I am looking for a circuit that takes a voltage to preselect a power and than outputs that power through a variable load. I have already seen a circuit that does this. The setup is easy: You take a small shunt resistor to get a voltage proportional to the output current, and you also take the the output voltage, and multiply the two voltages together with an analog multiplier IC. The multiplied voltages you compare with the input voltage and regulate the output voltage with an op amp accordingly. This might sound confusing but it's very easy. My problem is this: I need to do this in the microwatts range with a load that's [tex]R<100 \Omega[/tex] the multiplier chips I saw seem to be constructed for input of maybe U = 3V and the absolute errors look unacceptable. Can anyone recommend a circuit or a chip?