Q.What is feedback?

Feedback in Amplifiers

Feedback means sending a fraction of an amplifier's output signal back to its input. The fraction returned is set by the feedback factor $\beta$. Whether this helps or hurts depends on the phase of what is fed back.

Two kinds

• Positive (regenerative) feedback: the returned signal is in phase with the input, so it reinforces it. This raises gain but drives the system toward instability — the basis of oscillators, not amplifiers.
• Negative (degenerative) feedback: the returned signal opposes the input. Gain drops, but almost everything else improves.

The closed-loop gain

If the amplifier's own gain (open-loop) is $A$ and a fraction $\beta$ of the output is fed back negatively, the overall closed-loop gain becomes

• $A$ = open-loop (intrinsic) gain of the amplifier
• $\beta$ = fraction of output fed back (feedback factor)
• $A\beta$ = loop gain

Why give up gain

The denominator $(1 + A\beta)$ that shrinks the gain also stabilises it. If $A\beta \gg 1$, then $A_f \approx 1/\beta$ — the gain depends almost entirely on $\beta$ (usually fixed by precise passive components) and hardly at all on the transistor's temperature-sensitive, sample-to-sample $A$. Negative feedback therefore also widens the bandwidth, lowers distortion and noise, and lets us tailor input and output impedance. It is the reason well-designed amplifiers behave predictably.

Exam note: In the NCERT-aligned treatment, remember the trade-off in one line — negative feedback reduces gain but improves stability, bandwidth and fidelity. A common conceptual question asks why oscillators use positive feedback while amplifiers use negative feedback; the answer is the phase (in-phase reinforces and sustains oscillation, out-of-phase stabilises).