Combinational-Circuits

Question 1

In the following truth table, V = 1 if and only if the input is valid.

What function does the truth table represent?

A
Priority encoder
B
Decoder
C
Multiplexer
D
Demultiplexer
       Digital-Logic-Design       Combinational-Circuits       GATE 2013
Question 1 Explanation: 
It is a 22 × 2 encoder. The inputs have priorities. So, it is a priority encoder.
Question 2

The amount of ROM needed to implement a 4 bit multiplier is

A
64 bits
B
128 bits
C
1 Kbits
D
2 Kbits
       Digital-Logic-Design       Combinational-Circuits       GATE 2012
Question 2 Explanation: 
To implement a 4-bit multiplier we need to store all the possible combinations of 24 x 24 inputs and their corresponding 8 output bits. The total ROM size needed = 28 x 8 bits = 211 bits = 2 Kbits.
Hence option D is the answer.
Question 3

A circuit outputs a digit in the form of 4 bits. 0 is represented by 0000, 1 by 0001, ..., 9 by 1001. A combinational circuit is to be designed which takes these 4 bits as input and outputs 1 if the digit ≥ 5, and 0 otherwise. If only AND, OR and NOT gates may be used, what is the minimum number of gates required?

A
2
B
3
C
4
D
5
       Digital-Logic-Design       Combinational-Circuits       GATE 2004
Question 3 Explanation: 

= A + BD + BC
= A + B (D + C)
So minimum two OR gates and 1 AND gate is required. Hence, in total minimum 3 gates is required.
Question 4
The combinational circuit given below is implemented with two NAND gates. To which of the following individual gates is its equivalent?
A
NOT
B
OR
C
AND
D
XOR
       Digital-Logic-Design       Combinational-Circuits       Nielit Scientist-B CS 22-07-2017
Question 4 Explanation: 
[(a.b)'. (a.b)' ]'= ((a.b)')' + ((a.b)')'
=(a.b)+(a.b)
=(a.b)
There are 4 questions to complete.

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