Q.Explain how bandwidth of an amplifier is affected by negative feedback.

Negative feedback widens the amplifier's bandwidth by pushing the two cut-off frequencies apart. …

[!TLDR]

Negative feedback widens the bandwidth by $(1+A\beta)$: $f_1' = f_1/(1+A\beta)$, $f_2' = f_2(1+A\beta)$, $BW_f = BW(1+A\beta)$, with the gain-bandwidth product unchanged.

The frequency response of an amplifier (Fig. 4.4.4) is flat over a mid-band and falls off at low and high frequencies. The band is bounded by the two 3 dB frequencies — the lower cut-off $f_1$ and the upper cut-off $f_2$ — and the bandwidth without feedback is

Effect of negative feedback on the cut-off frequencies. Negative feedback reduces the gain most where it is largest (mid-band) and least where it is already small (near the cut-offs). This flattens and widens the response:

• the lower cut-off decreases: $f_1' = \dfrac{f_1}{1 + A\beta}$;
• the upper cut-off increases: $f_2' = f_2(1 + A\beta)$.

New bandwidth. The bandwidth with feedback is therefore

which is larger than $BW$ by the factor $(1 + A\beta)$. On the response graph the 'after feedback' curve is lower but stretches wider on both sides, with the axis marked $f_1', f_1, f_2, f_2'$. …