The oscillation is caused by a small part of the signal from the amplifier output being sent back to the input of the amplifier. This signal is amplified and again sent back to the input where it is amplified again. This process continues and the result is a loud noise out of the speaker. The process of sending part of the output signal of an amplifier back to the input of the amplifier is called feedback .

There are two types of feedback in amplifiers. They are positive feedback , also called regenerative feedback , and negative feedback , also called degenerative feedback . The difference between these two types is whether the feedback signal is in phase or out of phase with the input signal.