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Old 12th January 2012, 04:48 AM   #1
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Default Mosfet power amp Vbe multiplier

I would like to know: do mosfet power amps also use a bjt vbe multiplier attached to the heatsink for thermal tracking?
I thought with the mosfets having a neg temp coefficient, a vbe multiplier attached to the heatsink wouldn't be necessary?
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Old 12th January 2012, 05:00 AM   #2
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That can work. If you want to be true to the concept, use a Vgs multiplier. It works the same, only a bit better, as there's no base current error.
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Old 12th January 2012, 06:29 AM   #3
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"a vbe multiplier attached to the heatsink wouldn't be necessary?"

Correct, on lateral FETs only though.
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Old 12th January 2012, 06:49 AM   #4
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Actually a negative temperature of Vgs with temperature makes HS attachment of some kind of compensation necessary, as drain current will rise with temperature for a given fixed bias level. It's a positive tempco you're looking for, as is exhibited by lateral fets at modest bias current levels (a couple of hundred ma), and for vertical fets at bias current levels that blow up the bond wires.
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Old 12th January 2012, 09:17 AM   #5
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Not sure which one of the two coefficients it is then. The one thats opposite from Bjt's.......
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Old 12th January 2012, 10:05 AM   #6
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The IRF type (vertical) FETs have a slightly positive Rds, but have a negative Vgs, so they require compensation (unless running class A).
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Old 12th January 2012, 12:25 PM   #7
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Laterals mosFETs have the source PIN in the middle (or on the case for To3)
Vertical mosFETs have the drain PIN in the middle (or on the case for To3)
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