The GATE Grind

GATE 2015 EC – Question 47

Digital Circuits · Combinatorial Circuits · 2 marks · Numerical answer

All the logic gates shown in the figure have a propagation delay of 20 ns. Let $A = C = 0$ and $B = 1$ until time $t = 0$. At $t = 0$, all the inputs flip (i.e., $A = C = 1$ and $B = 0$) and remain in that state. For $t > 0$, output $Z = 1$ for a duration (in ns) of ________.

$B$ passes through a NOT gate and then, with $A$, into an AND gate. The AND output and $C$ feed an exclusive-OR gate whose output is $Z$.

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Correct answer: 40

Explanation

Before $t = 0$ the AND output is 0 and $C = 0$, so $Z = 0$. At $t = 0$, $C$ becomes 1, and after the exclusive-OR delay of 20 ns $Z$ becomes $1 \oplus 0 = 1$. The NOT gate output reaches 1 at 20 ns and the AND output reaches 1 at 40 ns. After the exclusive-OR delay, $Z$ becomes $1 \oplus 1 = 0$ at 60 ns. So $Z = 1$ from 20 ns to 60 ns, which is 40 ns.