High Side Constant Current Source
You can use this idea: -



The voltage across R1 sets the voltage that is forced across R2 by the op-amp's negative feedback. Let's say that voltage is 0.1 volts and let's say R2 is 1 ohm. This means that when 0.1 volts is forced across R2 there can only be 100mA flowing through it.

99%+ of that current flows through the load so it's basically a constant current generator. Less than 1% flows into the output of the op-amp to drive the base.

Sounds simple but you'll need a rail-to-rail op-amp with inputs capable of getting to within 100mV of the positive rail. Not too tricky of course. The voltage across R1 being set to 0.1V won't be stable if the V+ rail moves about so some kind of positive supply referenced shunt regulator can be used and this can be further potted down to provide 0.1 volts across R1.

Saturation of the PNP might be 100mV so overall, this design could be expected to "drop" about 200mV. It's a circuit I use a lot for the excitation of strain gauges.