Question

In a common emitter transistor amplifier, the output resistance is $500k\Omega$ and the current gain $\beta =49$. If the power gain of the amplifier is $5\times 10^6$, the input resistance is

• A. $325\Omega$
• B. $165\Omega$
• C. $198\Omega$
• D. $240\Omega$

Answer 1

Answer: (D)

Here, Current gain, $\beta =49$
Output resistance, $R_o=500k\Omega =500\times 10^3\Omega$
Power gain = $5\times 10^6$
Power gain ${\beta }^2\times \frac{R_o}{R_i},5\times 10^6={\left(49\right)}^2\times \frac{5\times 10^5}{R_i}$
$R_i=\frac{{\left(49\right)}^2\times 5\times 10^5}{5\times 10^6}\approx 240\Omega$