0to10v Convert to 4ma to 800mA

Programmable current sources work better if they include a control opamp.

This classic circuit is actually a current sink, rather than current source, but with a few more parts it could be expanded to be a true programmable current source. Often when one says current source, what they have in mind can be served just as well with a current sink. What are you working on?

The control voltage has been attenuated by 12.5x to provide more headroom (maximum compliance-voltage) and to reduce the power dissipated in the current-sense resistor Rs. The operating formula is Iout = Vin/Rs * 1k/12.5k

My suggested opamp, an LT1013, is an improved version of the jellybean LM358, with 4 to 10x lower offset voltage, 90uV typical, useful to deal with our reduced programming voltage. It's a single-supply opamp, which means it works well with Vin at ground. We limit its V+ supply voltage to +15V, because we don't want to expose the MOSFET's gate to an excessive voltage (edit the drawing to show about 7 to 8V minimum for the opamp supply, because Vgs for the IRF640 could be as high as 5V).

I've chosen a power MOSFET because these are often happier with high power dissipation, especially at high drain voltages, but you could just as well use a power Darlington BJT instead.

Power MOSFETs have high gate capacitance (the IRF640 (datasheet link has 1300 pF), which degrades the control loop's stability (it might even oscillate), so we add an R and C to isolate the opamp from the gate capacitance above 50kHz. The IRF640 ($2 at Mouser) is rated at 200 volts, and will give you that much compliance if you need it, but the high-voltage part is really chosen because it has a larger die (the rating is 125W with a 25ºC case temp) and is therefore better able to conduct heat to the required serious heatsink.

If you need more voltage capability, an IRF840 is rated at 500V, but you might find yourself running into power dissipation issues. How much compliance voltage do you need?