Question

For the combination of logic gates shown in the figure, the equivalent logic gate is

• A. AND
• B. NOT
• C. NAND
• D. NOR

Answer 1

Answer: (D)

$Y_1=A+B;Y_2=A+B$, as gate 1 and 2 are OR gates.
As gate 3 is NAND gate, its output will be
$Y=\overline{Y_1{Y}_2}={\bar{Y}}_1+{\bar{Y}}_2=\overline{A+B\cdot A+B}$
$=\overline{A+B}+\overline{A+B}=\overline{A+B}$
Using De-Morgan's theorem and identity $A+A=A$
So, given output is that of NOR gate.