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Effect of various capacitors on frequency response:
1. Effect of coupling capacitors:
The reactance of the capacitor is Xc = 1/2∏fc
At medium and high frequencies, the factor f makes Xc very small, so that all coupling capacitors behave as short circuits. At low frequencies, Xc increases. This increase in Xc 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 CE is not a short. So emitter is not at ac ground. Xc in parallel with RE 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, Cbe is the base emitter junction capacitance and Cbc is the base collector junction capacitance. Incase of FET, Cgs is the internal capacitance between gate and source and Cgd is the internal capacitance between gate and drain.
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