Question

The output (Y) of logic circuit given below is 0 only when :

• A. A = 1, B = 0
• B. A = 0, B = 0
• C. A = 1, B = 1
• D. A = 0, B = 1

Answer 1

Answer: (B)

A = 0, B = 0

Answer 2

To determine when the output $Y$ of the given logic circuit is 0, we analyze the circuit’s components and their behavior with inputs $A$ and $B$.

Step 1: Identifying Logic Gates
From the circuit:
- There is an OR gate taking inputs $A$ and $B$.
- Another OR gate takes input from the output of the first OR gate and a constant input 1.
- There is an AND gate with inputs $A$ and $B$, whose output feeds into the final OR gate.

Step 2: Analyzing the Behavior of the Circuit
1. The first OR gate outputs 1 if either $A = 1$ or $B = 1$. The output of this OR gate is 0 only if both $A = 0$ and $B = 0$.
2. The second OR gate has one input fixed as 1. Since an OR gate outputs 1 if any input is 1, this gate will always output 1 regardless of the input from the first OR gate.
3. The AND gate outputs 1 only if both $A = 1$ and $B = 1$. In all other cases, it outputs 0.
4. The output $Y$ is determined by the final OR gate, which takes inputs from the AND gate and the second OR gate. Since the second OR gate always outputs 1, the only way for $Y$ to be 0 is for all other inputs (AND gate output) to be 0.

Step 3: Condition for $Y = 0$
For the output $Y$ to be 0, the input conditions must be such that the output from the AND gate is 0 (which happens when $A = 0$ and $B = 0$).

Therefore, the correct condition is:

$A = 0, \, B = 0.$
Hence, the correct option is (2).