Modifying LM334 reference schematic to 100uA/500mV
Okay, R1 * I1 determines Vds and I2 independently determines your bias current by changing Vgs. 

Looking at the I1 current source - its purpose is to create a voltage 500mV above Vout. One easy way to do that is to use an LMV431 1.24V reference hung off the op-amp output and divide the 1.24V down to 500mV. The LMV431 needs at least 80uA to regulate. 







simulate this circuit &ndash; Schematic created using CircuitLab

This will work up to about 2.7V Vout with a 3.1V supply before the reference runs out of regulation room.