Question

Draw the labelled circuit diagram of a common-emitter transistor amplifie : Explain clearly how the input and output signals are in opposite phase.

Answer 1

The circuit details for using an npn transistor as common emitter amplifier are shown in the fig.

The input (base-emitter) circuit is forward biased and the output (collector-emitter) circuit is reverse biased.
When no a.c. signal is applied, the potential difference `V_(c)` between the collector and the emitter , is given by
`V_(C)=V_(CE)-I_(C)xxR_(L)` ...(i)
Where, `V_(C)` is the voltage of battery `V_(CE)`
When an a.c. signal is fed to the input circuit, the forward bias increases during the positive half cycle of the input. This results in an increase in `I_(C)` and a consequent decrease in `V_(C)`, as is clear from (i). Thus during positive half cycle of the input, the collector becomes less positive.
During the negative half cycle of the input, the forward bias is decreased resulting in a decrease in `I_(E)` and hence `I_(C)`. Therefore, from (i) `V_(C)` would increase, making the collector more positive. Hence, in a common-emitter amplifier, the output voltage is `180^(@)` out of phase with the input voltage.