I need a constant current of 10uA through a variable resistor in the 100k values. I need to inject the current into that resistor and I need the resistor to be connected to Ground such that I can measure the voltage across it with a microcontroller(writing this I figured I could just read the voltages in both points and substract them, but oh well). So how do I do this with discrete components? I heard that current mirrors don't work with discrete components. I don't know if using an operational amplifier and a sense resistor, I would be able to source/sink such a small current. Is a current limiting circuit better? I don't really need to inject the current, I just need constant 10uA.
What's a reliable, stable and fairly exact 10uA current source/sink that I can build with discrete components and without any IC's as I don't want to wait for themto ship to me?
Edit: I'm working with supply voltages in the range 5v-12v
Edit 2: 1% accuracy is just fine. Heck, even 5% accuracy is fine, anything below 10% is fine. My load is a 220k resistor at it's maximum (it's actually a 100k thermistor, but that's as high as it will get). And my supply would be 5V, so in order to get 10uA through it, I would need to supply about 2.2V, so no problem there.