Question

(A) Draw a circuit diagram of transistor as an amplifier in common emitter configuration.
(B) Obtain the expression for the voltage gain.

Answer 1

A. The circuit diagram of a transistor as an amplifier in common emitter configuration is shown.

B. The Voltage Gain of the common emitter amplifier is equal to the ratio of the change in the input voltage to the change in the amplifiers output voltage. Then $\Delta V_L$ is $V_{out}$ and $\Delta V_B$ is $V_{in}$. But voltage gain is also equal to the ratio of the signal resistance in the collector to the signal resistance in the Emitter and is given as

Voltage gain $=\frac{V_{out}}{V_{in}}=\frac{\Delta V_L}{\Delta V_B}=-\frac{R_L}{R_E}$

Voltage gain can also be written as

Voltage gain = $\beta \times \frac{R_L}{R_E}$ where $\beta =\frac{\alpha }{1-\alpha }$