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OR, AND, NOT, NAND and NOR logic gates: NEET MCQs with answers

51 questions on this topic of Electronic Devices (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following logic gates produces an output of logic 0 only when both of its inputs are logic 1?

  1. AND gate
  2. OR gate
  3. NOR gate
  4. NAND gate
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Answer: (D) NAND gate

A NAND gate performs an inverted AND operation. Its truth table shows that the output is 0 if and only if all inputs are 1; otherwise, the output is 1.

medium

2. The input waveforms applied to a two-input logic gate are A and B. In four successive clock periods, (A, B) takes values (0, 0), (1, 0), (0, 1), and (1, 1). The corresponding output sequence observed is 1, 0, 0, 0. Identify the logic gate.

  1. NAND gate
  2. AND gate
  3. NOR gate
  4. OR gate
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Answer: (C) NOR gate

The output sequence is 1 only for (0, 0) and 0 for (1, 0), (0, 1), and (1, 1). This is the exact truth table of a NOR gate, where the output is HIGH only when all inputs are LOW.

hard

3. A logic network has three inputs: A, B, and C. The function is defined such that the output Y is 1 if and only if an odd number of inputs are 1. If A = 1, B = 0, and C = 1, what is the output Y, and what type of gate network performs this operation?

  1. Y = 1, Odd parity (XOR) network
  2. Y = 0, Odd parity (XOR) network
  3. Y = 0, Majority logic network
  4. Y = 1, AND-OR network
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Answer: (B) Y = 0, Odd parity (XOR) network

For inputs A = 1, B = 0, C = 1, the total number of HIGH inputs is 1 + 0 + 1 = 2, which is an even number. Since the output is 1 only for an odd number of 1s, Y must be 0. This multi-input modulo-2 addition is implemented by an XOR (odd parity) network.

medium

4. Consider the following statements: Statement I: A NAND gate is obtained by connecting a NOT gate at the output terminal of an AND gate. Statement II: The Boolean expression for a two-input NAND gate with inputs A and B is Y = A + B. In the light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are true
  2. Both Statement I and Statement II are false
  3. Statement I is true but Statement II is false
  4. Statement I is false but Statement II is true
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Answer: (C) Statement I is true but Statement II is false

A NAND gate is a combination of an AND gate followed by a NOT gate, so Statement I is true. The Boolean expression for a NAND gate is Y = (A · B)' (NOT of AND), while Y = A + B represents an OR gate. Thus, Statement II is false.

easy

5. In the digital circuit where an input A is connected directly to one terminal of a 2-input AND gate, and the other terminal is connected to logic level '0' permanently, the output of the gate will be:

  1. Equal to NOT A
  2. Always 1
  3. Equal to A
  4. Always 0
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Answer: (D) Always 0

For an AND gate, the output is given by Y = A · B. Since one input B is permanently tied to 0, Y = A · 0 = 0 regardless of the value of A. Hence, the output remains logic 0 at all times.

hard

6. A digital signal defined by logic levels 0 V (LOW) and 5 V (HIGH) is fed to a circuit. Input A has a square wave of frequency 1 kHz and 50% duty cycle. Input B is held continuously at 5 V. If A and B are connected to a two-input NOR gate, the output signal is:

  1. A constant 5 V level
  2. A constant 0 V level
  3. A 1 kHz square wave inverted with respect to A
  4. A 2 kHz square wave
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Answer: (B) A constant 0 V level

For a NOR gate, the output is given by Y = (A + B)'. Since input B is continuously HIGH (5 V or logic 1), the sum (A + 1) is always 1, regardless of the state of A. The complement of 1 is always 0. Hence, the output remains continuously at 0 V.

easy

7. Consider the following two statements regarding semiconductor logic gates: Statement I: NAND and NOR gates are termed universal gates because any logic function or gate can be realized using only one of these types. Statement II: A NOT gate can be constructed using a single two-input NOR gate by joining its two inputs together. In the light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are true.
  2. Both Statement I and Statement II are false.
  3. Statement I is true but Statement II is false.
  4. Statement I is false but Statement II is true.
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Answer: (A) Both Statement I and Statement II are true.

Statement I is correct because repeated use of only NAND or only NOR gates can realize all basic logic operations (AND, OR, NOT). Statement II is also correct because tying the two inputs of a NOR gate together gives Y = (A + A)' = A', which is the NOT operation.

medium

8. A logic circuit produces an output Y = 0 only when both inputs A and B are 1. For all other combinations of inputs, Y = 1. What type of gate is this?

  1. AND gate
  2. NAND gate
  3. OR gate
  4. NOR gate
Show answer and explanation

Answer: (B) NAND gate

A NAND gate produces a LOW (0) output only when all its inputs are HIGH (1). When any or both inputs are 0, its output is 1.

medium

9. A two-input logic gate has the property that its output is logic 1 if and only if both inputs are identical (i.e., A = B). Which of the following expressions represents its output Y?

  1. (A · B)'
  2. A · B' + A' · B
  3. (A + B)'
  4. A · B + A' · B'
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Answer: (D) A · B + A' · B'

When A = 0 and B = 0, Y = 0·0 + 1·1 = 1. When A = 1 and B = 1, Y = 1·1 + 0·0 = 1. For unequal inputs (0,1 or 1,0), Y = 0. Thus, Y = A · B + A' · B' represents the XNOR gate which gives 1 when inputs are equal.

medium

10. In a logic gate circuit, input A is given the binary sequence 1010 and input B is given the binary sequence 1100. If these inputs are passed through a NOR gate, what is the resulting output sequence?

  1. 0101
  2. 1110
  3. 0001
  4. 0010
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Answer: (C) 0001

For each bit pair (A, B): (1, 1) gives (1+1)' = 0; (0, 1) gives (0+1)' = 0; (1, 0) gives (1+0)' = 0; (0, 0) gives (0+0)' = 1. The resulting output sequence is 0001.

medium

11. In a digital system, the inputs A and B undergo a sequence of states: (0, 0) → (0, 1) → (1, 0) → (1, 1). The corresponding measured outputs Y are 1, 0, 0, 0. If both inputs are now inverted before feeding them into this same gate, the sequence of outputs will be:

  1. 0, 1, 1, 1
  2. 1, 0, 0, 1
  3. 0, 0, 0, 1
  4. 1, 1, 1, 0
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Answer: (C) 0, 0, 0, 1

The initial gate produces 1 only for (0,0), which identifies it as a NOR gate: Y = (A + B)'. If inputs are inverted to A' and B', the new output is Y' = (A' + B')' = A · B (an AND gate). Thus, for the original sequence of inputs (0,0), (0,1), (1,0), (1,1), the new output will be 0, 0, 0, 1.

medium

12. A logic circuit receives two binary inputs A and B. Input A is inverted using a NOT gate to obtain A'. Then, A' and B are provided as inputs to a NAND gate. For which of the following combinations of (A, B) will the circuit produce a LOW (0) output?

  1. A = 1, B = 1
  2. A = 0, B = 1
  3. A = 0, B = 0
  4. A = 1, B = 0
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Answer: (B) A = 0, B = 1

The output of the NAND gate is Y = (A' · B)'. A NAND gate yields 0 only when both of its inputs are 1. Here, the two inputs are A' and B. For both inputs to be 1, we must have A' = 1 (which means A = 0) and B = 1. Thus, the only combination giving an output of 0 is A = 0, B = 1.

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