Stabilising a constant current load opamp feedback loop
Short answer: The closed loop has a resonance point, when you add the capacitor you are either flattening this point or eliminating it all together. It would be better to get spice out in a situation like this and get an idea (it won't be exact without the parasitics and it isn't worth the time to go through and find what they are on the board and model them). Once you have an idea, you can find ways to mitigate the problem. Or you could just add capacitors wherever and see if that fixes it.



It really depends on the system AND implementation. Sometimes board parasitics can make a difference. I've ran into a simmilar setup with a BJT for the current amplifier and the difference in the parasitics of a 10ohm resistor being cut and soldered on vs being soldered directly to a PCB with solder pads. The difference was nH's and pF's. Usually the oscillations are high frequency. The amplifiers can also cause problems since they have a cutoff and the mosfet usually has a higher frequency cutoff. If you don't do some kind of frequency analysis these problems will crop up more.