What is amplification? With a circuit diagram, explain the working of npn transistor as an amplifier in CE configuration.

Explanation

The circuit diagram of a CE amplifier using n-p-n transistor is as shown in figure.
The input (base-emitter) circuit is forward biased and the output (collector - emitter) circuit is reverse biased.
When the ac input signal to be amplified is fed to the base-emitter circuit. The output voltage `V_(0)` varies in accordance with the relation, `V_(0) = V_(CE) = V_(C C) - I_(C)R_(C)`. These variations in the collector voltage `V_(CF)` appears as amplified output.

During the positive half cycle of ac input signal, the forward bias of emitter - base junction increases. Due to this base current `I_(B)` increases and hence collector current `I_(C)` increases. As a result of this `I_(C)R_(C)` increases. Therefore, the output voltage `V_(0)` is negative. This indicates that the positive half cycle of input ac signal voltage is amplitude through negative half cycle.
During the negative half cycle of ac input signal, the forward bias of emitter base junction decreases. Due to this base curret I- decreases and hence collector current `I_(C)` decreases. As a result of this `I_(C)R_(C)` decreases. Therefore, the output voltage `V_(0)` is positive This indicates that the negative half cycle of input ac signal voltage is amplified through positive half cycle.
Thus, the weak input amplified and output signal is out of phase with the input signal by `180^(@)`.