If V A and V B are the input voltages (either 5V or 0V) and V₀ is the output voltage then the two gates represented in the following circuit (A) and (B) are :-

Solution

Physics sir · 3 steps

Circuit (A): any one high input lifts the output. Circuit (B): a high input pulls the output low.

1. Circuit A. Each diode points from its input towards the output node, and the output is tied to ground through R. A diode at 5 V is forward biased and carries about 5 V to V₀. A diode at 0 V passes nothing, so the resistor holds V₀ at 0 V only when both inputs are 0 V.

   V_A	V_B	Diodes conducting	V₀ in (A)
   0 V	0 V	none	0 V
   0 V	5 V	D₂	about 5 V
   5 V	0 V	D₁	about 5 V
   5 V	5 V	both	about 5 V

2. Circuit B. For V_B = 0 the npn transistor is cut off, so no collector current flows and V₀ = 5 V. For V_B = 5 V, I_B ≈ (5 - 0.7)/(100 kΩ) = 43 μA and β I_B ≈ 6.5 mA. That is more than the largest possible collector current (5 V)/(1 kΩ) = 5 mA, so the transistor saturates and V₀ ≈ 0.
3. So. Circuit (A) gives a high output whenever at least one input is high, so it is an OR gate. Circuit (B) inverts its single input, so it is a NOT gate. The answer is (b).

The idea to keep. Diodes pointing to the output with a resistor to ground make OR; reverse the diodes and send the resistor to the supply for AND. A transistor with output at its collector inverts: NOT.

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