Two p-n junction diodes D₁ and D₂ are connected as shown in figure. A and B are input signals and C is the output. The given circuit will function as a ________.

Solution

Physics sir · 4 steps

Look at which way the diodes point: any low input pulls C down through its diode.

1. Read the diodes. Each diode's anode faces the common point joined to C, and its cathode faces the input. The common point is pulled up to 5 V through R, so current can only flow out of that point towards an input.
2. Input at 0. If A or B is at 0 V, that diode is forward biased and conducts. Almost all of the 5 V drops across R, so C sits near 0 V, which is logic 0.
3. Both inputs high. If both A and B are at 5 V, neither diode has a forward voltage across it. No current flows through R, so C stays at 5 V, which is logic 1.

   A	B	Diodes conducting	C
   0	0	D₁ and D₂	0
   0	1	D₁	0
   1	0	D₂	0
   1	1	none	1

4. So. The output is 1 only when both inputs are 1. That is C = A · B, so the circuit is an AND gate, option (a).

The idea to keep. Diodes pointing away from the output, with a pull-up resistor to the supply, give AND. Diodes pointing towards the output, with a resistor to ground, give OR.

Answer from the NTA final answer key. Solution by PrepWiser.