Q18 of 19 Page 510

Write the truth table for circuit given in Fig. 14.47 below consisting of NOR gates and identify the logic operation (OR, AND, NOT) which this circuit is performing.


(Hint: A = 0, B = 1 then A and B inputs of second NOR gate will be 0 and hence Y = 1. Similarly, work out the values of Y for other combinations of A and B. Compare with the truth table of OR, AND, NOT gates and find the correct one.)

The given circuit is as follows:


The first NOR gate gives the output (A + B)’. The second NOR gate gives the output ((A + B)’ + (A + B)’)’.


Using De Morgan’s law,


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


So, the output is Y = A + B and the circuit acts as an OR gate.


The truth table for the given circuit is as follows:





























Input (A)



Input (B)



Output (Y)



0



0



0



0



1



1



1



0



1



1



1



1



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


More from this chapter

All 19 →
15

You are given the two circuits as shown in Fig. 14.44. Show that circuit (a) acts as OR gate while the circuit (b) acts as AND gate.

 16

Write the truth table for a NAND gate connected as given in Fig. 14.45.


Hence identify the exact logic operation carried out by this circuit.

 17

You are given two circuits as shown in Fig. 14.46, which consist of NAND gates. Identify the logic operation carried out by the two circuits.

 19

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.