Question

For a common emitter amplifier, the output resistance is $500\text{k}\Omega$, the current gain $\alpha =0.98$ and power gain is $6.0625\times {10}^6$. Find the input resistance of the amplifier.

• A. $400\Omega$
• B. $100\Omega$
• C. $198\Omega$
• D. $300\Omega$

Answer 1

The correct option is C $198\Omega$

Now,
$\beta =\frac{\alpha }{1-\alpha }=\frac{0.98}{1-0.98}=49$

Power gain is given by,
$P={\beta }^2\times \frac{R_{o/p}}{R_{i/p}}$

$\therefore 6.0625\times {10}^6=(49{)}^2\times \left(\frac{500\times {10}^3}{R_1}\right)$
$\therefore R_{i/p}=198\Omega$.

Hence, $\left(A\right)$ is the correct answer.