Question

Draw the circuit diagram of a common emitter amplifier using n-p-n transistor. What is the phase difference between the input signal and the output signal? State two reasons why a common amplifier is preferred over a common base amplifier.

Answer 1

The circuit of a common-emitter amplifier using an n-p-n transistor is shown in figure :

In a common emitter amplifier circuit, the input signal voltage and output collector voltage are in opposite phase. i.e 180° out of phase. Thus the phase difference between the input signal and output voltage is 180°.

The input and output wave forms are shown in fig.

Voltage gain $A_v=\beta \frac{R_L}{R_i}$

where $\beta$ is the current gain , $R_L$ the load resistance and $R_i$ the input resistance.

Reasons for using a common emitter amplifier over common base amplifier:

(i) Voltage gain is quite high without any phase change of signal voltage.

(ii) Voltage gain is uniform over a wide frequency range or power gain is high .