Power Mosfet with high transductance et low capacitance ?

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#5
High transconductance calls for a bigger die and hence higher capacitance. Toshiba FETs seem to achieve the highest transconductance numbers but at the expense of parasitic capacitance. Might be worth looking at TK7S10N1Z - gm looks fairly good at 2A ~5 but at 125mA its anyone's guess as there's no gm vs ID plot. Its only a 50W part but it doesn't show secondary breakdown on the SoA plot.
 
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#7
In fact it is for this amp :

The voltage amplifier is just here to try the amp... it could be replaced by a balanced BA2 or BA3

The output stage look like a circlotron BA3 output stage

a cascode connexion would add more mosfets, there are already enough

thanks for TK7S10N1Z, i look but DPAK is difficult to use with heatsink

https://circlotron.audio/sites/default/files/2020-09/RADIUS_2020.png
 
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#8
One thing you have to be careful with with modern power MOSFETs, especially the ones with huge transconductance, is Spirito instability. That's essentially the same as thermal second breakdown.

Looking at the TK7S10N1Z safe operating area, the current drops faster than inversely proportionally with increasing voltage, which must be due to Spirito instability. It can only handle 65 mA at 70 V if you somehow manage to keep its case at 25 degrees C, otherwise even less.
 
#9
Good point @Marcel - I got seduced by the lack of there being two slopes on the SoA, in fact there is just one, the secondary breakdown one. Normally MOSFETs have some area where they're thermally limited but this particular FET doesn't have that - in fact nowhere on the DC line does it reach its rated 50W dissipation - the closest is at 7A (* 4.5V).
 
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