Re: Need 30uA current source
See schematic of HP3478A. From Hewlet Packard (todays Agilent). There's best drawn schematic of current source with ranges of 100nA to 1mA (not sure for upper range)
Use oamps AD706 and for current source I will fill in today. (I don't remember exactly). Or see schematic Agilent 34401A. Watchout the lownoise, low input current oamp (unipolar input amplifier with low noise like TL051, AD706, ... ) or it will not work properly.
Good luck