Feedback Amplifier
Positive Feedback
Negative Feedback
Introduction
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.
Basic Feedback Amplifier
Positive feedback occurs when the feedback signal is
in phase with the input signal.
A block diagram of an amplifier with positive feedback is showm in figure .
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.
Positive feedback in a transistor amplifier
In a
common-base transistor amplifier , it is simple to provide positive feedback.
Since the input and output signals are in phase, you need only couple part of the output
signal back to the input. This is shown in figure below .
The feedback network in this amplifier is made up of
R2 and
C2. The value of
C2
should be large so that the capacitive reactance (XC) will be low and the capacitor
will couple the signal easily. The resistive value of
R2 should be
large to
limit the amount of feedback signal and to ensure that the majority of the
output signal goes on to the next stage through
C3 .
A more common configuration for transistor amplifiers is the
common-emitter configuration .
Positive feedback is a little more difficult with this configuration because the
input and output signals are
180º out of phase. Positive feedback can be accomplished
by feeding a portion of the output signal of the second stage back to the input of the first stage.
Positive feedback in two stages of transistor amplification
The figure shows that each stage of amplification has a
180º phase shift .
This means that the output signal of
Q2 will be in phase with the input signal to
Q1.
A portion of the output signal of
Q2 is coupled back to the input of
Q1 through
the feedback network of
C3 and
R3.
R3 should have a large resistance to limit the
amount of signal through the feedback network.
C3 should have a large capacitance
so the capacitive reactance is low and the capacitor will couple the signal easily .
Negative feedback is accomplished by adding part of the output signal
out of phase with
the input signal. The methods of providing negative
feedback are similar to those methods used to provide positive feedback. The phase relationship
of the feedback signal and the input signal is the only difference .
Negative feedback in a
common-emitter transistor amplifier is shown in the figure below .
The feedback network of
C2 and
R2 couples part of the output signal of
Q1 back to the input.
Since the output signal is
180º out of phase with the input signal, this causes negative feedback .
Negative feedback in a transistor amplifier
Negative feedback is used to improve fidelity of an amplifier by limiting the input signal.
Negative feedback can also be used to increase the frequency response of an amplifier.
The gain of an amplifier decreases when the limit of its frequency response is reached.
When negative feedback is used, the feedback signal decreases as the output signal
decreases . At the limits of frequency response of the amplifier the smaller feedback
signal means that the effective gain (gain with feedback) is increased . This will
improve the frequency response of the amplifier .