Logic Gates Questions and Answers

Q: How is the complement nature of NOR represented in its graphic symbol ?

The complement in a NOR symbol is shown by a small circle.A NOR gate is an OR gate followed by inversion, and its symbol is the OR shape with a small circle, called a bubble, at the output. The same bubble marks inversion everywhere in logic symbols: on the output of NAND and NOT gates, and on inputs that are active-low.The triangle is the body of the buffer and NOT symbols, not a mark of complement. Squares and diamonds are not used to show inversion in the standard gate symbols. A NOR is simply 'NOT OR', so the bubble that turns the OR symbol into a NOR is the graphic mark of its complement nature.

Q: The following diode circuit acts as a :

The circuit acts as an AND gate.The diodes' cathodes face the inputs and their anodes meet at the output, which is pulled up through R towards the positive supply. If either input is low, its diode conducts and holds the output low. Only when both inputs are high are both diodes reverse biased (or off), so the output rises to the high level.The output is high only when all inputs are high, the AND function. An OR gate would have the diodes reversed (anodes at the inputs) with the resistor to ground. NAND and NOR need an inverting stage, which a diode network alone cannot provide.

Q: Given Truth Table represents which of the following Logic Gate ?InputOutputABQ001011101110

The gate is the NAND gate, option (B).The table gives output 0 only when both inputs are 1, and output 1 for every other input pair. That is the NOT of the AND function: \(Q = \overline{A \cdot B}\). The NAND symbol is the AND shape with a small circle (the inversion bubble) on the output, which is option (B).An AND gate (A) would give 1 only for input 1, 1, the reverse of the table. An OR gate (C) would give 0 only for input 0, 0, and its output is 1 at 1, 1. A NOR gate (D) would give 1 only for input 0, 0. None of these matches the table, whose only 0 is at 1, 1.

Q: What type of gates are NAND and NOR gates considered to be ?

NAND and NOR are considered standard logic gates.NAND and NOR are among the basic gates available in every logic family, such as the 7400 (NAND) and 7402 (NOR) in TTL and the 4011 and 4001 in CMOS. They are also called universal gates, because any Boolean function can be built from NAND gates alone or from NOR gates alone, and in CMOS they are the simplest gates to build.They are not special-purpose or complex gates; complex gates (such as AND-OR-INVERT) combine several functions in one stage. They are certainly not outdated, since they are the most widely used gates in digital design.

Q: From the circuit of the following logic gates the basic logic gate obtained is ................

The circuit is a NAND gate.Follow the signals through. The NOT gates give \(\bar{A}\) and \(\bar{B}\). The NOR gate then gives, by De Morgan's theorem,\[ \overline{\bar{A} + \bar{B}} = A\cdot B \]The final NAND gate takes \(AB\) and \(B\):\[ Y = \overline{(AB)\cdot B} = \overline{AB} \]since \(AB\cdot B = AB\). So \(Y\) is A NAND B.The output depends on both inputs, so it is not a NOT gate, and it is the complement of AND, so it is neither AND nor OR: for \(A = B = 0\) it gives 1, which an AND or OR gate would not.

Q: What is the output of a 2-input AND gate if the inputes are A = 1, B = 0?

The output is 0.An AND gate outputs 1 only when all its inputs are 1. With A = 1 and B = 0, one input is 0, so the output is 0 (\(1 \cdot 0 = 0\)).1 would be the output of an OR gate, '10' is not a single-bit output, and a logic gate's output is always defined for valid inputs.The AND gate behaves like multiplication of bits, so its output is 1 only for the input pair 1, 1.

Q: Which logic gate can be used to implement any Boolean function ?

The gate is the NAND gate.NAND is a universal gate: NOT, AND and OR can each be built from NAND gates alone. NOT A is A NAND A, AND is a NAND followed by a NAND inverter, and OR follows from De Morgan's law as (NOT A) NAND (NOT B). So any Boolean function can be made from NAND gates. NOR is the other universal gate.AND and OR cannot produce inversion, so they cannot make NOT on their own. XOR alone cannot build every function either, so it is not universal.

Q: Which logic gate is called a universal gate ?

