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As shown in the above circuit diagram, the input ac signal Vs is given through resistance Rs to the base of transistor. Supply voltage VBB is give between base and emitter of transistor, which keeps the transistor in the center of its characteristics. At the output, Vcc supply is given through load resistance RL to the collector of transistor. The voltage between base and emitter is known as VBE and that of between collector and emitter is called VCE. It should also remember that VBB and VCC supply a dc current whereas Vs supply ac signal.
Initially when no input signal Vs, VBB will supply dc current into base IB so that corresponding dc current Ic will flow. The sum of two current IB and Ic will turn as emitter current IE i.e IE=Ic+IB. This is the condition when no input signal is applied.
When input signal Vs is applied to the base of transistor, corresponding ac current is will flow in the base which will be added to IB resulting in an ac component of current riding on dc called iB. Which in turn will give ac component of current ic to flow in the collector and this will be added to dc Ic.
The doping of junctions and width of base is so chosen that small base current iB will give large collector current ic. Therefore transistor has capability of current gain Ai. Since voltage from base to emitter is small and voltage in the collector circuit is large voltage gain Av is also obtained.
So for a transistor, iE=ic+iB and VCE=VCB+VBE