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 fH(n). It is the frequency for which the overall gain falls to 1/√2 (3dB) of its midband value.
Taking nth root on both the sides,
21/n = 1 + [ fH(n)/fH ]2
21/n -1 = [ fH(n)/fH ]2
Taking square root on both the sides,
√21/n -1 = fH(n)/fH
fH(n) = fH √21/n -1
In multistage amplifier fL(n) is always greater than fL and fH(n) is always less than fH. So the bandwidth of multistage amplifier is always less than single stage amplifier.