question_answer

Which logic gate is represented by the following combination of logic gates?

A) OR         

B)                       NAND                  

C)       AND

D)       NOR

Correct Answer: C

Solution :

                Key Idea: First two similar gates are NOT gates and the third one is NOR gate. The simple diagram can be shown as: The output of gate -\[1=\overline{A}\] The output of gate-2 = B The output of gate-3 \[y=\overline{\overline{A}+\overline{B}}\] Use Demorgans theorem \[\overline{\overline{A}+\overline{B}}=\overline{\overline{A}}.\overline{\overline{B}}\] Hence,    \[y=\overline{\overline{A}}.\overline{\overline{B}}\] \[y=A.B\] This is the Boolean expression of AND gate. Note: We can define NOR gate as: The OR gate followed by NOT gate as shown.