13 Jul 2026

Single-Stage RC-Coupled Transistor Amplifier in CE Mode

practical pg-ii transistor amplifier common-emitter

Aim

To study the voltage gain and frequency response of a single-stage RC-coupled transistor amplifier in common-emitter mode.

Apparatus

NPN transistor, DC supply, bias resistors, collector resistor, coupling capacitors, function generator, CRO, and connecting leads.

Experimental arrangement

The transistor is biased in the active region; the input is coupled to the base and the amplified output is observed at the collector.

Theory

The emitter is common to the input and output circuits. A small AC signal at the base changes the collector current. The changing collector current produces a larger voltage variation across the collector resistor. Coupling capacitors pass the AC signal while isolating the DC bias conditions.

The voltage gain is $A_v=V_o/V_i$. The lower and upper cutoff frequencies are the frequencies at which the gain falls to $0.707$ of its mid-band value. The bandwidth is $f_H-f_L$.

Observations

Frequency (kHz)	Input (mV)	Output (V)	Gain $A_v$
0.05	20	0.84	42.0
0.10	20	1.12	56.0
1.0	20	1.20	60.0
10	20	1.14	57.0
100	20	0.80	40.0

Result

The amplifier gives a mid-band voltage gain of approximately $60$. The gain decreases at low and high frequencies because of coupling/bypass capacitors and transistor stray capacitances.

Viva Questions

1. Why is the output phase reversed? An increase in collector current increases the voltage drop across the collector resistor and lowers the collector voltage.
2. What is bandwidth? The difference between upper and lower half-power frequencies.
3. Why is the emitter bypassed in a practical amplifier? To reduce AC negative feedback and increase voltage gain.