For a decoder, If the input is a N bit binary number, then the output will be one the lines of the 2^N output lines.

So, for a 2-bit binary input, there will be 2^2 output lines and only one of the output lines will be active according to the binary combination of the inputs. This type of decoder is known as 2 to 4 decoder.

A

B

 

Q0

Q1

Q2

Q4

0

0

 

1

0

0

0

0

1

 

0

1

0

0

1

0

 

0

0

1

 

1

1

 

0

0

0

1