Erm, thanks for that indulis, but I think I'm going to need a few other values and device numbers to go with that. Is that a precision constant current source circuit? Edit: I decided to simulate the circuit using SIMetrix, with a few components: http://vicious5id.zerodollars.com/eL-iSrc.GIF ^ Thats my version of the circuit. It works OK for a constant 20 Ohms, however the current will change if I replace the 20 Ohm load. Also I simulated it under varying temperatures and there's a drift of ~+5 mA, whereas in the circuit I have (with the appropriate heatsink in place) will drift by only ~+0.01 mA. I do hope that its the way I have set it up thats not letting it work, I like the idea of using a MOSFET. I'd be grateful, indulis, if you could upload a schematic of your circuit working with those specifications I just made; perhaps you could adjust mine or change yours. One final thing: it'd be really awesome if you could get a current source to supply 0 to 120 mA to a 20 Ohm load, which is independent of temperature change. That would be just kick-ass. I'm thinking about perhaps hi-power op-amps that allow upto 150 or 200 mA.