Semiconductor Electronics Materials Devices And Simple Circuits Ques 73
73. In a common emitter (CE) amplifier having a voltage gain $G$, the transistor used has transconductance 0.03 mho and current gain 25 . If the above transistor is replaced with another one with transconductance $0.02 mho$ and current gain 20 , the voltage gain will be
2013]
(a) $1.5 G$
(b) $\frac{1}{3} G$
(c) $\frac{5}{4} G$
(d) $\frac{2}{3} G$
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Answer:
Correct Answer: 73.(d)
Solution:
- (d) Voltage gain $A v=\beta \frac{R _{\text{out }}}{R _{\text{in }}}$
$\Rightarrow G=25 \frac{R _{\text{out }}}{R _{\text{in }}}$
Transconductance $g_m=\frac{\beta}{R _{\text{in }}}$
$\Rightarrow R _{\text{in }}=\frac{\beta}{g_m}=\frac{25}{0.03}$
Putting this value of $R _{\text{in }}$ in eqn. (i)
$ \begin{align*} & G=25 \frac{R _{\text{out }}}{25} \times 0.03 \tag{ii}\\ & \therefore \quad G^{\prime}=20 \frac{R _{\text{out }}}{20} \times 0.02 \tag{iii} \end{align*} $
From eqs. (ii) and (iii)
Voltage gain of new transistor $G^{\prime}=\frac{2}{3} G$
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