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My question is based on the MOSFET circuit shown in a question by uMinded.. "High Voltage Series Pass Transistor - Low Voltage Control" here

This circuit is a little unique in that it is not using a follower type load driver.. if you Google power supplies, 99% use an source- or emitter-follower based design driving the load..

My question is: yes a follower MOSFET or BJT has high current gain with no voltage gain and is simple, but adding a varying value of load capacitance and the error op-amp circuit becomes very unstable as the op-amp see's this capacitance on its output. So, why is the load, NOT Usually driven as it is in the circuit mentioned above (from the Drain or Collector) more isolating the op-amp from load capacitance... after all the op-amp has lots of gain.

I have simulated the circuit mentioned above with appropriate changes, and even when varying the load capacitance from 1 µF to 10000 µF it showed no sign of instability.

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  • \$\begingroup\$ I've edited your question to add a usable link. What in the linked question are you referring to, the OP, one of the answers? \$\endgroup\$ – Neil_UK Nov 9 '15 at 7:01
  • \$\begingroup\$ I am unsure what circuit you are referring to. Please embed the circuit in your question and be clearer about which type of circuit you are asking about. I'm talking about you main paragraph here that starts "my question". \$\endgroup\$ – Andy aka Nov 9 '15 at 9:39
  • \$\begingroup\$ Thanks for adding the link user44635, both the circuits in the link (bjt or mosfet) do not use the emitter follower at the load... Does this not better isolate the load capacitance from the op-amp.??? (op-amps don't like load capacitance) \$\endgroup\$ – Don Warr Nov 9 '15 at 20:27

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