GATE EC 2018 Set 1 — Question 11
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Analog Circuits → BJT & MOSFET Amplifiers → Multistage Amplifiers
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Question
Two identical nMOS transistors and are connected as shown below. The circuit is used as an amplifier with the input connected between G and S terminals and the output taken between D and S terminals. and are so adjusted that both transistors are in saturation. The transconductance of this combination is defined as while the output resistance is , where is the current flowing into the drain of . Let be the transconductances and be the output resistances of transistors and , respectively. 
Which of the following statements about estimates for and is correct?
Correct answer
(C) gₘ ≈ gₘ₁ and rₒ ≈ rₒ₁ · gₘ₂ · rₒ₂.
Solution
In a cascode configuration, the overall transconductance is determined by the input transistor , as the same current flows through both transistors: . The output resistance looking into the drain of is boosted by the intrinsic gain of : . Thus, option (C) is correct.
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