A decoder or decoder is a combinational circuit whose function is inverse to the encoder, ie converts a binary input (natural, BCD, etc.) of N input bits and M output lines (N can be any M is an integer and less than or equal to 2N integer), such that each line of output will be activated for only one of the possible combinations of input. These circuits normally are usually found as a decoder / demultiplexer. This is because a demultiplexer can behave as a decoder.
If for example we have a decoder 2 inputs with 22 = 4 outputs, their operation would be as shown in the following table, which has been found to activate outputs a logic "one":
