51. Which logic gate produces output 0 when all inputs are 0?
A) AND
B) OR
C) NAND
D) NOR
Answer: B
Explanation: OR gate gives output 1 when any input is 1; when all are 0, output is 0.
52. The output of NOR gate is same as
A) NOT(OR)
B) NOT(AND)
C) OR(AND)
D) AND(OR)
Answer: A
Explanation: NOR = NOT applied to OR output.
53. Boolean expression of NAND gate is
A) (AB)’
B) (A+B)’
C) A’B’
D) (A’+B’)
Answer: A
Explanation: NAND = NOT(AND) = (AB)’
54. A logic circuit that converts binary information into one of many output lines is called
A) Decoder
B) Encoder
C) Multiplexer
D) Demultiplexer
Answer: A
Explanation: Decoder activates one of many outputs according to binary input.
55. The logic expression AB + A’C represents
A) OR gate
B) Multiplexer
C) Decoder
D) Selector
Answer: B
Explanation: It’s a 2×1 multiplexer function with select input A.
56. A SR flip-flop constructed with NAND gates is active
A) High
B) Low
C) Both
D) None
Answer: B
Explanation: NAND-based SR flip-flop is active-low (inputs active at 0).
57. In a D flip-flop, when the clock is high
A) Output follows input
B) Output remains same
C) Output becomes zero
D) Output toggles
Answer: A
Explanation: D flip-flop transfers D input to Q when clock = 1.
58. A 4-to-16 line decoder will have how many inputs?
A) 2
B) 3
C) 4
D) 8
Answer: C
Explanation: 2⁴ = 16 outputs → requires 4 inputs.
59. A 3-bit binary counter can count from
A) 0 to 7
B) 0 to 8
C) 0 to 15
D) 0 to 3
Answer: A
Explanation: 3 bits → 2³ = 8 states → 0–7.
60. The complement of (A + B)’ is
A) A + B
B) AB
C) A’ + B’
D) A’B’
Answer: A
Explanation: Double complement returns original expression: (A + B)’’ = A + B.
61. The logic expression (A + B)(A + B’) simplifies to
A) A
B) B
C) AB
D) A + B
Answer: A
Explanation: Using Boolean law → A + BB’ = A + 0 = A.
62. The Boolean identity A + A’B = A + B is called
A) Absorption law
B) Distributive law
C) De Morgan’s law
D) Consensus theorem
Answer: D
Explanation: A + A’B = A + B is a consensus theorem simplification.
63. The expression A + AB simplifies to
A) A
B) AB
C) B
D) A + B
Answer: A
Explanation: A + AB = A(1 + B) = A.
64. The result of (A + B)(A + C) =
A) A + BC
B) AB + AC
C) A + B + C
D) (A + B + C)’
Answer: A
Explanation: Apply distributive law: (A + B)(A + C) = A + BC.
65. The number of logic levels in a circuit affects
A) Speed
B) Size
C) Color
D) Shape
Answer: A
Explanation: More logic levels increase propagation delay → reduces speed.
66. A flip-flop can store
A) 1 bit
B) 2 bits
C) 4 bits
D) 8 bits
Answer: A
Explanation: One flip-flop stores one binary digit.
67. Which logic gate output remains 1 unless all inputs are 1?
A) AND
B) OR
C) NAND
D) NOR
Answer: C
Explanation: NAND gives 0 only when all inputs are 1.
68. De Morgan’s theorem states
A) (A + B)’ = A’B’
B) (AB)’ = A’ + B’
C) Both A and B
D) None
Answer: C
Explanation: Both are standard De Morgan’s theorems.
69. In a JK flip-flop, when J = 0 and K = 1, output will
A) Toggle
B) Reset
C) Set
D) Hold
Answer: B
Explanation: J=0, K=1 resets the output (Q=0).
70. In binary subtraction, 1 – 1 =
A) 1
B) 0
C) 10
D) 11
Answer: B
Explanation: 1 minus 1 gives 0, no borrow.
71. The most commonly used code for representing characters in computers is
A) BCD
B) ASCII
C) Gray
D) Excess-3
Answer: B
Explanation: ASCII encodes alphabets, digits, and symbols.
72. Which of the following is a combinational circuit?
A) Flip-flop
B) Counter
C) Encoder
D) Register
Answer: C
Explanation: Encoder is a combinational circuit; others are sequential.
73. The maximum number of outputs in a 10-line decoder is
A) 100
B) 512
C) 1024
D) 256
Answer: C
Explanation: 2¹⁰ = 1024 outputs.
74. A digital-to-analog converter (DAC) converts
A) Digital to Digital
B) Analog to Digital
C) Digital to Analog
D) None
Answer: C
Explanation: DAC converts binary signals into continuous analog voltage.
75. An analog-to-digital converter (ADC) converts
A) Digital to Analog
B) Analog to Digital
C) Binary to Gray
D) Binary to Decimal
Answer: B
Explanation: ADC converts continuous signals into binary numbers.
