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Class A MOSFET amp with rapidly varying voltage supply ?

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TheBaconWizard:
This is likely not the efficient way to do this, but I would like to consider it with the others..

I am providing a top-rail voltage to a MOSFET-based voltage amplifier and then a push-pull. The voltage is provided by a charge-pump. If I omit the final smoothing-cap, there will be significant variation in the voltage, at the 5Mhz i’m Using to drive it.

I actually WANT that in the final waveform, and while yes, I could use a summing op-amp to combine the 5Mhz signal with the 500khz signal the final amp will have on its base, i’d still Like to investigate this option if only for education.

Could I bias the MOSFET in such a way as to set the bias point at Q no matter what Vcc happens to be at the time? feedback resistor perhaps? A very carefully chosen bootstrapping arrangement? These latter 2 options are new to me btw, I am still finding my feet.

Cerebus:
1) I think you might have missed part of the explanation as
--- Quote --- combine the 5Mhz signal with the 500khz signal
--- End quote ---
suddenly springs out with no introduction.

2) I take it you mean "push-pull" in the class-B/totem-pole sense, not the differential pair amplifier sense?

TheBaconWizard:
One signal (5Mhz) is driving the voltage pump which provides the top rail voltage, with a huge 5Mhz ripple if we deliberately omit the final smoothing cap.

The mosfet amplifier stages are  intended to be supplied from that moving voltage. They will of course have something to amplify. That something happens to be a 500khz sine wave. But as far as I know that's irrelevant: the question is, can I bias mosfets such that the bias follows the top-rail's voltage and thus continue to perform as an amplifier according to an input signal of whatever kind. It would of course vary in voltage at 5Mhz as well as following the input waveform. This is all assuming the mosfets are fast enough.

Yes, it's class B for the final output.

Cerebus:
Hmm, definitely beard scratching time. Normally one tries to design in as much PSRR to an amplifier as possible, so we're looking at chucking out all the usual design habits.

So, some more questions. What does the rail voltage look like? DC + some 5MHz signal on it, obviously, but how much of each? Do you have a sketch or something of what you're hoping your final output waveform looks like?

You'll have to forgo explicit and implicit negative feedback (e.g. degeneration) because that will raise the PSRR, yet stabilising bias in the face of wildly fluctuating supply voltages would seem to demand lots of negative feedback somewhere along the road. Let's kick the ball around for a bit, by all means, but I have a suspicion that a practical solution is going to be elusive. Doesn't hurt to try, it's always good to try and figure out novel solutions. (Plus a bit of proper analogue electronics is always welcome in the face of the preponderance of bit-twiddlers around here.  :))

Early thoughts are perhaps something more in the way of a common base/gate topology might be more do-able.

JS:
The two inout signals, at least an examplenof them, and the desired output should be clear, if you can grab a napkin, a lipstick and a camera and post it here would do.

I imagine a pretty known rail waveform, I imagine dc+squarewave, at 5MHz the bandwidth would probably be noticeably limited but let's say square wave. So its an elevated square, with base voltage half the value of the peaks. This is my first guess.
The second signal, the input signal let's call it a 500kHz sine, but would be whatever 500kHz BW limited signal.

Now, you want the sine to be chopped by the square, but keep the relation, not just clipped when the square is low and sine is high.

Is al that the case?

Then you need your output stage to behave as a constant resistance, variable with the sinewave input signal. The more controllable way would be to add the rail voltage in some way to the input, I'm thinking something like a conatant resistance dummy load right now, recently discussed around here. I don't know about that freq range with this loop but I guess a known load would help a lot compared to a test equipment where load could be almost anything, like an indian cow on new year...

If you want a more open loop approach and still get some kind of predictable, precise output it can get really tricky, as would probably be dependent on mosfet parameter, changing a lot from device to device, temperature and moon alignment.

JS

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