What are universal gates? Explain with suitable example.

Answer: – The NAND gate and nor gate are universal gate because its repeated use can produce other logic gates. The table below show how NAND gates can be connected to produce inverter (i.e., NOT gate) AND gate and OR gate.

universal.PNG

(1) NOT gate from NAND gate: – When two inputs to NAND gate are joined to gather so that has one input, the resulting circuit is NOT gate. The truth tables also show this fact.

A

BC=A

Y

0 0 1
1 1 0

(2) AND gate from NAND gates: – For this purpose, we use two gates in a manner as shown above. The output of first NAND gate is given to the second NAND gate. Acting as inverter (i.e., inputs of NAND gate joined) the resulting circuit is the AND gate. The output Y’ of first NAND gate (AND gate followed by NOT gate) is inverted output of AND gate. The second NAND gate acting as inverter further inverts it so that the final output Y is that of AND gate the truth table also show this fact.

A

B

Y’

Y

0 0 1 0
0 1 1 0
1 0 1 0
1 1 0 1

(3) OR gate from NAND gates: – For this purpose, we use three NAND gates in a manner as shown above. The first two NAND gates are operated as NOT gates and their output are fed to the third. The resulting circuit is OR gate. The fact is also indicated by the truth table.

A

B

Y’=A̅

Y”=B̅

Y

0 0 1 1 0
0 1 1 0 1
1 0 0 1 1
1 1 0 0 1
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