What is AND Gate in Digital Electronics

In this article, we will explore the AND gate, its definition, logic symbol, Boolean function, truth table, switching circuit diagram, and AND gate IC. So, let’s begin with the definition of AND gate.

What is an AND Gate?

The AND gate is a basic logic gate in digital electronics used to implement logical multiplication. An AND gate has two or more input lines and one output line.

The output of the AND gate is HIGH (Logic 1) only if all its inputs are HIGH (Logic 1). If any of the inputs to the AND gate is LOW (logic 0), the output of the AND gate will be LOW (logic 0).

The AND gate is basically used to implement the logical conjunction (^) operation in logical mathematics.

AND gate is widely used in digital systems for data transmission, alarm circuit implementation, push button implementation, etc.

AND Gate Symbols

The following figure shows the logic symbol of a two-input AND gate.

The logic symbol of a three-input AND gate is shown in the following figure.

AND Gate Truth Table

The truth table of a two-input AND gate is shown below.

Inputs		Output
A	B	Y
0	0	0
0	1	0
1	0	0
1	1	1

The truth table of the three-input AND gate is given below.

Inputs			Output
A	B	C	Y
0	0	0	0
0	0	1	0
0	1	0	0
0	1	1	0
1	0	0	0
1	0	1	0
1	1	0	0
1	1	1	1

AND Gate Logic Function

The logic function or Boolean expression of a two-input AND gate can be directly derived from its truth table given above. The following is the logical function or Boolean function of a two-input AND gate.

`\Y=A.B`

Where A and B are input variables, and Y is the output variable. This expression is read as Y is equal to A AND B.

Similarly, we can derive the Boolean expression or logical function of a three-input AND gate from its truth table given above. The Boolean function of a three-input AND gate is given below.

`\Y=A.B.C`

It is important to note that we can extend this expression to any number of input variables.

Transistor AND Gate

The circuit diagram of a transistor AND gate, i.e., AND gate transistor circuit is shown in the following figure.

Operation:

This transistor AND gate circuit operates as follows:

• When both A and B are LOW, the transistors T₁ and T₂ are off. The output line will connect to the ground, i.e. output will be LOW.
• When the input A is LOW and B is HIGH, the transistor T₁ is off and T₂ is on. In this case, also, the output line will be connected to the ground and not to the power supply due to off transistor T₁. Hence, the output of the circuit is LOW.
• When the input A is HIGH and B is LOW, the transistor T₁ is on and T₂ is off. In this case, also, the output line will be connected to the ground and not to the power supply due to off transistor T₂. Thus, the output of the circuit is LOW.
• When both A and B are HIGH, then both transistors T₁ and T₂ will be on and the output line will connect to the power supply through the transistors. Hence, the output of the circuit will be HIGH, in this case.

This way the above shown transistor circuit implements the AND operation.

AND Gate Switching Circuit

The switching circuit of a two-input AND gate is shown in the following figure.

In this circuit, when both switches A and B are closed, the lamp will glow. If any of the two switches is open, the circuit will become incomplete and the lamp will go off.

Hence, this is the switching circuit implementation of the AND gate.

AND Gate IC

The AND Gate IC 7408 is a very commonly used AND gate IC. It is a TTL (Transistor-Transistor Logic) IC which has four two-input AND gates. Hence, it is a Quad-2 AND Gate IC.

The pin diagram of the AND gate IC 7408 is shown in the following figure.

It has a total of 14 pins and requires a 5.2 V DC maximum power supply. A supply voltage greater than 5.2 V DC can damage the IC circuit.