GATE EC 2019 Set 1 — Question 25
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Digital Circuits → Combinational Logic → Static CMOS Logic Gates
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Question
In the circuit shown, A and B are the inputs and F is the output. What is the functionality of the circuit?
Correct answer
(B) XNOR
Solution
Read the circuit from the figure:
F = 1 exactly when A = B, so .
- Two pMOS transistors in series connect to F; their gates are driven by A and B. They pull F high only when .
- The left nMOS connects A to F and its gate is B; it passes A to F when .
- The right nMOS connects B to F and its gate is A; it passes B to F when .
| A | B | Conducting path | F |
|---|---|---|---|
| 0 | 0 | both pMOS ON | 1 |
| 0 | 1 | left nMOS passes A = 0 | 0 |
| 1 | 0 | right nMOS passes B = 0 | 0 |
| 1 | 1 | both nMOS pass 1 | 1 |
- It is combinational with no feedback, so it is not a latch (A) or an SRAM cell (C).
- The output is the complement of XOR, so (D) is wrong.
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