i am looking to design a circuit that gives continious 20 mA DC output using battery source

 

An LM317 3-pin regulator with a resistor chosen to give 1.25v @ 20mA (R=E/I, R=1.25/0.020, R=62.5 ohms) will make a constant current source.

Headroom will be approx 2.75v when it is regulating, so a 9v battery will produce 20mA up to about 6.25v.