It is important to memorize logic symbols, Boolean equations, and truth tables for logic gates.
Answer: A
Correct answer: True
The symbol, the Boolean expression and the truth table are three views of the same gate, and reading a circuit means moving between them without hesitation, which is why they are worth knowing by heart.
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Which of the following circuit can be used as parallel to serial converter ?
Answer: A
Correct answer: Multiplexer
Stepping a multiplexer's select lines through their values in turn presents its parallel inputs one after another at the single output, which is parallel to serial conversion. A demultiplexer performs the reverse.
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Which of the following logical operations is represented by the + sign in Boolean algebra?
Answer: C
Correct answer: OR
In Boolean algebra the plus sign denotes OR and the dot, or juxtaposition, denotes AND. Complementation is written with a prime or an overbar.
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Output will be a LOW for any case when one or more inputs are zero for a(n):
Answer: C
Correct answer: AND gate
An AND gate outputs 0 whenever any input is 0, since it needs all of them to be 1. An OR gate would be LOW only if every input were 0.
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The output of an exclusive-OR gate is HIGH if ________.
Answer: C
Correct answer: the inputs are unequal
XOR is the inequality detector: it gives 1 when its two inputs differ and 0 when they agree. With more than two inputs it gives 1 when an odd number of them are 1, which is the same rule generalised.
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The Boolean expression for a 3-input AND gate is ________.
Answer: B
Correct answer: X = ABC
AND is written as a product, so a three-input AND is the product of all three variables. The plus sign would make it an OR, and \( AB+C \) mixes the two operations.
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The time required for a gate or inverter to change its state is called
Answer: C
Correct answer: Propagation time
Propagation delay is the interval between a change at a gate's input and the corresponding change at its output. Rise and decay times describe the shape of the transition itself rather than the lag between input and output.
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The power dissipation, PD, of a logic gate is the product of the ________.
Answer: B
Correct answer: dc supply voltage and the average supply current
Power dissipation is
\[ P_D = V_{CC}\times I_{CC(avg)} \]
The average current is used because the current drawn varies as the gate switches; the peak value would overstate the power, and the supply is dc, not ac.
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The logic gate that will have a LOW output when any one of its inputs is HIGH is the:
Answer: C
Correct answer: NOR gate
NOR gives 0 whenever any input is 1, since OR would give 1 there. An OR gate would give a HIGH output in that case, and NAND goes LOW only when all inputs are HIGH.
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Which of the following adders can add three or more numbers at a time ?
Answer: C
Correct answer: Carry-save-adder
A carry-save adder reduces three numbers to two - a partial sum and a carry vector - without propagating carries, so several numbers can be added at once and the carries resolved only at the final step. The other adders take two operands at a time.
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