Current amplifier to raise 0-5 mA to 0-20 mA range
You need add this to your circuit, see picture below. 
You can power this addition with 3V (instead of 5V as show in picture), maybe you'll have to adjust the R2 value.

You can learn more about it in the following link:
https://learn.sparkfun.com/tutorials/transistors/applications-i-switches

There's an animated GIF at this page that is very good to help you understand the concept.