Electronics > Projects, Designs, and Technical Stuff
OP Supply Bootstrapping: Substituting the p-Mosfet against an n-Mosfet possible?
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Micke:
To return to the subject using all N-MOSFET, newer versions of the calibrator had MOSFET HV amplifier instead. Apparently it was prone to errors, the guy that helped me has repaired his HV amplifier several times. He did some reverse engineering, I have just drawn it from his handwritten notes to CAD. Less component count rather than the bipolar version with 48 NPN transistors! (But bipolar is perhaps more robust after all?) Anyhow, might be useful to get inspiration from!  ;)
David Hess:
The schematic is not quite right but I can see the general idea.  It would not surprise me if it had dynamic stability problems with all of those high value unbypassed resistor chains.  High voltage amplifiers usually include either series (transconductance) or shunt (transimpedance) local feedback.  Cascode amplifiers like that can have surprisingly high bandwidth and clean response.

Bipolar transistors have a big cost advantage at high voltages so I am surprised anybody would use that many MOSFETs.  With a 3600 volt total supply, even p-channel MOSFETs could have been used which gives the possibility of a higher performance design.

Micke:
Thanks for the analysis David!
Hmm, might have done some errors redrawing or the original had some errors...
Anyhow, I stick with my 48pcs NPN bipolar version!   :)
Having a proper PCB, not that difficult with that many components anyway.
But with a quiescent current of 7mA at 3600V that is 25W just in idle power  :scared:
David Hess:
Oscilloscope z-axis amplifiers which drive the grid to control CRT brightness run at about 100 volts with a similar configuration with up to two transistors on the bottom and two transistors on the top to provide a delay in the 10s of nanoseconds range and rise/fall times in the range of several nanoseconds.  The big difference is that they use local shunt feedback so operate as a current in and voltage out transimpedance amplifier.

Two transistors are required because the high frequency transistors are not high voltage and the high voltage transistors are not high frequency.

I have seen at least one 50 MHz class-AB output amplifier for a function generator which also used a similar configuration.  Very high power audio amplifiers may also use cascode output stages.
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