### LOGIC GATES BASIC INFORMATION AND TUTORIALS

The logic gate is the most basic building block of any digital system, including computers. Each one of the basic logic gates is a piece of hardware or an electronic circuit that can be used to implement some basic logic expression.

While laws of Boolean algebra could be used to do manipulation with binary variables and simplify logic expressions, these are actually implemented in a digital system with the help of electronic circuits called logic gates.

The three basic logic gates are the OR gate, the AND gate and the NOT gate.

OR GATE
An OR gate performs an ORing operation on two or more than two logic variables. The OR operation on two independent logic variables A and B is written as Y = A+B and reads as Y equals A OR B and not as A plus B.

An OR gate is a logic circuit with two or more inputs and one output. The output of an OR gate is LOW only when all of its inputs are LOW. For all other possible input combinations, the output is HIGH. This statement when interpreted for a positive logic system means the following.

The output of an OR gate is a logic ‘0’ only when all of its inputs are at logic ‘0’. For all other possible input combinations, the output is a logic ‘1’. Figure 4.3 shows the circuit symbol and the truth table of a two-input OR gate. The operation of a two-input OR gate is explained by the logic expression Y = A+B

AND GATE
An AND gate is a logic circuit having two or more inputs and one output. The output of an AND gate is HIGH only when all of its inputs are in the HIGH state. In all other cases, the output is LOW.

When interpreted for a positive logic system, this means that the output of the AND gate is a logic ‘1’ only when all of its inputs are in logic ‘1’ state. In all other cases, the output is logic ‘0’.

The AND operation on two independent logic variables A and B is written as Y = A B and reads as Y equals A AND B and not as A multiplied by B. Here, A and B are input logic variables and Y is the output.

NOT GATE
A NOT gate is a one-input, one-output logic circuit whose output is always the complement of the input. That is, a LOW input produces a HIGH output, and vice versa.

When interpreted for a positive logic system, a logic ‘0’ at the input produces a logic ‘1’ at the output, and vice versa. It is also known as a ‘complementing circuit’ or an ‘inverting circuit’.

The NOT operation on a logic variable X is denoted as X or X . That is, if X is the input to a NOT circuit, then its output Y is given by Y = X or X and reads as Y equals NOT X. Thus, if X = 0 Y = 1 and if X = 1 Y = 0.