Q. The input resistance of a common emitter transistor amplifier, if the output resistance is $500\ K\Omega$, the current gain $\alpha = 0.98$ and power gain is $6.0625 \times 10^{6}$, is
A) 198 $\Omega$
B) 300$\Omega$
C) 100$\Omega$
D) 400$\Omega$
Explanation
(a) Voltage gain $= A_{v} = \beta\frac{R_{2}}{R_{1}}$
Also, current gain $\beta = \frac{\alpha}{1 - \alpha} = \frac{0.98}{1 - 0.98} = 49$
$A_{V} = (49)\left\lbrack \frac{500 \times 10^{3}}{R_{1}} \right\rbrack$
Power gain
$= 6.0625 \times 10^{6} = 49 \times \left\lbrack \frac{500 \times 10^{3}}{R_{1}} \right\rbrack \times 49$
$\therefore R_{1} = 198\Omega$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.