What is a decoder? 

Decoder 

Basically a decoder is a combinational logic circuit used to convert a piece of coded information into another signal pattern. And the decoder converts 'n' number of inputs into 'm = 2^n' number of unique outputs. Such decoders are generally described as N-line to M-line decoders. A decoder has only one high output for any given code. This is the opposite of an encoder and a special case of a demultiplexer without an input line. And the decoder can be used to perform code conversions or to detect the presence of a particular binary state and thereby activate other circuits based on the input. And simplest decoder is a single logic gate such as an AND or NAND gate with an input inverter, whose outputs are active when the all inputs are high. When combined with an inverter, a NAND or AND gate can produce an output for a unique combination of inputs. So, the decoder used in practical applications is an extension of the simplest decoder and consists of many NOT and AND gates. The number of unnecessary gates will be equal to the number of inputs ie., n. The number of AND gates required will be equal to the number of outputs.,m = 2^n.