Question

Two p-n junction diodes \(D_1\) and \(D_2\) are connected as shown in the figure. \(A\) and \(B\) are input signals and \(C\) is the output. The given circuit will function as a _______. 

• A. NOR Gate
• B. NAND Gate
• C. AND Gate
• D. OR Gate

Hint:

In diode logic with a pull-up resistor, diodes pulling the output LOW usually implement an AND gate.

Answer 1

The Correct Option is C

The given circuit is an arrangement of diodes connected to implement a logic gate. Let's analyze the behavior of the circuit with respect to input signals \(A\) and \(B\) and determine the logic function it performs: 

1. In the given circuit, both diodes \(D_1\) and \(D_2\) are positioned such that when a voltage is applied, they permit current flow if they are forward-biased.
2. For a diode to be forward-biased, the anode must be at a higher potential than the cathode.
3. Consider the scenarios for inputs:
   • When both \(A = 1\) (high) and \(B = 1\) (high): Both diodes \(D_1\) and \(D_2\) are reverse-biased because their anodes are connected to high potential, blocking any current flow from the \(5V\) source to ground through the diodes. Hence, a current path through the resistor \(R\) is established, pulling \(C\) to ground (logic high).
   • When \(A = 0\) (low) or \(B = 0\) (low) or both: At least one diode becomes forward-biased, allowing current flow from the \(5V\) source through the diode to ground. This action keeps the \(C\) at ground potential (logic low).
4. From this analysis, the output \(C\) is high (\(1\)) only when both inputs \(A\) and \(B\) are high. If any one or both inputs are low, the output \(C\) remains low (\(0\)).

The truth table for this operation is:

A	B	C (Output)
0	0	0
0	1	0
1	0	0
1	1	1

This is the characteristic behavior of an AND Gate. Therefore, the given circuit functions as an AND Gate.