By Mike Engelhardt, creator of QSPICE
Charley Moser, EE Ph.D., was one of my earliest mentors for analog design. While I learned many things from him - hybrid pi transistor modeling, the limitation of Bode plots for stability analysis to minimum phase systems, why use a snubber and how to design one, pulling current of the base of a bipolar to prevent breakdown, SCR use at high temperature - the most strikingly novel was how to regulate low power at high voltage. This was particularly valuable to me because I was the go-to guy for designing the high-voltage power supplies used in charged-particle optics at various instrumentation companies.
He taught me to use shunt regulation with a grounded-base high-voltage transistor and drive the emitter from an op-amp. The transistor's gain bandwidth would be 3dB down at its fT. Controlling an HV output with a low voltage input entailed a large feedback divider ratio, killing the open loop gain, so the transistor's huge contribution in voltage gain in grounded-base configuration was useful and shouldn't be too hard to tame for a stable loop.