Mosfet power amp Vbe multiplier

[diy didi]

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?

 

[wrenchone]

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.

 

[djk]

"a vbe multiplier attached to the heatsink wouldn't be necessary?"

Correct, on lateral FETs only though.

 

[wrenchone]

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.

 

[diy didi]

Not sure which one of the two coefficients it is then. The one thats opposite from Bjt's.......

 

[djk]

The IRF type (vertical) FETs have a slightly positive Rds, but have a negative Vgs, so they require compensation (unless running class A).

 

[AndrewT]

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)