Q.Explain how stability is achieved in negative feedback amplifier.

Stability means the gain stays constant even when the amplifier's own parameters change, and negative feedback achieves this by making the gain depend on the passive feedback network instead of the transistor. …

[!TLDR]

Because $A_f \approx 1/\beta$ when $A\beta \gg 1$, the gain depends only on the passive feedback network, so it stays constant despite changes in the amplifier itself.

The closed-loop gain of a negative-feedback amplifier is

When the loop gain is large, $A\beta \gg 1$, the '1' is negligible and

The gain now depends only on $\beta$, which is fixed by stable passive resistors, so it is unaffected by changes in transistor parameters, supply voltage or ageing.

This can be shown quantitatively by differentiating $A_f$ with respect to $A$:

$\frac{dA_f}{A_f} = \frac{dA}{A} \times \frac{1}{1 + A\beta}.$ …