current source circuit nanoamp
I don't see a problem to operate a transistor current source in the nA range. You have to check for sufficient low leakage currents of course. But I wonder, if it's an appropriate way to build a current source for a precise measurement. The current transfer ratio even of monolithic current sources is rather variable. I think that high ohmic resistors in the 100 Mohm upto Gohm range allow to design higher accuracy current source as expected in instrumentation applications.