1-18
AMPLIFIER FEEDBACK
Perhaps you have been around a public address system when a squeal or high-pitched noise has
come from the speaker. Someone will turn down the volume and the noise will stop. That noise is an
indication that the amplifier (at least one stage of amplification) has begun oscillating. Oscillation is
covered in detail in NEETS Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits. For
now, you need only realize that 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.
Positive feedback occurs when the feedback signal is in phase with the input signal. Figure 1-15
shows a block diagram of an amplifier with positive feedback. Notice that the feedback signal is in phase
with the input signal. This means that the feedback signal will add to or "regenerate" the input signal. The
result is a larger amplitude output signal than would occur without the feedback. This type of feedback is
what causes the public address system to squeal as described above.
Figure 1-15.—Positive feedback in an amplifier.
Figure 1-16 is a block diagram of an amplifier with negative feedback. In this case, the feedback
signal is out of phase with the input signal. This means that the feedback signal will subtract from or
"degenerate" the input signal. This results in a lower amplitude output signal than would occur without
the feedback.
