LED current regulator has low dropout | EDN

http://www.edn.com/design/led/4442091/LED-current-regulator-has-low-dropout?utm_content=buffer1967d&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

This article has an intetesting LED current regulator circuit idea built using transistors.

The voltage drop on the current sensing resistor r is less than 40mV. 
The nominal LED current here is 7.2 mA at 9V. Increasing to 20V causes a current change of +15%, giving a dynamic resistance of about 10kΩ.