I need a small (somewhat) voltage independent current (about 10 mA), with a voltage range of 3-30V. I came up with the following circuit.
I drew the resistor U/I characteristic in the Id-Vgs characteristic of the MOSFET:
The idea is the circuit will quiesce at the intersection of both characteristics.
The MOSFET is a depletion mode N channel type. Circuitlab doesn't seem to have distinctive symbols for enhancement and depletion mode.
I choose a type that can handle over 1W of power to keep well away from SOA limits.
I am aware that a similar solution exists for JFET's, but I could not find a JFET type that can handle the voltage requirement. Also I know similar devices with JFET's exist as discrete components (constant current diodes), but these are rather expensive ($5 or more. The aim is to both keep component count and price low. A Widlar source is not an option, because it needs a reference current, which is not available.
Current constancy is less of an issue, it is meant to limit the current of an unknown voltage, supplied to a low resistance (20-50 ohm), 10 - 20 mA is fine. Neither are response times very important.
I don't feel really comfortable working at the lower end of the characteristic. Does anyone see a problem with this setup, something I overlooked, thermal stability issues, and maybe points for improvement? Maybe I am too much reliant on constancy of performance of individual devices?