With the help of a labelled circuit diagram, explain how an n-p-n transistor can be used as an amplifier in common-emitter configuration. Explain how the input and output voltages are out of phase by 180 for a common-emitter transistor amplifier.

Explanation

Principle : The collector current in a transistor can be controlled by a small change in the base current.
Electric ciruit : Consider the use of pnp transistor as an amplifier in the common-emitter (CE) configuration in which the emitter is common to the input and output circuit. The emitter-base junction is forward blased by the battery `V_(BB)` while the collector-base junction is reverse biased by the battery `V_(CC)`
The signal voltage `V_(i)` to the amplified is applied between the base and the emitter.
Working : The conductor characterisitics shows that in the active region. the collector current `I_(c)` is determined almost entirely by the base current `I_(B)`, and collector potential `V_(C)` has relatively little effect on it.
The applied signal voltage causes small changes in the emitter-base p.d. thereby producing variations in the base current. These cause proportionately larger variations in the collector current because `I_(C) = beta I_(B),` where the common-emitter current gain (`beta)` is always greater than 50 .

The time-varying collector current produces a time-varying output voltage `V_(0)` across the load resistance `R_(L)`.
`V_(0) = I_(C) * R_(L) = betaI_(B) * R_(L)`
Thus, `V_(0) gt V_(1)`, so that the circuit has produced a voltge gain. The amplifier's voltage gain `(A_(V))` is defined as the ratio of the input voltage to the input voltage.
`A_(V) = (V_(0))/(V_(1))`