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 $({\beta }_{AC}$ is $AC$ 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 = ${\beta }_{AC}\times \frac{200}{100}$
$\therefore$ ${\beta }_{AC}=25$
Now, AC power gain = $A_V\times {\beta }_{AC}$ = 50 $\times$ 25 = 1250