Designing AND gate using Diodes

An AND gate has two or more inputs and a single output. The definition of AND gate states that if one of the inputs is 0 the output will be 0. The figure below shows the structure of the AND gate.

For a two-input AND gate, the relationship between input and output is shown in the truth table below.

You will notice that if any-one of the input is 0 then the output is 0.

Now let us try to understand how you can build AND gate using diodes. But first, you must understand the basic principle on which the diode works.

Look at the figure of the diode below. If you apply some positive voltage V(in) which is greater than Vth (Breakdown voltage of diode) at point A then the diode will act as a closed switch or it will be forward biased. Thus the diode will start conducting. On the other hand, if you apply some negative voltage V(in) which is less than Vth, the diode will not conduct and will act as an open switch. This is called reverse biased.

Having understood the working of the diode, let us now take a look at the circuit diagram of the AND gate.

Construction:- V1 and V2 are input voltages. R2 and R3 are the source resistances. R2 and R3 are equal and R1 >> R2, R3. D1 and D2 are the diodes. R1 is a pull-up resistor. Vr is clamped to 5 volts. Vout is the output voltage.

Working:-

Case 1:- V1 = 0, V2 = 0

In this case, since the V1 and V2 are at 0 levels the current flows from Vr to V1 and V2 and goes to ground. Diodes D1 and D2 are forward biased in this case. Thus the output voltage is 0.

Case 2:- V1 = 0 V2 = 5 V

In this case, diode D2 is reverse biased providing a high impedance path. But diode D1 is forward biased since V1 = 0 providing low impedance path. Thus the current flow through V1 to the ground and again Vout is 0.

Case 3:- V1 = 5 V, V2 = 0

This case is similar to case 2. Here instead of D2, D1 is reverse biased and D2 is forward biased generating 0 v output signal.

Case 4:- V1 = 5 V, V2 = 5 V

In this case both D1 and D2 are reverse biased providing high impedance path. Thus the only way for the current to flow is to Vout. Thus the output generated is HIGH (5 v) in this case.

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