Q. 195.0( 9 Votes )

Write the truth table for the circuits given in Fig. 14.48 consisting of NOR gates only. Identify the logic operations (OR, AND, NOT) performed by the two circuits.


Answer :

The circuit (a) is as follows:


The circuit gives the output (A + A)’ = A’.


So, the output is Y = A’ and the circuit acts as a NOT gate.


The truth table for the circuit is as follows:



The circuit (b) is as follows:



Similar to the circuit (a), the first two gates give inverted output as A’ and B’. The third NOR gate gives the output (A’ + B’)’.


Using De Morgan’s law,


(A’ + B’)’ = (A’)’∙(B’)’ = A∙B (Since, (A’)’ = A)


Hence, the output is Y = A∙B and the circuit acts as an AND gate.


The truth table is as follows:



NOTE: NOR gate is a universal gate because it can be used to implement any Boolean function without using any other gates.


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