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

The bandwidth of an amplifier is the span between its two $3\text{ dB}$ (0.707 of maximum) cut-off frequencies — the lower cut-off $f_1$ and the upper cut-off $f_2$:

When negative feedback is applied, the response flattens and widens: the lower cut-off decreases and the upper cut-off increases, each by the factor $(1+A\beta)$,

so the new bandwidth is

The passband therefore grows by the same factor $(1+A\beta)$ by which the gain shrank. On a gain-versus-frequency graph the "after feedback" curve sits lower (peak $A_{f,max} < A_{max}$) but stretches wider on both sides. …