Digital Circuit Systems

Logic Gates Viva Questions

Logic Gates Viva Questions

Logic Gates Viva Questions, Viva Questions on Logic Gates, Short questions and answer on Logic Gates, Logic Circuit Viva Questions, Engineering Viva Questions, Digital Electronics Viva Questions

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Short Questions and Answers

Q.1. What are logic gates?

Ans. Logic gates are the fundamental building blocks of digital systems. They are circuits in which the output depends upon the inputs according to some logic rules.

Q.2. What is a logic circuit?

Ans. A logic circuit is an electronic circuit that operates on digital signals in accordance with a logic function.

Q.3. What is logic design?

Ans. The interconnection of gates to perform a variety of logical operations is called logic design.

Q.4. What is a truth table?

Ans. A truth table is a table which lists all possible combinations of inputs and the corresponding outputs.

Q.5. What is a positive logic system?

Ans. A positive logic system is one in which the higher of the two voltage levels represents logic 1 and the lower of the two voltage levels represents logic 0.

Q.6. What is a negative logic system?

Ans. A negative logic system is one in which the higher of the two voltage levels represents logic 0 and the lower of the two voltage levels represent logic 1.

Q.7. What is the maximum number of outputs a logic gate can have?

Ans. All logic gates can have only one output.

Q.8. What do you mean by level logic?

Ans. Level logic means voltage levels represent logic 0 and logic 1.

Q.9. What is an AND gate?

Ans. An AND gate is a logic gate with two or more inputs and only one output. It outputs a 1 only when each one of its in is 1. It outputs a 0 even if one of its inputs is a 0. It is a basic gate. It is also called an all or nothing gate. It performs AND operation.

Q.10. Which gate is called an all or nothing gate? Why?

Ans. An AND gate is called an all or nothing gate; because it produces a 1 only in one case when all its inputs are a 1. In all other cases its output is a zero.

Q.11. What is an OR-gate?

Ans. An OR gate is a logic gate with two or more inputs and only one output. It outputs a 1 even if one of its inputs is a 1. It outputs a 0 only when each one of its input is a 0. It is also called an any or all gate. It is a basic gate. It performs OR operation.

Q.12. Which gate is called any or all gate? Why?

Ans. An OR gate is called any or all gate; because it produces a 1 even if one of its inputs is a 1. It produces a 0 only when all the inputs are a 0.

Q.13. What is a NOT gate?

Ans. A NOT gate is a logic gate with only one input and one output. Its output is always the complement of its input. It is a basic gate. It performs NOT operation.

Q.14. What is an inverter?

Ans. An inverter is nothing but a NOT gate.

Q.15. Which gates can be used as inverters in addition to the NOT gate?

Ans. NAND gate, NOR gate. X-OR gate and X-NOR gate can also be used as inverters in addition to the NOT gate.

Q.16. Why AND, OR and NOT gates are called basic gates?

Ans. AND, OR, and NOT gates are called basic gates or basic building blocks because any digital circuit of any complexity can be built by using only these three gates.

Q.17. What is a NAND gate?

Ans. A NAND gate is a logic gate with two or more inputs and only one output. Its output is 0 only when each one of its input is a 1. Its output is 1 even if one of its input is a 0. It performs NAND operation. It is a universal gate. It can be used as an inverter. It is an AND gate followed by an Inverter. Or a NAND gate is a logic circuit whose output is logic 0 if and only if all of its inputs are logic 1.

Q.18. What do you mean by a bubbled OR gate?

Ans. The OR gate with inverted inputs is called a bubbled OR gate. It is also called a negative OR gate. It is an active-LOW OR gate. It is equivalent to a NAND gate.

Q.19. What type of gate is equivalent to a NAND gate followed by an inverter?

Ans. A NAND gate followed by an inverter is equivalent to an AND gate.

Q.20. A bubbled NAND gate is equivalent to which gate?

Ans. A bubbled NAND gate is equivalent to an OR gate.

Q.21. How can a NAND gate be used as an inverter?

Ans. A NAND gate can be used as an inverter by tying all the input terminals together and feeding the signal to be inverted to the common terminal, or by tying all but one input terminal to logic 1 and feeding the signal to be inverted to the remaining terminal.

Q.22. What is a NOR gate?

Ans. A NOR gate is a logic gate with two or more inputs and only one output. Its output is 1 only when each one of its inputs is a 0. Its output is 0 even if one of its inputs is a 1. It performs NOR operation. It is a universal gate. It can be used as an inverter also. It is an OR gate followed by an Inverter. Or a NOR gate is a logic gate whose output is logic 1 if and only if all of its inputs are logic 0.

Q.23. How can a NOR gate be used as an inverter?

Ans. A NOR gate can be used as an inverter by tying all the input terminals together and feeding the signal to be inverted to the common terminal, or by tying all but one input terminal to logic 0 and feeding the signal to be inverted to the remaining terminals.

Q.24. What do you mean by a bubbled AND gate?

Ans. The AND gate with inverted inputs is called a bubbled AND gate. It is also called a negative AND gate. It is an active-LOW AND gate. It is equivalent to a NOR gate.

Q.25. A bubbled NOR gate is equivalent to which gate?

Ans. A bubbled NOR gate is equivalent to an AND gate.

Q.26. What type of gate is equivalent to a NOR gate followed by an inverter?

Ans. A NOR gate followed by an inverter is equivalent to an OR gate.

Q.27. Why are NAND and NOR gates called universal gates?

Ans. NAND and NOR gates are called universal gates or universal building blocks because any digital circuit of any complexity can be built by using only NAND gates or only NOR gates.

Q.28. What is an X-OR gate?

Ans. An X-OR gate is a logic gate with only two inputs and one output. Its output is 1 only when its inputs are not equal, i.e. when the inputs do not coincide. Its output is 0 when the inputs are equal, i.e. when the inputs coincide. It is called an anticoincidence gate or inequality detector. It can be used as an inverter. Or an X-OR gate is a two input gate whose output is logic 0 when both the inputs are equal and logic 1 when they are unequal.

Q.29. How can an X-OR gate be used as an inverter?

Ans. An X-OR gate can be used as an inverter by connecting one of its two terminals to logic 1 and feeding the signal to be inverted to other is terminal. It is a controlled inverter.

Q.30. What is an X-NOR gate?

Ans. An X-NOR gate is a logic gate with only two inputs and one output. Its output is 1 only when its inputs are equal, i.e. when the inputs coincide. Its output is 0 when the inputs are not equal, i.e. when the inputs do not coincide. It is called an equality detector or a coincidence gate. It can be used as an inverter. Or an X-NOR gate is a two-input gate whose output is logic 1 when both the inputs are equal and logic 0 when they are unequal.

Q.31. How can an X-NOR gate be used as an inverter?

Ans. An X-NOR gate can be used as an inverter by connecting one input terminal to logic 0 and feeding the signal to be inverted to the other terminal. It is a controlled inverter.

Q.32. How do you realize three or more input X-OR and X-NOR gates?

Ans. Three or more input X-OR and X-NOR gates do not exist. When three or more variables are to be X-ORed or X-NORed, a number of two input X-OR and X-NOR gates may be used.

Q.33. What do you mean by an active-LOW input gate?

Ans. An active-LOW input gate is one for which the inputs are normally in high state and become effective only when they go to the LOW-state.

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