The universal gate is NAND.A gate is called universal when every other logic function can be built from it alone. Tying the inputs of a NAND gate together gives NOT; a NAND followed by a NOT gives AND; and NOT-ing the inputs of a NAND gives OR (by De Morgan's theorem). From NOT, AND and OR any Boolean function follows. NOR is the other universal gate.AND, OR and XOR cannot do this on their own: none of them can produce inversion (NOT) from its inputs, so they are not universal.

Q: Which logic gate gives output 1 only when the inputs are different ?

The gate is XOR (exclusive OR).An XOR gate outputs 1 when its two inputs differ (0 and 1, or 1 and 0) and 0 when they are the same (both 0 or both 1). That is why it is used to compare bits and to form the sum bit in an adder.OR gives 1 whenever at least one input is 1, including when both are 1. AND gives 1 only when both inputs are 1, which is when they are the same. NOT has a single input and simply inverts it.

Q: Which one of the following is an universal logic gate ?

The universal gate is NAND.A universal gate can be used alone to build any Boolean function. With NAND, tying both inputs together gives NOT, a NAND followed by NOT gives AND, and NOT on each input followed by NAND gives OR (by De Morgan's law). Since AND, OR and NOT are complete, so is NAND. NOR is the other universal gate.AND and OR cannot produce inversion on their own, and NOT has only one input, so none of them is universal.

21.

How is the complement nature of NOR represented in its graphic symbol ?

A.A small square
B.A small triangle
C.A small circle
D.A small diamond

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: C

The complement in a NOR symbol is shown by a small circle.

A NOR gate is an OR gate followed by inversion, and its symbol is the OR shape with a small circle, called a bubble, at the output. The same bubble marks inversion everywhere in logic symbols: on the output of NAND and NOT gates, and on inputs that are active-low.

The triangle is the body of the buffer and NOT symbols, not a mark of complement. Squares and diamonds are not used to show inversion in the standard gate symbols. A NOR is simply 'NOT OR', so the bubble that turns the OR symbol into a NOR is the graphic mark of its complement nature.

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22.

The following diode circuit acts as a :

Diodes D1 and D2 have their cathodes at the inputs Vin1 and Vin2 and their anodes joined at the output Vout, which is connected through a resistor R to a +0.5 supply

A.AND gate
B.OR gate
C.NAND gate
D.NOR gate

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: A

The circuit acts as an AND gate.

The diodes' cathodes face the inputs and their anodes meet at the output, which is pulled up through R towards the positive supply. If either input is low, its diode conducts and holds the output low. Only when both inputs are high are both diodes reverse biased (or off), so the output rises to the high level.

The output is high only when all inputs are high, the AND function. An OR gate would have the diodes reversed (anodes at the inputs) with the resistor to ground. NAND and NOR need an inverting stage, which a diode network alone cannot provide.

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23.

Given Truth Table represents which of the following Logic Gate ?

Input		Output
A	B	Q
0	0	1
0	1	1
1	0	1
1	1	0

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: B

The gate is the NAND gate, option (B).

The table gives output 0 only when both inputs are 1, and output 1 for every other input pair. That is the NOT of the AND function: \(Q = \overline{A \cdot B}\). The NAND symbol is the AND shape with a small circle (the inversion bubble) on the output, which is option (B).

An AND gate (A) would give 1 only for input 1, 1, the reverse of the table. An OR gate (C) would give 0 only for input 0, 0, and its output is 1 at 1, 1. A NOR gate (D) would give 1 only for input 0, 0. None of these matches the table, whose only 0 is at 1, 1.

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24.

What type of gates are NAND and NOR gates considered to be ?

A.Special-purpose gates
B.Complex gates
C.Standard logic gates
D.Outdated gates

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: C

NAND and NOR are considered standard logic gates.

NAND and NOR are among the basic gates available in every logic family, such as the 7400 (NAND) and 7402 (NOR) in TTL and the 4011 and 4001 in CMOS. They are also called universal gates, because any Boolean function can be built from NAND gates alone or from NOR gates alone, and in CMOS they are the simplest gates to build.

They are not special-purpose or complex gates; complex gates (such as AND-OR-INVERT) combine several functions in one stage. They are certainly not outdated, since they are the most widely used gates in digital design.

