Explanation

(b) The band width is defined as the frequency band in which the amplifier gain remains above $\frac{1}{\sqrt{2}} = 0.707$ of the mid frequency gain $(A_{\max})$. The low frequency $f_{1}$ at which the gain fall to $\frac{1}{\sqrt{2}},\ i.e.,\ 0.707$ times it’s mid frequency value is called lower cut off frequency and the high frequency $f_{4}$ at which the gain falls to $\frac{1}{\sqrt{2}},\ i.e.,\ 0.707$ times of it’s mid frequency is known as higher cut off frequency so band width $= f_{4} - f_{1}$