GATE EC 2023 Set 1 — Question 21
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Analog Circuits → BJT & MOSFET Amplifiers → Multistage Amplifiers
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Question
In the circuit shown below, and are bias voltages. Based on input and output impedances, the circuit behaves as a
Correct answer
(D) current controlled current source.
Solution
Reading the circuit. An NMOS transistor (gate biased at ) has its source driven by through ; its drain is the output node. A PMOS transistor (gate biased at , source at ) connects to the same output node and acts as a current-source load.Input impedance. The signal enters at the source of the NMOS whose gate is at AC ground — a common-gate stage. The impedance looking into the source is about , which is low. A low input impedance means the circuit is driven by (senses) an input current.Output impedance. The output node sees the drain of the common-gate NMOS (its output resistance is boosted by source degeneration : ) in parallel with the drain of the PMOS current source (). Both are large, so the output impedance is high — the output behaves as a current source.Low input impedance + high output impedance ⇒ the circuit acts as a current buffer, i.e. a current-controlled current source.
- (A) VCVS: needs high , low .
- (B) VCCS: needs high , high .
- (C) CCVS: needs low , low .
- (D) CCCS: low , high — matches.
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