Types of feedback

Depending on whether the feedback signal increases or decreases the net input signal, there are two types of feedback in amplifiers: (1) positive feedback and (2) negative feedback.

Positive feedback

If the feedback voltage (or current) is applied so as to increase the input, the feedback is called positive, regenerative or direct feedback. Here the feedback voltage $V_f$ is in phase with the source $V_s$, so the net input to the basic amplifier is

and the net input increases.

Advantage: high gain.

Disadvantages: (1) more distortion, (2) poor gain stability, (3) poor frequency response, (4) more drift.

Application: used in oscillator circuits.

Negative feedback

If the feedback voltage (or current) is applied so as to reduce the input, the feedback is called negative, degenerative or inverse feedback. Here $V_f$ is $180^\circ$ out of phase with $V_s$, so the net input to the basic amplifier is

and the net input decreases.

Advantages: (1) highly stabilised gain, (2) less distortion, (3) reduced noise, (4) increased bandwidth, (5) input and output impedances can be modified as desired.

Disadvantage: less gain. …