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[Amps] Using small switching MOSFETs

Subject: [Amps] Using small switching MOSFETs
From: Manfred Mornhinweg <>
Date: Sun, 06 Feb 2011 17:40:40 +0000
List-post: <">>
Hi all,

regarding the DL9AH design based on many small cheap switching MOSFETs 
in parallel, I have a question that maybe someone can answer.

Older MOSFETs, like the IRF710 he used, typically have capacitances that 
increase at a rather even rate as the drain voltage gets lower, reaching 
a sort of soft knee somewhere around 8V or so, where the capacitances 
start increasing at a higher rate. Many more modern MOSFETs, instead, 
have a much flatter capacitance curve from the high voltages down to 
about 10V, but then the capacitance increases much more sharply than 
with the older MOSFETs, with a very pronounced knee.

My question is which of the two behaviors is better suited to linear 
amplification. The older FETs with the softer curve will inevitably have 
some capacitance variation induced distortion, even if the drain never 
pulls lower than 15V or so, but if overdriven (down to 5V or so), the 
distortion will increase only moderately. Instead the more modern FETs, 
with the sharp knee, should deliver an extremely low level of 
capacitance variation induced distortion when driven all the way down to 
12 or 10V, but if driven even a little bit further, the distortion will 

What would you think is the better approach? Using FETs with the softer 
capacitance variation curve, accepting some more distortion in normal 
operation, or using the FETs with the flatter curve and sharp knee, 
depending on a very effective ALC circuit to keep them out of the highly 
nonlinear zone?

Mathematical analysis of this is beyond my interest level. And to 
experiment it, I would first have to import a set of each kind of 
MOSFETs, because locally I can't get any of those...


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