Logic Gates

In electronics, there are mainly two types of wave forms. The information carried by these wave forms is called signal. When the signal takes any value within a range of amplitude at any instant of time, it is called a continuous signal. When the signal takes the value only at certain times, it is called a discrete signal. When the signal takes only particular finite number of amplitude values, it is called a digital signal.

The digital signal varies in steps and typically has only two widely separated values 0 and 1. These are called bits. Normally 0 V corresponds to bit 0 and 5 V corresponds to bit 1. Since the levels are so widely separated, any noise riding on the signal within the range of almost 2 V, [(0V + 2V) for level 0 and (5V – 2V) for level 1] does not affect the signal value, Hence these signals are immune to noise.

The signals used in a computer are digital. The information is coded in the form of digital signals by a series of bits arranged in different order. Each bit is a pulse of fixed time duration.

Different mathematical operations can be performed on the digital signal. The mathematics governing these operations is called Boolean algebra. In Boolean algebra, the basic operations are addition and multiplication. If it is a digital data that takes value 0 or 1, the following identities hold:

A × 0 = 0

A + 1 = 1

The circuits which perform these operations are called logic gates.

Basic Logic Gates

Logic gates are devices which have one or more inputs and one output. They give different output when the input bits differ in their arrangement. The output produced by these gates follows the laws of Boolean logic. There are three basic types of logic gates:

  1. AND Gate
  2. OR Gate
  3. NOT Gate

These gates perform multiplication, addition and inversion (negation) operations, respectively.