An amplifier is represented by the circuit shown in Fig. Here `r_(i)` is the input resistance of the amplifier and the voltage `V_(i)` is appearing across it. This voltage is amplified by a factor `A_(V)` and appears across the load as voltage `V_(0)`

An external voltage `V_(s)` is applied at the input terminals of the amplifier via series resistance `R_(S)`. What will be the apparent gain `A_(V)(=V_(0)//V_(s))` of the ampilifier in terms of `A_(V), R_(S)` and `r_(i).`

Explanation

Here, `V_(s)` is the source voltage applied to input circuit. Let `r_(i)` be the input resistance of transistor. Then current in the input circuit,
`I=V_(s)/(r_(i)+R_(s))`
Voltage across the input terminal of amplifier
is `V_(i)=(V_(s))/(r_(i)+R_(s))xxr_(i)`
Voltage gain, `A_(V)=V_(0)/V_(i)=(V_(0)(r_(i)+R_(s)))/(V_(s)xxr_(i))`
or `V_(0)/V_(s)=r_(i)/((r_(i)+R_(s)))A_(V)`
Apparent voltage gain,
`A_(V)^(')=V_(0)/V_(s)=r_(i)/((r_(i)+R_(s))A_(V)`