Digital Circuits - Two Questions on Logic Gates

“God used beautiful mathematics in creating the world.”

– P. A. M. Dirac

All India Pre-Medical/Pre-Dental Entrance Test (AIPMT) held on 3^rd May, 2015 has been cancelled by the Supreme Court. A fresh test will be conducted by the Central Board of Secondary Examination (CBSE), in accordance with the Supreme Court Judgment of June 15.

            Today we shall discuss two questions on digital circuits. The first question appeared in the test paper of the cancelled AIPMT 2015. The second question is a variation of the first question. Both are single correct answer type multiple choice questions. Here are the questions:

(i) Which logic gate is represented by the following combination of logic gates?

(1) NAND

(2) AND

(3) NOR

(4) OR

Note that A and B are the inputs applied to two inverters (NOT gates) and Y₁ and Y₂ are their outputs.which are applied to the input side of a two input NOR gate. Y is the final output of the combination. You can easily solve this question by writing the truth table which is given below:

A    B     Y

0     0    0

0     1    0

1     0    0

1     1     1

The combination evidently implements AND operation. So the correct option is (2).

(ii) Which logic gate is represented by the following combination of logic gates?

(1) AND

(2) NAND

(3) NOR

(4) OR

As in question (i) A and B are the inputs applied to two inverters (NOT gates). Y₁ and Y₂ are their outputs which are applied to the input side of a two input NAND gate. The truth table for the combination is given below:

A     B     Y

0     0     0

0      1     1

1      0     1

1      1     1

The combination evidently implements OR operation. So the correct option is (4).