Question

A common emitter amplifier has a voltage gain of 50, an input impedance of 100 $\Omega$ and an output impedance of 200 $\Omega.$ The power gain of the amplifier is

• A. 500
• B. 1000
• C. 1250
• D. 100

Answer 1

Answer: (C)

AC power gain $=\frac{\text { Change in output power }}{\text { Change in input power }}$
$ =\frac{\Delta V_{c} \times \Delta i_{c}}{\Delta V_{i} \times \Delta i_{b}}=\left(\frac{\Delta V_{c}}{\Delta V_{i}}\right) \times\left(\frac{\Delta i_{c}}{\Delta i_{b}}\right)=A_{V} \times \beta_{ AC } $
where $A_{V}$ is voltage gain and $\left(\beta_{ AC }\right.$ is $A C$ current gain.
Also,
$A_{V}=\beta_{ AC } \times$ resistance gain $\left(=\frac{R_{0}}{R_{i}}\right)$
Given, $A_{V}=50, \, R_{o}=200 \, \Omega, \, R_{i}=100 \, \Omega$
Hence, 50 = $\beta _{A C}\times \frac{200}{100}$
$\therefore $ $\beta _{A C}=25$
Now, AC power gain = $A_{V}\times \beta _{A C}$ = 50 $\times $ 25 = 1250