# What is logic gates and classification of logic gate

In today’s article What is logic gates and classification of logic gate, we are going to discuss the basics of a gate and its Introduction. We will also discuss its various types, there circuit diagram, truth table, different symbols for different gates, transistor diode diagram etc in detail.

The aim of this article is to clarify the basics of gate in easiest way possible.

## Logic gate

The operation in which the input can be operated on the basis of input that is known as logic gate.

## Types of logic gate-

**1. AND gate**

Let us consider there are two input terminals A and B then the output is given by Y.

**Symbol**–

A.B = Y

**Logic gate truth table**

**Circuit diagram**

**Diagram**

**Transistor diode diagram**

**Note**–

If base have zero input or i.e. low input then the Transistor will be works in cut off region. And if base have one (1) input or i.e. high input then Transistor will works in saturation region.

**Theory**–

- If A and B both are zero then the diode will be in forward bias condition. So that current will flow in output part then output is zero and hence LED will not glow.
- If A and B both are 1. Then the diode will be in reverse bias condition and the output current will go the LED works it glow.
- If A is zero and B is one or vise versa then the current will again go to diode and hence LED will not glow.

### 2. OR gate-

Let us consider there are two input terminals A and B then the output is given by Y.

**Symbol**

A+B = Y

**Truth table**

**Circuit diagram-**

**Diode diagram-**

**Transistor diode diagram-**

**Theory**

- If A and B are zero. Then diode will be in reverse bias and will not conduct any current. So consequently, LED will not glow. (Note- this is because A and B both inputs are zero i.e. low hence there should be no current. But on the other hand we have connected a Vcc external supply due to which in have 1 as an output. But in OR gate Y = A+B that’s why we have got zero output)
- If A is zero and B is one or (A= 1, B=0) then diode conducts current and LED will glows.
- If A and B both are one, then A and B both are in forward bias. And hence they conducts current and LED will glows.

#### 3. NOT gate

When A is input and Y shows its output then NOT gate can be expressed as

Y = A‾ ( A-bar)

**Symbol**

**Circuit diagram**

**Truth table**

**Transistor diagram**

**Theory**

- When input A is zero then the Transistor behave as OFF switch and the output will be one.
- When the input A is one, then the transistor behave as ON switch and output will be zero.

##### 4. NAND gate

NAND gate is basically the combination of NOT and AND gate. If A and B are the input and Y represents the output then NAND gate can be expressed as

Y = A.B‾‾

**Symbol**

**Circuit diagram**

**Truth table**

**Transistor diode diagram**

**Theory**–

- When input A and B is zero. Then both diode D1 and D2 will be in forward bias. Then transistor behave as OFF switch and the output will be one.
- When (A =0) and (B= 1) then diode D1 will be in forward bias and D2 will be in reverse bias. Then transistor behave as OFF switch and the output will be one.
- When A is one and B is zero. Then diode D2 will be in forward bias and D1 will be in reverse bias. Then Transistor behave as OFF switch so, the output will be one.
- When A and B both are one. Then D1, D2 will be in reverse bias. So the Transistor will behave as ON switch and hence output will be zero.

##### 5. NOR gate

NOR gate is the combination of NOT and OR gate. If A and B are the input and Y represents the output then NOR gate can be expressed as

Y =( A + B)‾‾

**Symbol**

**Truth table**

**Circuit diagram**

**Transistor diode diagram**

**Theory**

- When A and B is zero. Then D1 and D2 will be in reverse bias i.e. both diode conduct and Transistor behave as OFF switch and the output will be one.
- When A is zero and B is one, then D1 will be in reverse bias and D2 will be forward bias so, the Transistor work as ON switch and output will be zero.
- When A is one and B is zero, then D1 works in forward bias and D2 works in reverse bias. So, that the Transistor works as ON switch and the output will be zero.
- When A and B are one, then diode D1 and D2 works in forward bias and Transistor will works as ON switch and the output will be zero.

##### 6. Exclusive OR gate (X-OR)

When both inputs terminal A and B are at different voltage then the output of the gate is maximum (1).

When both input are at same voltage the output will be zero.

**Symbol**

**Truth table**

*Diagram*

**Transistor logic gate (AND)**

**Transistor logic gate (OR)**

**Theory**

When both input are in low state then Transistor Q1 and Q2 behave in cut off region so, a wire point higher value then Transistor Q3 work in saturation region. And connection for high voltage connection +Vcc power supply, the result output in low state.

###### Conclusion

In this article we have discussed about Gates and there classification, operation, circuit diagram, truth table etc in Detail. And now we are able to conclude that Gates have an great importance in field of digital analogue.

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