback will modify the harmonic and it will be closer to normal levels. If we fed back positively, it would increase the gain, and thus the problem.

In general we use negative feedback in both transmitters and receivers. Positive is used only when trying to pick up a weak signal, and then sparingly.