This is a current source designe I have seen alot and have been trying to understand for weeks. I now do.
This is how it works:
V_be (The voltage across the base to emitter junction) of left transistor sets the voltage across and hence the current through the right resistor. The current through the right resistor sets I_e (emitter current) for the right transistor.
Notice for large voltage swings of the input, the output current remains relatively flat.
