Current Sink circuit on the Right: the lower npn sets .7V on its base that can be used to set the current in upper npn darlington. The upper 100k resistor has 5-2 V on it, and is used to bias the darlington.
Current Source circuit on the Left: everything is flipped around, except who has a reference from the high rail? So lets use a mirror from a low side reference.
Update: The bias leg does not really need a reference voltage so it can connect to the 7V and when the 7V goes up to 25V the current won't change much... perhaps I should nuke the mirror.
Update2: now shown with a shunt reference that is feed by the current source. Also added AC to show how to get high line rejection. Bummer now it won't start without an open switch.
Update3: Vbe of npn used for mirror change with temperature, so added diode to keep the voltage on 100k nearly constant.
Update4: removed Darlington from current source, just make sure the mirror has enough current current to keep the current source very well biased.
Update5: switch added to source to flip between mirror and a resistor bias.
Update6: Zener is broken on Chrome desktop at this time
