Q132 / 180 · AIIMS 2018 (26 May) · Physics

In a common emitter (CE) amplifier having a voltage gain $A_v$, the transistor used has transconductance 0.03 mho and current gain 25. If this transistor is replaced with another one with transconductance $0.02 \mathrm{mho}$ and current gain 20 , the voltage gain will be

  • Correct answer
    A
    $\frac{2}{3} A_v$
  • B
    $A_v$
  • C
    $\frac{3}{2} A_v$
  • D
    $\frac{A_v}{2}$

Solution

$\begin{aligned} & \text A_v=\beta \frac{R_{\text {out }}}{R_{\text {in }}}=25 \frac{R_{\text {out }}}{R_{\text {in }}} \\ & \because \quad G_m=\frac{\beta}{R_{\text {in }}} \Rightarrow R_{\text {in }}=\frac{\beta}{G_m}=\frac{25}{0.03} \\ & \therefore \quad A_v=25 \frac{R_{\text {out }}}{25} \times 0.03 \\ & \text { Now, } A_v^{\prime}=20 \frac{R_{\text {out }}}{20} \times 0.02 \\ & A_v^{\prime}=\frac{2}{3} A_v\end{aligned}$

Hence, the correct answer is option: 'A'.

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