Question

You are given two circuits as shown in following figure, which consist of NAND gates. Identify the logic operation carried out by the two circuits.

(a)

(b)

Answer 1

In both the given circuits, A and B are the inputs and Y is the output.

a) The output of the left NAND gate will be `bar("A"."B")`, as shown in the following figure.

Hence, the output of the combination of the two NAND gates is given as:

`"Y" = bar((bar("A.B")).(bar("A.B")))= bar(bar("AB")) + bar(bar("AB")) = "AB"`

Hence, this circuit functions as an AND gate.

(b) `bar"A"` is the output of the upper left of the NAND gate and `bar"B"` is the output of the lower half of the NAND gate, as shown in the following figure.

Hence, the output of the combination of the NAND gates will be given as:

`"Y"  = bar"A".bar"B" = bar(bar"A") + bar(bar"B") = "A"+"B"`

Hence, this circuit functions as an OR gate.