# Digital Circuits for GATE: 20 MCQ Questions with Solutions

1. Question: Which logic gate produces a HIGH output when any of its inputs is HIGH? a) AND gate b) OR gate c) NOT gate d) XOR gate Solution: b) OR gate
2. Question: The complement of a binary number is obtained by flipping all its bits. What is the result when the binary number “10101010” is complemented? a) 10101010 b) 01010101 c) 11111111 d) 00000000 Solution: b) 01010101
3. Question: How many NAND gates are required to implement a 4-input OR gate? a) 4 b) 6 c) 8 d) 10 Solution: b) 6
4. Question: What is the Boolean expression for the exclusive-OR (XOR) operation? a) AB + A’B’ b) AB + A’B c) AB’ + A’B d) AB’ + A’B’ Solution: c) AB’ + A’B
5. Question: Which of the following gates is called the “Universal Gate” as it can be used to implement any other logic gate? a) AND gate b) OR gate c) NAND gate d) NOR gate Solution: c) NAND gate
6. Question: In a half adder circuit, what are the outputs called for the sum and carry? a) S and C b) S and C_out c) SUM and C d) SUM and C_out Solution: b) S and C_out
7. Question: Which logic gate operation is represented by the Boolean expression: F = (A + B)(C + D)? a) AND b) OR c) NAND d) NOR Solution: b) OR
8. Question: What is the minimum number of two-input AND gates required to implement the Boolean expression: F = AB + CD + EF? a) 3 b) 4 c) 5 d) 6 Solution: a) 3
9. Question: The output of a 4-bit binary counter is connected to the input of a 4-to-1 multiplexer. How many control inputs are required in the multiplexer? a) 1 b) 2 c) 3 d) 4 Solution: b) 2
10. Question: What is the output of an RS flip-flop when both R and S inputs are HIGH? a) Set b) Reset c) Undefined d) Toggle Solution: c) Undefined
11. Question: Which gate is equivalent to a combination of an AND gate followed by an inverter (NOT gate)? a) NAND gate b) NOR gate c) XOR gate d) XNOR gate Solution: a) NAND gate
12. Question: What is the truth table for an 8-to-1 multiplexer (MUX) with inputs A, B, C, D, E, F, G, H, and selection lines S2, S1, S0? a) 8 rows and 1 column b) 1 row and 8 columns c) 2 rows and 4 columns d) 4 rows and 2 columns Solution: a) 8 rows and 1 column
13. Question: What is the output of a D flip-flop when the clock signal is LOW? a) Q = 0, Q’ = 1 b) Q = 1, Q’ = 0 c) Q and Q’ hold their previous states d) Q and Q’ toggle their states Solution: c) Q and Q’ hold their previous states
14. Question: What is the Boolean expression for the exclusive-NOR (XNOR) operation? a) AB + A’B’ b) AB + A’B c) AB’ + A’B d) AB’ + A’B’ Solution: d) AB’ + A’B’
15. Question: In a full adder circuit, how many inputs are there for the sum output? a) 1 b) 2 c) 3 d) 4 Solution: b) 2
16. Question: Which logic gate operation is represented by the Boolean expression: F = (A + B)’? a) AND b) OR c) NAND d) NOR Solution: c) NAND
17. Question: What is the output of an RS flip-flop when both R and S inputs are LOW? a) Set b) Reset c) Undefined d) Hold Solution: c) Undefined
18. Question: How many input combinations are possible for a 3-input AND gate? a) 2 b) 3 c) 4 d) 8 Solution: d) 8
19. Question: What is the output of a JK flip-flop when both J and K inputs are HIGH? a) Set b) Reset c) Toggle d) Hold Solution: c) Toggle
20. Question: Which gate is equivalent to a combination of an OR gate followed by an inverter (NOT gate)? a) NAND gate b) NOR gate c) XOR gate d) XNOR gate Solution: b) NOR gate
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