Question

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

Answer 1

In both the given circuits, $A$ and $B$ are the inputs and $Y$ is the output.

For first circuit, The output of the left NAND gate will be $\overline{AB}$.

Hence, the output of the combination of the two NAND gates is given as:

$Y=\overline{\overline{A.B}.\overline{A.B}}=AB$

Hence, this circuit functions as an AND gate.

For the second circuit, $\overline{A}$ is the output of the upper left of the NAND gate and $\overline{B}$ is the output of the lower half of the NAND gate, as shown in the following figure.

Hence, the output of the combination of the NAND gates will be given as:

$Y=\overline{\overline{A}+\overline{B}}=A+B$

Hence, this circuit functions as an OR gate.