Simplistic MosFET HV Shunt Regs

diyAudio Chief Moderator
Joined 2002
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Was it a different B+ voltage when you checked those constant current loads ma max maybe? Because the cascode Mosfet M1 in them creates a Vgs level which is VDS for the JFET T1 to reach some max IDSS. At different HV across the cascode Mosfet it's Vgs changes a little.
 
diyAudio Chief Moderator
Joined 2002
Paid Member
When you first confirmed 15mA maybe it was in a hotter ambient. If you have 2SK170V >15mA they should reach. Probably some J112 or J113 can do it too. Its the JFET that produces the current in that active load CCS you are trying for. M1 IXTP D-Mosfet is just a cascode to protect the JFET from HV. You don't need to match the JFETs because you have trimmers to regulate their IDSS. But they must be able to reach beyond your target ceiling. Measure them for IDSS in few Volts VDS, as much as the VDS they see now in that circuit. Measure that across the J309s before you pull them.