Q. Explain low-frequency response of RC coupled amplifier. What are the reasons for reduction in gain at low frequencies?

The RC coupled amplifier circuit contains capacitors. Capacitors (and inductors) are frequency-dependent elements. Since they are present in the circuit, the output immediately becomes a function of frequency. Shown above is the frequency response curve of an RC coupled amplifier. The curve illustrates how the magnitude of voltage gain of an amplifier varies with input signal.

The gain is constant only for a limited set of frequencies known as the mid-frequency range. In mid-frequency range, the coupling capacitor and the bypass capacitor behaves as though short circuited.

When frequency is low, the capacitors have high reactance (Xc=1/2?fc) which results in a less gain as compared to the mid-frequency region. At zero frequency, the reactance of the capacitor will be infinitely large and there will be no output.