Semiconductor Electronics Materials Devices and Simple Circuits - Result Question 63

65. 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: 65. (d)

Solution:

  1. (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 $G^{\prime}=\frac{2}{3} G$