PlacementPrepWhich logic gate gives output 1 only when all its inputs are 1?
Solution
AND outputs 1 only if every input is 1.
Answer: (A) AND
Technical · Electronics and Communication (ECE)
Digital Electronics MCQs for placement tests
20 technical questions of moderate difficulty, as asked in campus placement and company tests. Each has a short explanation.
Which gates are called universal gates?
Solution
Any Boolean function can be built using only NAND gates or only NOR gates.
Answer: (B) NAND and NOR
What is the binary equivalent of decimal 25?
Solution
25 = 16 + 8 + 1 = 11001₂.
Answer: (D) 11001
What is the decimal value of hexadecimal 2F?
Solution
2F = 2 × 16 + 15 = 47.
Answer: (B) 47
What is the 2's complement of the 4-bit number 0101?
Solution
Invert the bits (1010) and add 1: 1011.
Answer: (C) 1011
Simplify the Boolean expression A + AB.
Solution
By the absorption law, A + AB = A(1 + B) = A.
Answer: (D) A
By De Morgan's theorem, (A · B)' equals:
Solution
The complement of a product is the sum of the complements.
Answer: (C) A' + B'
The output of an XOR gate is 1 when:
Solution
XOR gives 1 for an odd number of 1s; for two inputs, when they differ.
Answer: (D) The inputs are different
How many select lines does an 8-to-1 multiplexer need?
Solution
2³ = 8, so three select lines choose one of eight inputs.
Answer: (B) 3
A half adder has how many outputs?
Solution
A half adder produces a sum (A ⊕ B) and a carry (A · B).
Answer: (C) 2 (sum and carry)
Which flip-flop toggles its output when its input is 1 on each clock edge?
Solution
A T (toggle) flip-flop changes state when T = 1.
Answer: (A) T flip-flop
Which flip-flop has an invalid (forbidden) input state?
Solution
S = R = 1 is not allowed in an SR flip-flop; the JK flip-flop removes this problem.
Answer: (A) SR flip-flop
A 4-bit ripple counter can count how many distinct states?
Solution
4 bits give 2⁴ = 16 states (0 to 15).
Answer: (B) 16
Combinational circuits differ from sequential circuits because they:
Solution
The output of a combinational circuit depends only on the present inputs.
Answer: (D) Have no memory
How many 2-input NAND gates are needed to build an XOR gate?
Solution
The standard NAND-only XOR uses four 2-input NAND gates.
Answer: (D) 4
What is the Gray code for binary 0110?
Solution
Gray bits: keep the MSB (0), then XOR adjacent bits: 0⊕1 = 1, 1⊕1 = 0, 1⊕0 = 1, giving 0101.
Answer: (B) 0101
A decoder with n inputs has at most how many outputs?
Solution
Each input combination activates one output.
Answer: (C) 2ⁿ
Which logic family is known for very low static power consumption?
Solution
CMOS draws almost no current except while switching.
Answer: (B) CMOS
What is the BCD representation of decimal 59?
Solution
Each decimal digit is coded separately: 5 = 0101 and 9 = 1001.
Answer: (C) 0101 1001
A shift register that accepts data one bit at a time and outputs all bits together is:
Solution
Serial input, parallel output.
Answer: (C) SIPO (serial-in parallel-out)
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