Digital Circuits Questions and Answers – Logic Gates and Networks – 2

This set of Digital Electronic Circuits Interview Questions and Answers for freshers focuses on “Logic Gates and Networks-2”.

1. A single transistor can be used to build which of the following digital logic gates?
a) AND gates
b) OR gates
c) NOT gates
d) NAND gates

Explanation: A transistor can be used as a switch. That is when base is low collector is high (input zero, output one) and base is high collector is low (input 1, output 0).

2. How many truth table entries are necessary for a four-input circuit?
a) 4
b) 8
c) 12
d) 16

Explanation: For 4 inputs: 24 = 16 truth table entries are necessary.

3. Which input values will cause an AND logic gate to produce a HIGH output?
a) At least one input is HIGH
b) At least one input is LOW
c) All inputs are HIGH
d) All inputs are LOW

Explanation: For AND gate, the output is high only when both inputs are high. That’s why the high output in AND will occurs only when all the inputs are high. However, in case of OR gate, if atleast one input is high, the output will be high.

4. Exclusive-OR (XOR) logic gates can be constructed from what other logic gates?
a) OR gates only
b) AND gates and NOT gates
c) AND gates, OR gates, and NOT gates
d) OR gates and NOT gates

Explanation: Expression for XOR is: A.(B’)+(A’).B
So in the above expression, the following logic gates are used: AND, OR, NOT.
Thus, 2 AND gates with two-inputs and 1 OR gate with two-inputs will be required for constructing a XOR gate.

5. The basic logic gate whose output is the complement of the input is the ___________
a) OR gate
b) AND gate
c) INVERTER gate
d) XOR gate

Explanation: It is also called NOT gate and it simply inverts the input, such that 1 becomes 0 and 0 becomes 1.

6. The AND function can be used to ___________ and the OR function can be used to _____________
a) Enable, disable
b) Disable, enable
c) Synchronize, energize
d) Detect, invert

Explanation: The AND gate and OR gate are used for enabling and disabling respectively because of their multiplicity and additivity property. The AND gate outputs 1 when all inputs are at logic 1, whereas the OR gate outputs 0 when all inputs are at logic 0.

7. The dependency notation “>=1” inside a block stands for which operation?
a) OR
b) XOR
c) AND
d) XNOR

Explanation: The dependency notation “>=1” inside a block stands for OR operation.

8. If we use an AND gate to inhibit a signal from passing one of the inputs must be ___________
a) LOW
b) HIGH
c) Inverted
d) Floating

Explanation: AND gate means A*B and OR gate means A+B and to inhibit means to get low signal, one of the input must be low. It means (0*1=0 or 1*0=0) we will get low output signal. Thus, AND gate outputs 1 only when all inputs are at logic level 1 else it outputs 0.

9. Logic gate circuits contain predictable gate functions that open theirs ____________
a) Outputs
b) Inputs
c) Pre-state
d) Impedance state

Explanation: Logic gate circuits contain predictable gate functions that open their inputs because we are free to give any types of inputs.

10. How many NAND circuits are contained in a 7400 NAND IC?
a) 1
b) 2
c) 4
d) 8

Explanation: 7400 IC’s pin has total 14 pin. Pin no 7 use for GND and pin no 14 used for +vcc and remaining pins used for connections. For a NAND gate two inputs are required and one output is obtained means for NAND gate 3 pin connections are required. Thus, a 7400IC contains 4 NAND gates with each having 3 pins. Therefore, total 12 pins dedicated for the NAND operation. Rest 2 pins for power supply.

Sanfoundry Global Education & Learning Series – Digital Circuits.

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