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

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 .
Top
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

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 .
Top
Clicky Web Analytics