ISRO2017-23

Question

What is the minimum number of two-input $\text{NAND}$ gates used to perform the function of two-input $\text{OR}$ gate?

• A. One
• B. Two
• C. Three
• D. Four

Comments

It is direct question.

Ans =3

Answer 1

Need only 3 NAND Gate : A+B = ((A + B)')' = ( A' .B' )'

Comments

I think answer is 1. Since it is asked minimum number of NAND GATE  and also it is not mentioned that we should use only the NAND GATEs.......so we can use 1 NAND GATE and two NOT GATEs...but if maximum numbers of NAND GATEs would have been asked...then the answer would have been 3.

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Question doesn't say that we can use another gate except Nand gate.

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In th question it is just asked minimum no. of NAND Gates....It is not said that we are bound to use only NAND gates.

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ok thankyou :)

Answer 2

	by using NAND	by using NOR
NOT	1	1
OR	3	2
AND	2	3
NAND	-	4
NOR	4	-
EX-OR	4	5
EX-NOR	5	4

so by using 3 NAND gate we can perform OR gate operation

Comments

@pawan KumarIn Sir, if we have gates with different fan-in and fan-out. will this table be same....?

This table is standard for 2 fan-in and 2 fan-out...?

Answer 3

Gate	Min NAND	MIn NOR
AND	2	3
OR	3	2
NAND	1	4
NOR	4	1
EX-OR	4	5
EX-NOR	5	4

For NOT gate only 1 NAND & NOR gate are enough to realize.

& IF from 2 Input NAND gate we want to realize n-input NAND gate then it will be 2n-3. 

Answer 4

3 nand gates for or and 2 for and.