In transistor the emitter − − base junction is forward bias while the base − − collector junction is given a large reverse bias. Under forward bias the charge carriers move towards base causing the flow of current.

The following shows the circuit of a common − − emitter amplifier circuit using an n − − p − − n transistor. The input (base − − emitter) circuit is forward biased by a low voltage battery , so that the resistance of the input circuit is small.

The output (collector − − emitter) circuit is reverse biased by means of a high voltage battery so that resistance of output is high. The weak input AC signal is applied across the base − − emitter circuit and the amplified output signal is obtained across the collector − − emitter circuit. Input impedance of common − − emitter configuration

Here, voltage across base and emitter, base current ofthe order of few micro ampere.