Voltage controlled current source using op-amps
In the end I switched to a different solution, which has been tested both in simulation and in the real world.







simulate this circuit &ndash; Schematic created using CircuitLab

I chose the AD797 as the op-amp simply because I had them laying around, so that part isn't necessarily part of the final implementation. It also appears to be able to drive quite a resistive load with a maximum output voltage of 13V and a maximum output current of 50mA.

RV is the variable load (resistance of the cables, of the current sensor, etc...) and could be anywhere between 100Ω and 400Ω (I used a potentiometer in the circuit for simulation purposes)

R1 has been chosen to be 250Ω (precision resistor) since V = IR or I = V/R, which owuld give us I = 20mA for the 5V input, or I = 4mA for a 1V input.

Should the load's resistance (RV) increase dramatically I'll have to look for another op-amp or a different solution alltogether but for now this will do!