What is a universal gate? Give examples. Realize the basic gates with any one universal gate.

Ans:

Universal Gates:  NAND and NOR are termed as Universal gates. The OR, AND and NOT gates can be realized by using any of such two gates.  The complete logic system can be implemented using any of such two gates. These gates are simple to realize and consume less power than the other gates.

Realizations of NOT, AND and OR gates using NAND gates

NOT GATE: Fig. (a) Demonstrates the realization of Inverter (NOT) gate by using NAND gate. Both the inputs to the NAND gates are tied together hence the gate works like an inverter (NOT) gate.

Fig.(a) Realization of Inverter (NOT) gate using NAND gate

AND GATE: Fig. (b) Demonstrates the realization of AND gate by using two NAND gates. This has combination of two NAND gates provides AND operation. The first NAND gate has two inputs as A and B. The 2-inputs to the second NAND gate are tied together and the output (AB)‾ of the primary gate is fed to this universal terminal. The output is AB therefore giving AND operation.

Fig. (b) Realization of AND gate using NAND gates

OR GATE: Fig. (c) Demonstrates the realization of OR gate by using NAND gates. The two inputs from the entire first two NAND gates are tied together and fed through A and B as demonstrated in the figure. The outputs are as A‾ and B‾. The outputs are fed to as inputs to third NAND gate. The last output is A OR B therefore giving OR operation.

Fig. (c) Realization of OR gate using NAND gates