Q.What is the effect of negative feedback on stability of an amplifier?

One of the chief merits of negative feedback is that it makes the overall gain almost constant, even though an amplifier's internal gain $A$ varies with device parameters, supply voltage, temperature and ageing.

The closed-loop gain is $A_f = \dfrac{A}{1+A\beta}$. When the loop gain is large, $A\beta \gg 1$, the $1$ in the denominator is negligible, so

The gain now depends only on $\beta$, which is fixed by stable passive components in the feedback network, and is essentially independent of the amplifier's own parameters.

The improvement can be quantified. Differentiating $A_f$ with respect to $A$ gives

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