First slide

Transistor

Question

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

Moderate

A

500
B

1000
C

1250
D

100

Solution

AC power gain is the ratio of change in output power to the change in input power.
AC power gain $= \frac{Change in output power}{Change in input power}$
$= \frac{∆ V_{c} \times ∆ i_{c}}{∆ V_{i} \times ∆ i_{b}} = (\frac{∆ V_{c}}{∆ V_{i}}) \times (\frac{∆ i_{c}}{∆ i_{b}}) = A_{v} \times β_{AC} Where A_{v} is voltge gain and {(β)}_{Ac} is AC current gain . Also, A_{v} / β_{Ac} = resistance gain (= \frac{R_{0}}{R_{1}}) Given, A_{v} = 50, R_{0} = 200 Ω, R_{i} = 100 Ω Hence, 50 / β_{Ac} = \frac{200}{100} β_{Ac} = 25 Now, AC power gain = A_{v} \times β_{Ac} = 50 \times 25 = 1250$

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