Effect of various capacitors on frequency response

Effect of various capacitors on frequency response:

1. Effect of coupling capacitors:

The reactance of the capacitor is X_c = 1/2∏f_c

At medium and high frequencies, the factor f makes X_c very small, so that all coupling capacitors behave as short circuits. At low frequencies, X_c increases. This increase in X_c drops the signal voltage across the capacitor and reduces the circuit gain. As signal frequencies decrease, capacitor reactance increase and gain continues to fall, reducing the output voltage.

2. Effect of Bypass capacitors:

At lower frequencies, bypass capacitor C_E is not a short. So emitter is not at ac ground. X_c in parallel with R_E creates an impedance. The signal voltage drops across this impedance reducing the circuit gain.

3. Effect of internal transistor capacitances:

At high frequencies, coupling and bypass capacitors act as short circuit and do not affect the amplifier frequency response. At high frequencies, internal capacitances, commonly known as junction capacitances. The following figure shows the junction capacitances for both BJT and FET. Incase of BJT, C_be is the base emitter junction capacitance and C_bc is the base collector junction capacitance. Incase of FET, C_gs is the internal capacitance between gate and source and C_gd is the internal capacitance between gate and drain.