Depletion mosfets are expensive so...

[throbscottle]

What's a simple way to create a switch which is only off if it has a negative input? A depletion mode n-mosfet would be ideal but buying just one is expensive. It's to shunt a (up to) 45v constant current source to ground, with -6v available to turn the shunt off.

Thanks, hope you all are well

 

[throbscottle]

Huh, I think I found a way to do it. My arrangement had been a pd between the high voltage supply and negative supply with a normal mosfet's gate tied to that. Almost worked but for a "hump" in the output as the high voltage dropped. Replaced the bottom resistor with a zener, looks much better in LTSpice.