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25.

From the circuit of the following logic gates the basic logic gate obtained is ................

Logic circuit: inputs A and B each pass through a NOT gate; the two NOT outputs feed a NOR gate; the NOR output and B itself feed a NAND gate with output Y

A.NOT gate
B.AND gate
C.OR gate
D.NAND gate

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: D

The circuit is a NAND gate.

Follow the signals through. The NOT gates give \(\bar{A}\) and \(\bar{B}\). The NOR gate then gives, by De Morgan's theorem,

\[ \overline{\bar{A} + \bar{B}} = A\cdot B \]

The final NAND gate takes \(AB\) and \(B\):

\[ Y = \overline{(AB)\cdot B} = \overline{AB} \]

since \(AB\cdot B = AB\). So \(Y\) is A NAND B.

The output depends on both inputs, so it is not a NOT gate, and it is the complement of AND, so it is neither AND nor OR: for \(A = B = 0\) it gives 1, which an AND or OR gate would not.

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26.

What is the output of a 2-input AND gate if the inputes are A = 1, B = 0?

A.1
B.0
C.10
D.Undefined

Exam Year(s)

Tripura Public Service Commission - 2025,

Answer: B

The output is 0.

An AND gate outputs 1 only when all its inputs are 1. With A = 1 and B = 0, one input is 0, so the output is 0 (\(1 \cdot 0 = 0\)).

1 would be the output of an OR gate, '10' is not a single-bit output, and a logic gate's output is always defined for valid inputs.

The AND gate behaves like multiplication of bits, so its output is 1 only for the input pair 1, 1.

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27.

Which logic gate can be used to implement any Boolean function ?

A.AND gate
B.OR gate
C.NAND gate
D.XOR gate

Exam Year(s)

Tripura Public Service Commission - 2026,

Answer: C

The gate is the NAND gate.

NAND is a universal gate: NOT, AND and OR can each be built from NAND gates alone. NOT A is A NAND A, AND is a NAND followed by a NAND inverter, and OR follows from De Morgan's law as (NOT A) NAND (NOT B). So any Boolean function can be made from NAND gates. NOR is the other universal gate.

AND and OR cannot produce inversion, so they cannot make NOT on their own. XOR alone cannot build every function either, so it is not universal.

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28.

Which logic gate is called a universal gate ?

A.AND
B.OR
C.NAND
D.XOR

Exam Year(s)

Tripura Public Service Commission - 2026,

Answer: C

The universal gate is NAND.

A gate is called universal when every other logic function can be built from it alone. Tying the inputs of a NAND gate together gives NOT; a NAND followed by a NOT gives AND; and NOT-ing the inputs of a NAND gives OR (by De Morgan's theorem). From NOT, AND and OR any Boolean function follows. NOR is the other universal gate.

AND, OR and XOR cannot do this on their own: none of them can produce inversion (NOT) from its inputs, so they are not universal.

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29.

Which logic gate gives output 1 only when the inputs are different ?

A.OR
B.AND
C.NOT
D.XOR

Exam Year(s)

Tripura Public Service Commission - 2026,

Answer: D

The gate is XOR (exclusive OR).

An XOR gate outputs 1 when its two inputs differ (0 and 1, or 1 and 0) and 0 when they are the same (both 0 or both 1). That is why it is used to compare bits and to form the sum bit in an adder.

OR gives 1 whenever at least one input is 1, including when both are 1. AND gives 1 only when both inputs are 1, which is when they are the same. NOT has a single input and simply inverts it.

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30.

Which one of the following is an universal logic gate ?

A.AND
B.OR
C.NAND
D.NOT

Exam Year(s)

Tripura Public Service Commission - 2026,

Answer: C

The universal gate is NAND.

A universal gate can be used alone to build any Boolean function. With NAND, tying both inputs together gives NOT, a NAND followed by NOT gives AND, and NOT on each input followed by NAND gives OR (by De Morgan's law). Since AND, OR and NOT are complete, so is NAND. NOR is the other universal gate.

AND and OR cannot produce inversion on their own, and NOT has only one input, so none of them is universal.

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