What is the Frequency Response of RC Coupled Amplifier

Frequency response of RC Coupled Amplifier

Frequency response of R.C coupled amplifier is uniform over mid frequency range of 50 Hz to 20 KHz. The voltage gain drops off at low < 50 Hz and high > 20 KHz frequencies. The frequency response curve of RC coupled amplifier is shown in the below fig. the behaviour of the amplifier is given below.

Frequency Response of RC Coupled Amplifier

1.       At low frequencies (<50Hz), the reactance of coupling capacitor C_C is quite high and hence very small part of signal will pass from one stage to the next stage. The reactance of C_E also high at low frequencies. This high reactance cannot shunt the emitter resistance R_E effectively. Due to these two factor, the voltage gain falls at low frequencies.

2.       At high frequencies (>20 KHz), the reactance CV_C is very small and it behaves as short circuit. This increases the loading effect of next stage and serves to reduce the voltage gain. Also the capacitive reactance of base emitter junction is low, at high frequencies. It increases the base current. Therefore the β value reduces. Due to these two reasons, the voltage gain falls at high frequencies.

3.       At mid frequencies (50 Hz to 20 KHz), the voltage gain of the amplifier is constant. The effect of coupling capacitor in this frequencies range is such so as to maintain a uniform voltage gain. Thus, as the frequency increases in this range, reactance of C_C decreases which tends to increases the gain. However, at the same time, lower reactance means higher loading of first stage and hence lower gain. These two factors almost cancel each other, resulting in a uniform gain at mid frequency.