A gate is a digitcal circuit which follows certain logical relationship between input and output voltage. Therefore, they are generally known as ‘logic gate’, gates because they central the flow of information. Logic gates are used in calculators, digital watches, computers, robots, industrial control system and telecommunication etc.
The five common logic gates used are OR, AND, NOT, NAND and NOR.
(i) OR Gate
An OR gate has two inputs related by one output. The truth table and logic symbol (circuit symbol) are shown below. The output Y is 1 when either input A or B or both are 1. That is if any of the input voltage is high input is high.
The Booliean representation of OR gate is
Y = A + B
(ii) AND Gate
An AND gate has two inputs related by one output. The output Y is 1 only when both inputs A and B are 1. That is if both the input voltages are high, then only output voltage is high. The logic symbol and truth table are given as below.
The Booliean representation of AND gate is
Y = A.B or Y = A x B
(iii) NOT Gate
This is the most basic gate with one input related to one output. It gives inverted version of input voltage as output voltage. This is the reason it is also known as inverter. The logic symbol and truth table are given as below
The Booliean representation of NOT gate is
Y = A
(iv) NAND Gate
This is an AND gate followed by a NOTgate as shown in Fig. If the inputs A and B are both1 the output Y is not 1.
NAND gates are also called universal gates since by using these gates we can realize other basic gates like OR, AND and NOT. The logic symbol and truth table are given below
The Booliean representation of NAND gate is
Y = A.B
(v) NOR Gate
This is an OR gate followed by a NOT gate as shown in Fig. Its output Y is only 1, when both inputs one 0. Thelogic symbol and truth table are given below
The Booliean representation of NOR gate is
Y = A+B
