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GATE CSE 2023 Q21: 2-input multiplexer with output fed back to input 0

Question

The output of a 2-input multiplexer is connected back to one of its inputs as shown in the figure.

A 2-input multiplexer: its output Q is wired back to input 0, input 1 is an external input, and S is the select line
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Match the functional equivalence of this circuit to one of the following options.

Try it: multiple choice: exactly one correct option

Select one option
Wrong answer costs 1/3 mark in GATE.
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Answer: B (official GATE 2023 final answer key)

Concept used: A level-sensitive storage element: pass the input while enabled, hold the old value otherwise.

First step: Write QQ for S=1S = 1 (follows input 1) and for S=0S = 0 (keeps its own value).

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Source: official GATE 2023 CS question paper and final answer key, organised by IIT Kanpur. Question paper · Answer key

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