Simple high-side current sources that can be controlled by a ground referenced signal are not common. Below is the schematic of such a circuit, a high-side current source the delivers a constant current to a grounded load as controlled by the voltage difference between the differential inputs. The circuit uses a P-MOSFET controlled by an op amp differential circuit. This works because the input resistor that normally goes to ground in such a circuit (R5) instead goes the the V+ supply....