Logic Gates | Digital Electronics

 LOGIC GATES

·       Logic gates are the basic building block of the Digital System.

·       It is an Electronic circuit having one or more inputs, and only one output.

·       The relation between input and outputs is based on certain Logic. Based on this logic gates are named as AND gate, OR gate, Not gate.

·       Total there are Seven Gates, three are basic and other are derived from basic logic gates.

ja1.  NOT Gate

2 2. AND Gate

3 3. OR Gate

   4. NOR Gate

    5. NAND Gate

    6. XOR Gate

    7. XNOR Gate

      

   1. NOT GATE:

NOT gate is also known as inverter.

The symbols used for NOT gate are given as under,

The Boolean expression for NOT gate is:
        X = A'

Where 

X is output

A' is the inverted version of the input A.

When the input will be LOW the output will be HIGH.

When the input will be HIGH the output will be LOW.

TRUTH TABLE OF NOT GATE:

NOT GATE USING TTL:

2. AND GATE:

The AND gate is the basic logic gate which can be combine to form any logic function.

AND Gate can have two or more than two inputs and it performs logical multiplication.

The Boolean expression for AND gate is given as,

                                    X = A. B (Read as X= A and B)

Symbolically it is represented as,

The AND gate produce a HIGH output when all the inputs are HIGH.

The AND gate produces LOW Input when any of the input is LOW.

TRUTH TABLE FOR AND GATE:

DIODE LEVEL Circuit of AND GATE;

Transistor Level Circuit Diagram of AND gate:

3. OR GATE:
Now we have third basic fundamental logic gate which is OR Gate.

OR Gate has two or more than two inputs and one output.

It performs logical addition.

Symbolically it can be represented as,

The Boolean expression for OR gate is:

X= A + B (Read as X= A or B)

The OR gate produces a HIGH output when any of the inputs is HIGH.

The oOR Gate produces a LOW output, when all the inputs are LOW.

TRUTH TABLE FOR 2-INPUT OR GATE:

Diode Level Circuit Diagram of 2-inputs OR Gate:

Transistor Level OR Gate:

4. NAND GATE:
NAND is the contraction of the AND and NOT gate, performs AND operation with complemented output.

The equivalent symbols for the NAND gate are as under,

It functions opposite of the AND gate when the output of AND gate will be HIGH, the output of the NAND gate will be LOW.

When the output of the AND gate will be LOW, the output of the NAND gate will be HIGH.

TRUTH TABLE OF 2-inputs NAND Gate:

Diode Level Circuit Diagram of 2-input NAND Gate:

5. XOR GATE:

XOR gate has two inputs and it performs modulo-2-addition.

The output of XOR gate is HIGH, when two inputs are at opposite logic levels.

"When input A is HIGH and input B is LOW, the output of XOR gate will be HIGH. 

When input A is LOW and input B is HIGH, the output of XOR gate will be HIGH.

When input A = Input B =1/0, the output will be LOW."

Truth Table of 2-input XOR Gate:

Diode Level Circuit Diagram of 2-input XOR gate:

6. XNOR Gate:

      XNOR GATE has only two inputs. The bubble indicate that its output will be inverted      When the two input logic levels are opposite, the output of the exclusive-NOR gate is LOW.

        For an exclusive-NOR gate, output X is LOW when input A is LOW  and input B is HIGH, or when A is HIGH and B is LOW; X is HIGH when A and B are both HIGH or both LOW.

T  Truth Table: