Combinational Circuits MCQs

Practice Combinational Circuits MCQs with four options, answers and explanations for focused Electronics & Telecommunication Engineering revision.

Question 1

Digital ElectronicsCombinational Circuits

A half adder adds?

ATwo single-bit binary inputs
BThree single-bit binary inputs
CTwo 4-bit numbers only
DOne binary input and one decimal input

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Correct Answer: A. Two single-bit binary inputs

Explanation: A half adder adds two one-bit inputs and produces Sum and Carry outputs.

Reference: Half-adder fundamentals

Question 2

Digital ElectronicsCombinational Circuits

A full adder has how many binary input variables?

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Correct Answer: C. 3

Explanation: A full adder accepts A, B and carry-in, for a total of three binary inputs.

Reference: Full-adder fundamentals

Question 3

Digital ElectronicsCombinational Circuits

A full subtractor has inputs A, B and Bin. Its difference output is?

AA XOR B XOR Bin
BA + B + Bin
CAB + Bin
DA XNOR B XNOR Bin

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Correct Answer: A. A XOR B XOR Bin

Explanation: The difference bit of a full subtractor is the modulo-2 sum A XOR B XOR Bin.

Reference: Standard digital electronics: full subtractor

Question 4

Digital ElectronicsCombinational Circuits

For a half adder with inputs A = 1 and B = 1, the Sum and Carry outputs respectively are?

A0 and 0
B0 and 1
C1 and 0
D1 and 1

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Correct Answer: B. 0 and 1

Explanation: A half adder has Sum = A XOR B and Carry = AB. For A = B = 1, Sum = 0 and Carry = 1.

Reference: Standard digital electronics: half adder

Question 5

Digital ElectronicsCombinational Circuits

For a full adder with A = 1, B = 1 and Cin = 1, the Sum and Carry-out are?

A0 and 0
B0 and 1
C1 and 0
D1 and 1

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Correct Answer: D. 1 and 1

Explanation: 1 + 1 + 1 = binary 11, so Sum = 1 and Carry-out = 1.

Reference: Standard digital electronics: full adder

Question 6

Digital ElectronicsCombinational Circuits

The output of a 2-bit magnitude comparator indicating A > B should be HIGH for A = 10 and B = 01 because?

A2 is greater than 1
B1 is greater than 2
CBoth numbers are equal
DBinary 10 represents zero

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Correct Answer: A. 2 is greater than 1

Explanation: Binary 10 equals decimal 2 and binary 01 equals decimal 1, so the A > B output is asserted.

Reference: Standard digital electronics: magnitude comparators

Question 7

Digital ElectronicsCombinational Circuits

For a full adder, the carry-out expression can be written as?

AA XOR B XOR Cin
BAB + ACin + BCin
CA + B + Cin
DABCin only

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Correct Answer: B. AB + ACin + BCin

Explanation: The full-adder carry is 1 whenever at least two of the three inputs are 1, giving Cout = AB + ACin + BCin.

Reference: Standard digital electronics: full adder

Question 8

Digital ElectronicsCombinational Circuits

A magnitude comparator cascades several comparison blocks from least-significant to most-significant groups. Which information must be propagated between blocks to obtain the final comparison?

AA>B, A=B, and A<B conditions
BOnly parity
COnly clock frequency
DOnly carry-out from an adder

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Correct Answer: A. A>B, A=B, and A<B conditions

Explanation: Cascaded magnitude comparison requires greater-than, equal-to, and less-than status so higher-order groups can determine the overall relation correctly.

Reference: Standard digital electronics: cascaded magnitude comparators