Current controlled current source, collector resistor from 0 to 500 Ohm, the current flowing thru the led will remain constant with a fixed base current of 199 micro amp, change collector resistor from 0 to 500 ohm, collector current remains at 20 ma.
The variable base current can be changed from 0 to 198 microamps, output changes from 0 to 20 ma.
Simplified model on top!