76. The 2’s complement of 0101 is
A) 1010
B) 1011
C) 0110
D) 1101
Answer: B
Explanation: 1’s complement → 1010; add 1 → 1011.
77. The main advantage of CMOS logic is
A) High power
B) Low speed
C) Low power
D) High current
Answer: C
Explanation: CMOS has low power dissipation.
78. TTL logic stands for
A) Transistor Transistor Logic
B) Transistor Transfer Logic
C) Two Transistor Logic
D) Triode Transistor Logic
Answer: A
Explanation: TTL = Transistor–Transistor Logic, uses BJTs.
79. A counter which counts both up and down is
A) Mod counter
B) Asynchronous counter
C) Up/Down counter
D) Ring counter
Answer: C
Explanation: Up/down counter can count incrementally or decrementally.
80. A BCD counter has maximum count of
A) 9
B) 10
C) 15
D) 16
Answer: B
Explanation: BCD counter counts 0000–1001 (0–9).
81. The main function of an encoder is to
A) Decode signals
B) Convert multiple inputs to binary code
C) Perform addition
D) Generate parity
Answer: B
Explanation: Encoder converts active input into coded binary output.
82. A 7-segment display can display
A) Only numbers
B) Only letters
C) Numbers and some letters
D) Symbols only
Answer: C
Explanation: Displays digits (0–9) and few letters (A, b, C, etc.)
83. The term “setup time” in flip-flop means
A) Time before clock when input must be stable
B) Time after clock
C) Clock duration
D) Propagation delay
Answer: A
Explanation: Setup time = minimum time input must be stable before clock edge.
84. The “hold time” of a flip-flop means
A) Time after clock edge when input must remain stable
B) Time before clock
C) Pulse width
D) Delay
Answer: A
Explanation: Hold time = time after clock during which input must not change.
85. Which logic gate is used as a basic building block for memory circuits?
A) XOR
B) NAND
C) NOR
D) Both B and C
Answer: D
Explanation: NAND and NOR gates are used to build latches and flip-flops.
86. A register is a group of
A) Gates
B) Counters
C) Flip-flops
D) Multiplexers
Answer: C
Explanation: Registers are made of flip-flops to store multiple bits.
87. The output of a T flip-flop toggles when
A) T = 1
B) T = 0
C) Clock = 0
D) Reset = 1
Answer: A
Explanation: T=1 → output toggles at each clock pulse.
88. The speed of a digital circuit is mainly determined by
A) Power supply
B) Propagation delay
C) Number of inputs
D) Fan-out
Answer: B
Explanation: Propagation delay defines how fast output changes.
89. “Fan-out” means
A) No. of inputs to a gate
B) No. of outputs from a gate
C) No. of gates driven by one output
D) Power consumption
Answer: C
Explanation: Fan-out = number of inputs that can be driven by a gate output.
90. A 4-bit shift register can store
A) 1 bit
B) 2 bits
C) 4 bits
D) 8 bits
Answer: C
Explanation: Each flip-flop stores one bit → 4-bit register stores 4 bits.
91. The primary function of parity bit is
A) Error detection
B) Error correction
C) Data storage
D) Speed improvement
Answer: A
Explanation: Parity bits help detect single-bit errors in data transmission.
92. The output of XOR gate for inputs (1,1,0,0) is
A) 0
B) 1
C) Undefined
D) Depends on circuit
Answer: A
Explanation: XOR gives 1 if odd number of 1’s; here 2 → even → 0.
93. Which logic family has highest speed?
A) ECL
B) CMOS
C) TTL
D) RTL
Answer: A
Explanation: Emitter Coupled Logic (ECL) has very high switching speed.
94. A 2-bit comparator gives high output when
A) A > B
B) A = B
C) A < B
D) None
Answer: B
Explanation: Comparator outputs 1 when both binary inputs are equal.
95. The clock frequency of a 3-bit ripple counter is 8 kHz. Output frequency at last flip-flop = ?
A) 4 kHz
B) 2 kHz
C) 1 kHz
D) 500 Hz
Answer: C
Explanation: Each stage divides frequency by 2 → 8/8 = 1 kHz.
96. The binary value 11111111 equals
A) 255 (decimal)
B) 256
C) 127
D) 128
Answer: A
Explanation: 11111111₂ = 2⁸−1 = 255.
97. The octal equivalent of binary 110010 is
A) 62
B) 72
C) 64
D) 61
Answer: A
Explanation: 110010 → 110 010 → 6 2 → 62₈.
98. The Gray code equivalent of binary 1011 is
A) 1110
B) 1101
C) 1001
D) 1010
Answer: A
Explanation: Gray code → MSB same; each next bit = XOR of previous two → 1110.
99. A 10-bit register can store up to
A) 1024 combinations
B) 512 combinations
C) 256 combinations
D) 100 combinations
Answer: A
Explanation: 2¹⁰ = 1024 possible binary states.
100. The basic function of a digital system is
A) Amplification
B) Signal conversion
C) Information processing
D) Frequency generation
Answer: C
Explanation: Digital systems process information using binary logic.
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