Question

Explain using circuit diagram the working of n-p-n transistor as CE amplifier. Hence obtain expression for its voltage gain in common emitter configuration.

Answer 1

In common emitter configuration of the amplifier, the Emitter of BJT is common to both input and output of the applied signal.

When a signal is applied across the emitter-base junction, the forward bias across the junction increases during the upper half of the cycle. This leads to an increase of flow of electrons from the emitter to the collector through the base terminal. It leads

to increases in collector current. The increase in collector current makes more voltage drop across the load resistor R.

The negative cycle decreases the forward bias voltage across the emitter-base junction. The decrease in collector-base voltage decreases the collector current i the collector resistor. Thus amplified load resistor appears across the collector resistor.

The voltage gain is defined as the product of the current gain and the ratio of the output resistance of the collector to the input resistance of the base circuits.

Current gain, $\beta =\frac{\Delta I_C}{\Delta I_B}$

$\Delta I_C$= Change in collector current

$\Delta I_B$= Change in base current.

Voltage gain for common emitter configuration is, $A_V=\beta \frac{R_C}{R_B}$