Frequency Response of Multistage Amplifiers

Frequency Response of Multistage Amplifiers:

The bandwidth of multistage amplifier is always less than that of the bandwidth of single stage amplifier.

1. Overall Lower Cut-off Frequency of Multistage Amplifier:

2. Overall Higher Cut-off Frequency of Multistage Amplifier:

Let us consider the upper 3dB frequency of n identical cascaded stages as f_H(n). It is the frequency for which the overall gain falls to 1/√2 (3dB) of its midband value.

Taking n^th root on both the sides,

2^1/n = 1 + [ f_H(n)/f_H ]²

2^1/n -1 = [ f_H(n)/f_H ]²

Taking square root on both the sides,

√2^1/n -1 = f_H(n)/f_H

f_H(n) = f_H √2^1/n -1

In multistage amplifier f_L(n) is always greater than f_L and f_H(n) is always less than f_H. So the bandwidth of multistage amplifier is always less than single stage amplifier.