bdccomputers: Logic Gates

Logic Gates

Basics

Handout-1

Definition:
A logic gate is an elementary building block of a digital circuit . Most logic gates have two inputs and one output. At any given moment, every terminal is in one of the two binary conditions low (0) or high (1), represented by different voltage levels.
There are five basic logic gates: AND, OR, NOT, NAND and NOR
AND gate:

The AND gate is so named because, if 0 is called "false" and 1 is called "true," the gate acts in the same way as the logical "and" operator. The following illustration and table show the circuit symbol and logic combinations for an AND gate. (In the symbol, the input terminals are at left and the output terminal is at right.) The output is "true" when both inputs are "true." Otherwise, the output is "false."

AND gate

Input 1	Input 2	Output
0	0	0
0	1	0
1	0	0
1	1	1

OR gate:

The OR gate gets its name from the fact that it behaves after the fashion of the logical inclusive "or." The output is "true" if either or both of the inputs are "true." If both inputs are "false," then the output is "false."

OR gate

Input 1	Input 2	Output
0	0	0
0	1	1
1	0	1
1	1	1

NOT gate/ logical inverter:

A logical inverter , sometimes called a NOT gate to differentiate it from other types of electronic inverter devices, has only one input. It reverses the logic state.

Inverter or NOT gate

Input	Output
1	0
0	1

NAND gate:

The NAND gate operates as an AND gate followed by a NOT gate. It acts in the manner of the logical operation "and" followed by negation. The output is "false" if both inputs are "true." Otherwise, the output is "true."

NAND gate