Voltage gain of common emitter amplifier
The voltage gain (Av) of an amplifier is equal to a.c. output voltage (vout) divided by a.c. input voltage (vin) i.e. Av = vout/vin. We have already seen that voltage gain of a CE amplifier is given by;
Voltage gain, Av= β ×RC/Rin... for unloaded amplifier
Voltage gain, Av= β ×RAC/Rin... for loaded amplifier
Remember that RAC = RC || RL
The above formula for Av can be use if we know the values of RC (or RAC), β and Rin. Generally, all these values are not know. In that case, we can find the value of Av 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 RE), it can be prove that the voltage gain is give by ;
Voltage gain, Av = RC /re ... for unloaded amplifier
RAC /re ... for loaded amplifier
Voltage Gain of Unloaded CE Amplifier Fig. 1shows the circuit of unloaded CE amplifier (i.e. no load RL is connect to the circuit ).
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 (Av) 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
emitter is no longer at the ac ground as show in Fig. 2.
Therefore, for the a.c. signal, both e r′ and RE are in series. As a result, the voltage gain of the amplifier becomes :
Voltage gain, Av=RC/re+RE... for unloaded amplifier
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