Question

In a common emitter transistor amplifier, the output resistance is $500\, k\Omega$ 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, $R_o = 500\, k\Omega = 500 \times 10^3\, \Omega$
Power gain = $5 \times 10^6$
Power gain $\beta^{2 }\times\frac{R_{o}}{R_{i}}, 5\times10^{6} =\left(49\right)^{2} \times\frac{5\times10^{5}}{R_{i}}$
$ R_{i}=\frac{\left(49\right)^{2}\times5\times10^{5}}{5\times10^{6}}\approx240 \Omega$