Question

The combination of logic gates shown in circuit is given input $A=0$ and $B=1$.


The output at $Y$ will be

• A. $0$
• B. $1$
• C. $1$ always irrespective of the inputs
• D. $0$ always irrespective of the inputs

Answer 1

The correct option is B $1$

Let us give the values of inputs $A=0$ and $B=1$ for the given circuit.

The output of each gate is shown in figure below.


$\therefore y=1$

If $A=0$ and $B=0$, then $y=\overline{1.1}=\overline{1}=0$

Therefore, output will not be always 1.

$\text{Alternative method:}$

Output from first gate $\Rightarrow \overline{A}$

Output from 2nd gate $\Rightarrow \overline{B}$

Input to the final gate are $\overline{A}$ and $\overline{B}$

Thus, $y=\overline{\overline{A}.\overline{B}}$

By Boolean algebra,

$\Rightarrow y=\overline{\overline{A}}+\overline{\overline{B}}$

$\Rightarrow y=A+B$

If $A=0,B=1$
$\Rightarrow Y=0+1=1$

Hence, option $\left(B\right)$ is correct.

Why this question? Tip: In the problems involving combination of logic gates, the outputs can be obtained by feeding the input in a manner equivalent to flow of current in a circuit.