Draw the circuit diagram of a common emitter transistor amplifier. Write the expression for its voltage gain. Explain , how the input and output signal differ in phase by `180^(@)`

Explanation

During positive half cycle of input single, forward bias of `E_(B)` junction decreases casuing. `I_(e)` and hence `I_(c)` decreases. It results in voltage drop across load resistance `R_(L)`. So, from the relation.
`V_(CE) = V_("CC") - I_(C) R_(L), V_(0)` increases.
As collector is connected to the negative terminal of battery, increase in collector voltage indicates that collector becomes more negative. As in common ammeter amplifier, the output voltage decreases with increases in input signal and increases with decreasing input signal, so both input and output signals are out of phase with phase difference `pi`. During negative half cycle of input signal, the forward bias of `E_(B)` junction decreases. As a result, `I_(E)` and hence `I_(C)` decreases. Voltage Drop across `R_(L) = I_(C)R_(L)` also decreases. So, the output voltage increases. As collector is connected to the positive terminal of `V_("CC")`, so, increase in `V_(0) rArr` collector voltage will be more positive. An amplified positive signal is obtained across the output.