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Official PYQ · 2002

GATE CS 2002 Q2-1 — Combinational Circuits

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Consider the following logic circuit whose inputs are functions f1,f2,f3f_1, f_2, f_3 and output is ff

Given that

  • f1(x,y,z)=Σ(0,1,3,5)f_1(x,y,z) = \Sigma (0,1,3,5)
  • f2(x,y,z)=Σ(6,7),f_2(x,y,z) = \Sigma (6,7), and
  • f(x,y,z)=Σ(1,4,5).f(x,y,z) = \Sigma (1,4,5).

f3f_3 is 

Choose one option
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Answer: A

Based on the logic circuit diagram, the system consists of two gates:

  1. An AND gate with inputs f1f_1 and f2f_2.
  2. An OR gate with inputs being the output of the AND gate and f3f_3, producing the final output ff.

Thus, the Boolean expression for the output ff is:

f=(f1⋅f2)+f3f = (f_1 \cdot f_2) + f_3

We are given the minterm representations of the functions:

  • f1(x,y,z)=Σ(0,1,3,5)f_1(x,y,z) = \Sigma (0,1,3,5)
  • f2(x,y,z)=Σ(6,7)f_2(x,y,z) = \Sigma (6,7)
  • f(x,y,z)=Σ(1,4,5)f(x,y,z) = \Sigma (1,4,5)

First, let's find the output of the AND gate, which corresponds to the intersection (∩\cap) of the minterms of f1f_1 and f2f_2:

f1⋅f2=Σ(0,1,3,5)∩Σ(6,7)=∅f_1 \cdot f_2 = \Sigma (0,1,3,5) \cap \Sigma (6,7) = \emptyset

Now, substitute this result back into the expression for ff:

f=∅+f3f = \emptyset + f_3 f=f3f = f_3

Therefore, we have:

f3=f=Σ(1,4,5)f_3 = f = \Sigma (1,4,5)

This matches Option A.

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