We need to determine the correct truth table for the given diode logic circuit.
1. Analyze the Circuit Components and Logic Levels
From the schematic :
- The inputs $$A$$ and $$B$$ are connected to the cathodes (the pointy/bar side) of the ideal diodes $$D_1$$ and $$D_2$$.
- The anodes (p-side) of both diodes are tied together to the output terminal $$Y$$.
- The output terminal $$Y$$ is connected through a resistor $$R$$ to a $$+5\text{ V}$$ supply voltage.
- Logical High (1): $$+5\text{ V}$$
- Logical Low (0): $$0\text{ V}$$ (Ground)
2. Evaluate the State Combinations
- Case 1: Both inputs are low ($$A = 0\text{ V}, B = 0\text{ V}$$)
Both diodes have $$5\text{ V}$$ on their anode side (via resistor $$R$$) and $$0\text{ V}$$ on their cathode side. This makes both diodes forward-biased (conducting). They act as short circuits to ground, pulling the output voltage at $$Y$$ down to $$0\text{ V}$$.
$$\implies \text{Output } Y = 0$$
- Case 2: One input is low and the other is high ($$A = 5\text{ V}, B = 0\text{ V}$$ or $$A = 0\text{ V}, B = 5\text{ V}$$)
The diode connected to the $$0\text{ V}$$ input becomes forward-biased and conducts, creating a direct path to ground. This forces the entire shared anode junction voltage down to $$0\text{ V}$$. The other diode (connected to $$5\text{ V}$$) will be reverse-biased (non-conducting).
$$\implies \text{Output } Y = 0$$
- Case 3: Both inputs are high ($$A = 5\text{ V}, B = 5\text{ V}$$)
Both sides of the diodes are at a potential of $$5\text{ V}$$. Because there is no potential difference across them to turn them on, no current flows through the diodes (they remain non-conducting). Since no current flows through the resistor $$R$$, there is no voltage drop across it, leaving the output terminal $$Y$$ at the supply potential of $$+5\text{ V}$$.
$$\implies \text{Output } Y = 1$$
3. Establish the Truth Table
The circuit behaves exactly like a standard digital AND gate:
| $$A$$ |
$$B$$ |
$$Y$$ |
| 0 | 0 | 0 |
| 1 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 1 | 1 |
Conclusion
The truth table corresponds directly to Option A.