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Old 16th October 2012, 06:56 PM   #1
Dura is offline Dura  Canada
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Default Practical MOSFET Amp Design/Speculation/Questions:

I'm working on "designing" and (depending on how far I get) building a simple class-A 2-stage MOSFET amp with an output of 20W/channel. The circuit will be have the same layout as Nelson Pass's Zen preamp(single ended(SE) class A) -> Complementary(PP) Zen(version 5) with the only different being some of the resistor values for more gain from the first stage and possibly a different output power.

Since this is a two stage amp I'm confused about how I should divide the gain between the two stages. I have found it difficult to find practical information. I would think that the first stage should have more gain than the power stage. Does anyone know if low level amp stages tend to be designed for more gain? I know preamps tend to not have much gain but I think that is more to do with the fact that they are designed to go in front of an amp not necessarily designed for a preamp. Here is at present how I have the gain divided: 40mW in(I think this is a typical source power level) -> IRF510 SE 1.25W(15dB) 30W draw -> IRF240/IRFP9240 PP 20W(12dB) 90W draw, efficiency=20W/120W*100%=16.7%. Mr. Pass seems to limit the gain to around 15dB for his Zen series so is this a good practical limit on the amount of gain for a single stage for these types of amps?

I've been flipping back and forth from the above design to a class-A PP common source(voltage amp stage) feeding a common drain PP(current amp) final stage. The advantage of the common drain final is a lower output impedance(any ball park figures?) which results in a higher dampening factor and makes the amp more efficient (less loss in lower impedance output) although I'm not sure if these things are very important? Any opinions on the better sounding circuit?

For determining the CCS resistor, the bias, the load resistors(if any) and the output impedance I wonder if making a chart of gate voltage vs. drain/source resistance is a standard practice or is it something an amp designer knows from experience or uses other data/calculations?

I have read that the output resistance of a common source stage is around 150ohms. I assume this is a general value and that for a TO-92 MOSFET it would be higher, for a TO220(IFR510) it might be about right and for a TO247(IFR240) it might be lower? Does anyone have any ball park numbers or is their a relatively simple way to calculate this from the datasheets or other methods?

I welcome any constructive criticism/advice. I don't want some else to do my work so please post a reference if you know of a source that can help and don't want to explain the basics. Once I have my design questions figured out I will post a schematic for review.

Thanks for any help.

Brock
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Old 16th October 2012, 07:29 PM   #2
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Old 16th October 2012, 10:52 PM   #3
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have you seen the Pass F5 amp?

might fit your description and specs.

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