Positive and negative feedback amplifier

When a part of output signal is fed back to the input of circuit through a suitable network, the process is called feedback. Feedback is of two types

Positive and Negative feedback in amplifier

Negative feedback in amplifier

In negative feedback, the feedback signal is opposite in phase to the input signal and this lead to a net decrease in the input signal resulting in a reduced gain. Negative feedback is also called degenerative or inverse feedback. It is used in amplifiers to improve the stability of gain, reduce distortion, transform input and output impedances and improve frequency response.

Positive feedback in amplifier

If the feedback signal is in the phase with the input signal, the effective input to the circuit is increased and this type of feedback is called positive, regenerative or direct feedback. It provides increased gain but it also increases distortion and leads to poor stability of gain. Positive feedback is used in oscillators where it is needed to sustain the oscillations in the output.

Types of feedback connections in an amplifier

Since both voltage and current can be fed back to the input using a series or parallel connection, there are four different types of feedback connections:

A. Voltage series feedback.

B. Current series feedback.

C. Voltage shunt feedback.

D. Current shunt feedback.

Series feedback generally increases the input impedance whereas shunt feedback decreases the input impedance. Further, current feedback generally increases the output impedance whereas voltage feedback decreases the output impedance. Most multistage amplifiers require high input and low output impedances and voltage series feedback connection fulfils this requirement.

Voltage gain in amplifier

Consider a voltage series negative feedback amplifier as shown in fig. If there is no feedback then the voltage gain is called internal gain and is given by

Positive and negative feedback amplifier

If the feedback signal Vf is applied to the input , then

Positive and negative feedback amplifier

Hence eqn, (1) can be written as

The overall voltage gain with feedback which is also called closed loop gain is given by

Since (1+Aβ)>A, therefore Af <1. Thus, the overall gain in case of negative feedback decreases. Similarly, it can be shown that in case of positive feedback, the input signal Vi is given by

The output voltage Vo is given by

The closed loop gain for a positive feedback circuit is given by

Positive and negative feedback amplifier

This shows that positive feedback increases the voltage gain.

The factor (-Aβ) is called the loop gain or loop transmission or return ratio or feedback factor.

The difference between unity and feedback factor (1+Aβ) is called return difference.

Advantages of negative feedback

Inspite of reduction of gain, negative feedback is preferred in amplifiers because it improves the functioning of the amplifier in many ways. Its advantages are:

  • It improves the stability of gain.
  • It increases the input impedance and reduces the output impedance.
  • It increases the bandwidth of the amplifier.
  • It reduces distortion and noise.

difference between positive and negative feedback amplifier