Voltage Gain of Common Emitter Amplifier

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Voltage gain of common emitter amplifier

The voltage gain (A_v) of an amplifier is equal to a.c. output voltage (v_out) divided by a.c. input voltage (v_in) i.e. A_v = v_out/v_in. We have already seen that voltage gain of a CE amplifier is given by;

Voltage gain, A_v= β ×R_C/R_in... for unloaded amplifier

Voltage gain, A_v= β ×R_AC/R_in... for loaded amplifier

Remember that R_AC = R_C || R_L

The above formula for A_v can be use if we know the values of R_C (or R_AC), β and R_in. Generally, all these values are not know. In that case, we can find the value of A_v in terms of total a.c. collector resistance and total a.c. emitter resistance. For the circuit shown in Voltage gain of common emitter amplifier Fig. 1 (with CE connected across R_E), it can be prove that the voltage gain is give by ;

Voltage gain, Av = R_C /r_e ... for unloaded amplifier

R_AC /r_e ... for loaded amplifier

Voltage gain of common emitter amplifier — Fig-1

Voltage Gain of Unloaded CE Amplifier Fig. 1shows the circuit of unloaded CE amplifier (i.e. no load R_L is connect to the circuit ).

Note that

emitter bypass capacitor CE is connect in parallel with emitter resistance RE. The capacitor CE acts as a *short to the a.c. signal so that it bypasses the a.c. signal to the ground. Therefore, the emitter is effectively at a.c. ground. It is important to note that CE plays an important role in determining the voltage gain (A_v) of the CE amplifier. If it is remove, the voltage gain of the amplifier is greatly reduce (soon you will see the reason for it).

Voltage Gain of CE Amplifier Without CE

When we remove the emitter bypass capacitor from the CE amplifier shown in Fig. 2, the voltage gain of the circuit is greatly reduce. The reason is simple. Without the emitter bypass capacitor CE, the