Question

Write the truth table for circuit given in Fig. 14.47 below consistingof NOR gates and identify the logic operation (OR, AND, NOT) whichthis circuit is performing.(Hint: A = 0, B = 1 then A and B inputs of second NOR gate will be 0and hence Y=1. Similarly work out the values of Y for othercombinations of A and B. Compare with the truth table of OR, AND,NOT gates and find the correct one.)

Answer 1

Let Y 1 be the output of first NOR gate.

The output of the first NOR gate is given as,

Y 1 = A+B ¯

The output of the second OR gate is given as,

Y 2 = A+B ¯

The net output is given as,

Y= Y 1 ⋅ Y 2 ¯

By substituting the value of Y 1 and Y 2 in above equation, we get

Y= A+B ¯ ⋅ A+B ¯ ¯ = A ¯ ⋅ B ¯ ¯ + A ¯ ⋅ B ¯ ¯ = A ¯ ¯ + B ¯ ¯ =A+B

The truth table is given as,

A	B	Y=( A+B )
0	0	0
0	1	1
1	0	1
1	1	1

The above truth table represents OR gate.

Thus, the circuit behaves like an OR Gate.