GATE EC 2025 Set 1 — Question 21
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Analog Circuits → BJT & MOSFET Amplifiers → Small-Signal & Hybrid-Pi Model
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Question
A simplified small-signal equivalent circuit of a BJT-based amplifier is given below.The small-signal voltage gain (in V/V) is ___________.
Correct answer
(A) (-β R_L)/(Rₛ + r_π)
Solution
From the input loop of the small-signal circuit:
From the output side of the circuit:
The output voltage is developed across by the dependent current source . Note that the current flows from ground through towards the output node, or equivalently, the arrow points downwards, making the top node negative relative to ground.
Substituting the expression for :
The voltage gain is:
This matches option (A).
From the output side of the circuit:
The output voltage is developed across by the dependent current source . Note that the current flows from ground through towards the output node, or equivalently, the arrow points downwards, making the top node negative relative to ground.
Substituting the expression for :
The voltage gain is:
This matches option (A).
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