Logic gates are one of the fundamental units of a digital circuit. They can be made from only a handful of transitors each, and they can implement any logical function, and therefore build any digital device. At the most basic level, all digital circuits are combination of logic gates.
AND, OR and NOT gates are the fundamental functions, out of which the other functions are built. The other functions available are
- NAND: an inversion of AND's output.
- NOR: an inversion of OR's output.
- XOR (exclusive-OR): outputs high only if just one input is high.
- XNOR: inversion of the output of XOR.
In total then, we have seven basic gates. Although there are sixteen possible outputs from two input logic gates, the other functions not represented here are usually made up of these seven. This idea can even be extended further - NOR or NAND are each capable of producing all the other functions and are sometimes called the "universal gates" (see NOR Logic and NAND Logic for more information). At the transistor level, many devices consist only of one of these gates. That said, by using the other gates, we make conceptualising and designing digital circuits much easier, and loses no generality, as the conversion to the relevant type of gate will be done by automatic tools if needed.
Only the NOT gate and the buffer (which just copies the input to the output) have one input. The other basic gates have two inputs, although we will later look at how to extend the concept to more inputs.
|NOT gate (Inverter)|
|NAND gate (NOT−AND)|
|NOR gate (NOT−OR)|
|XOR gate (Exclusive-OR)|
|XNOR gate (NOT−exclusive−OR)|
The truth tables for the seven common logic gates are given below:
|A||B||Logic Gate Output|
|NOT A||A AND B||A NAND B||A OR B||A NOR B||A XOR B||A XNOR B|
Logic gates with more than 2 inputs
We can separate and make a group of 2 and work on it. for eg:
Y=A+B+C then Y is also equal to X+C where X=A+B.
For further reading
- The Apollo Guidance Computer, the first computer ever to use integrated circuits, was built from 4,100 ICs, each containing a single 3-input NOR logic gate.