Q.What is meant by feedback in amplifier? Explain the types of feedback.

Feedback is the technique of returning a part of the amplifier's output to its input, and the sign of that returned signal defines the two types.

[!ANSWER]

Feedback is the process of taking a fraction of the output signal and feeding it back to the input. The two types are positive feedback ($V_f$ in phase, $V_i = V_s + V_f$, gain increases, used in oscillators) and negative feedback ($V_f$ $180^{\circ}$ out of phase, $V_i = V_s - V_f$, gain decreases but becomes stable, used in amplifiers).

[!TLDR]

Feedback = feeding part of the output back to the input; it is positive (in phase, raises gain — for oscillators) or negative (out of phase, lowers but stabilises gain — for amplifiers.

What is feedback?

Feedback is the process of taking a part of the output signal of an amplifier and feeding it back to the input circuit. The feedback network samples the output, and the returned signal $V_f$ is combined with the source signal $V_s$ at a mixer to form the net input $V_i$ to the basic amplifier. Two useful quantities describe the loop: the feedback factor $\beta = V_f/V_o$ (the fraction of output fed back) and the loop gain $A\beta$ (the product of open-loop gain and feedback factor).

Types of feedback. Depending on whether the feedback signal increases or decreases the net input, there are two types.

1. Positive (regenerative / direct) feedback:

• $V_f$ is in phase with $V_s$, so the net input increases: $V_i = V_s + V_f$.
• Advantage: high gain.
• Disadvantages: more distortion, poor gain stability, poor frequency response, more drift.
• Application: used in oscillator circuits.

2. Negative (degenerative / inverse) feedback:

• $V_f$ is $180^{\circ}$ out of phase with $V_s$, so the net input decreases: $V_i = V_s - V_f$.
• Advantages: highly stabilised gain, less distortion, reduced noise, increased bandwidth, and input and output impedances that can be modified as desired.
• Disadvantage: reduced gain.
• Application: used in amplifier circuits.

[!ANSWER]

Feedback is feeding a fraction of the output back to the input. Positive feedback returns $V_f$ in phase ($V_i = V_s + V_f$), raising the gain but worsening stability — used in oscillators; negative feedback returns $V_f$ out of phase ($V_i = V_s - V_f$), lowering the gain but stabilising it and improving distortion, noise, bandwidth and impedances — used in amplifiers.