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For the given circuit, the input digital signals are applied at the terminals A, B and C. What would be the output at the terminal y?

Asked in NEET 2021 · Waveforms, switches and diode gates

Figure: Waveforms, switches and diode gates
Answer: (3) Graph (C)

Step-by-step solution

Idea: write the output as an expression, then look at what the waveforms make each term do.

The upper AND gives A· B and the lower NAND gives B· C, and the OR adds them: y=A· B+B· C.

Read B and C off the waveforms: whenever B is high C is low, and whenever B is low C is high. They are complements.

So B· C=0 at every instant, and B· C=1 at every instant.

A logical sum with a term stuck at 1 is itself stuck at 1, whatever A· B does.

The output stays high throughout, at 5 V, which is Graph (C).

Why the other options are wrong

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