First slide

Transistors

Question

In a common emitter amplifier the load resistance of the output circuit is 792 times the resistance of the input circuit. If $α = 0.99$ , then voltage gain is

Moderate

A

78408
B

39600
C

7920
D

3960

Solution

$V o l t a g e g a i n = \frac{O u t p u t v o l t a g e}{I n p u t v o l t a g e} = \frac{V_{o / p}}{V_{i / p}}$
$= \frac{i_{o / p} R_{o / p}}{i_{i / p} R_{i / p}} = β \frac{R_{o / p}}{R_{i / p}}$
$= \frac{α}{1 - α} \frac{R_{o / p}}{R_{i / p}} [P u t β = \frac{α}{1 - α}]$
$\begin{array}{l} = \frac{0.99}{1 - 0.99} \times \frac{792 R_{i / p}}{R_{i / p}} = \frac{0.99}{0.01} \times 792 [G i v e n : R_{o / p} = 792 R_{i / p}] \end{array}$
$= \frac{99}{1} \times 792 = 99 \times 792$
$= 78408$

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