Amplification and Feedback

Most of us are already familiar with audio 'feedback' caused when some of the audio from a loudspeaker is fed back into the microphone which is picking up the original sound in the first place. In the case of audio, it is sometimes called 'howl-around' - with some justification!

A similar process occurs intentionally in the case of a radio wave generator. Some of the output from an amplifier is fed back into its own input. If the signal is fed back such that it is "in phase" - ie: the peaks and troughs of the incoming signal correspond exactly with those of the fed-back signal - oscillation will occur. This is called positive feedback.

Negative feedback (where the fed-back signal and the input are not in phase) is also used in situations where high gain (ie good amplification) is wanted but without the unwanted tendancy for the circuit to go into spontaneous oscillation.