Common-Emitter Transistor Amplifier: Common-emitter transistor amplifier gives the highest gain and hence it is the most commonly employed circuit. Fig. depicts the circuit for a p-n-p transistor. In this circuit, the emitter is common to both the input (emitter-base) and output (collector-emitter) circuits and is grounded. The emitter-base circuit is forward biased and the basecollector circuit is reverse biased.
In a common-emitter circuit, the collector-current is controlled by the base-current rather than the emitter-current. Since in a transistor, a large collector-current corresponds to a very small basecurrent, therefore, when input signal is applied to base, a very small change in base-current provides a much larger change in collector-current and thus extremely large current gains are possible.
Referring to fig., when positive half cycle is fed to the input circuit, it opposes the forward bias of the circuit which causes the collector current to decrease. It decreases the voltage drop across load RL and thus makes collector voltage more negative. Thus when input cycle varies through a positive half cycle, the output voltage developed at the collector varies through a negative half cycle and vice versa. Thus the output voltage in common-emitter amplifier is in antiphase with the input signal or the output and input voltages are 180° out of phase.