Electronics > Beginners
Class A MOSFET amp with rapidly varying voltage supply ?
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JS:
  Opamp's PSRR at high freq gets worse, and once out of opamp's bandwidth there's not much the opamp can do to reject the PS noise. You might build the thing as you are thinking and get it working close to what you expect, so why not to try it... this is one of the cases where simulator probably won't help much (unless really good opamp models) but prototype can solve the riddle. You could pick an opamp with the right bandwidth you need or try one with external compensation terminals so you can tweak the BW after the fact.

JS
T3sl4co1l:
It sounds like you're making a class D amp, and therefore just moving the losses back a stage.  To control the output amplitude, the ~5MHz charge pump needs to be modulated.  But charge pumps can't be modulated*, they are driven with resistive switches and you can only burn the difference as heat in said resistance.

*With a great many stages (impractical for a discrete design, but I'm sure it's been used in ICs before), you can get ratios spaced closely enough to deliver high efficiency.  If the ratios still aren't close enough (e.g., you need continuously variable output, not just a steppy "power DAC"), you only need one stage to burn the difference; which, because the ratios are closely spaced, the difference is small, so the efficiency hit is small.

So you could in turn control the voltage into the charge pump, say with an inductive SMPS.  But then you could remove the charge pump entirely, and drive the output from the SMPS.  Which, really, isn't a bad idea, when it works out (in this case, probably not at 500kHz).

Whatever the application is, it still screams of X-Y problem, which I may've felt (or hinted at?) in a previous thread... :)

Tim
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