Question

Draw the circuit diagram of AND gate using diodes.

Answer 1

AND Gate:

1. An AND gate is a sort of logic gate in which the output achieves state $1$ only if and only if all of the inputs are in the state $1$.
2. The logic AND gate and its circuit diagram using diodes is as follows,
   

The Boolean expression of AND gate is $\mathrm{Y}=\mathrm{A}.\mathrm{B}$

The truth table of a two-input AND basic gate is given as follows:

An illustrative diagram of a wired-AND connection is as follows:

(i) When both inputs are 1, the two diodes are reverse biased and there is no current flowing to the ground. So the output is 1 as there is no decrease in voltage around the R resistor.

(ii) When one of the logic inputs is ‘0’, the current flows through the corresponding diode and through the resistor. Thus, the ‘0’ logic will be the diode anode or the output.

The working of the circuit can be explained as follows:

Case(i) A = 0 and B = 0.

When A and B are zero, both diodes are in forward bias condition and they conduct and hence the output will be zero because the supply voltage V will be dropped across R only.

Therefore Y = 0.

Case(ii) A = 0 and B = 1.

When A = 0 and B is high, diode D₁​ is forward biased and diode D₂​ is reverse biased. The diode D₁​ will now conduct due to forward biasing.

Therefore, output Y= 0.

Case(iii) A = 1 and B = 0.

In this case, diode D_2​ will be conducting, and hence the output Y = 0.

Case(iv) A = 1 and B = 1.

In this case, both the diodes are not conducting. Since D₁​ and D₂​ are in OFF condition, no current flows through R. The output is equal to the supply voltage. Therefore Y = 1.

Thus the output will be high only when the inputs A and B are high.