13 Jul 2026

Frequency Response of an RC-Coupled Transistor Amplifier

practical ug-iv electronics amplifier frequency-response

Experimental arrangement

RC-coupled amplifier test path: bias supply and input signal feed the transistor stage, while the output is measured across the device.

Aim

To determine the frequency response and bandwidth of a single-stage RC-coupled transistor amplifier.

Apparatus

RC-coupled amplifier circuit, signal generator, CRO, DC supply, and connecting leads.

Theory

The voltage gain of an amplifier depends on frequency. It is nearly constant in the mid-frequency region and decreases at low frequency because of coupling capacitors and at high frequency because of transistor and stray capacitances. The bandwidth is

\[\text{Bandwidth}=f_H-f_L,\]

where $f_L$ and $f_H$ are the lower and upper half-power frequencies.

Observations

Input amplitude: $20\,\text{mV}$.

Frequency (Hz)	Output amplitude (V)	Voltage gain
50	0.42	21
100	0.76	38
500	1.02	51
1000	1.05	52.5
5000	1.04	52
10000	0.98	49
20000	0.74	37
50000	0.43	21.5

The half-power gain is approximately $52.5/\sqrt2=37.1$.

Result

From the response curve, $f_L\approx100\,\text{Hz}$ and $f_H\approx20\,\text{kHz}$. Therefore,

\[\boxed{\text{Bandwidth}\approx19.9\,\text{kHz}}.\]

Precautions

1. Keep the input amplitude constant while changing frequency.
2. Avoid overloading the amplifier input.
3. Use a common ground for the generator and CRO.
4. Measure the output after the waveform becomes stable.

Viva Questions

1. Why is the coupling capacitor used? It passes AC while blocking DC bias.
2. Why does gain fall at low frequency? The reactance of coupling and bypass capacitors becomes large.
3. Why does gain fall at high frequency? Transistor and stray capacitances become significant.
4. What is the mid-band gain? It is the nearly constant gain in the central frequency range.