GATE 2025 EC – Question 21
A simplified small-signal equivalent circuit of a BJT-based amplifier is given below.
The small-signal voltage gain $v_o/v_s$ (in V/V) is ________.
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Correct answer: (D) $\dfrac{+\beta R_L}{R_S+r_\pi}$
Explanation
The base current is $i_b=v_s/(R_S+r_\pi)$. The dependent current source $\beta i_b$ is drawn with its arrow pointing up into the output node, so it drives $\beta i_b$ through $R_L$ to ground. Hence $v_o=+\beta i_bR_L$ and $v_o/v_s=\dfrac{+\beta R_L}{R_S+r_\pi}$.