How much VAS current to drive a pair of lateral FETs?

[Henry8]

How much VAS current is needed to drive a pair of lateral FETs (k1058/j162)?
The Fets are driven from the collector of a CE BJT.
Is current choice different for a low Cob device?

[richie00boy]

Current is demanded by the FET gates, so the Cob of the VAS transistor is not really of any relevance to this part of the design.

I found IRFP240/9240 vertical MOSFETs pull about 3mA at 20kHz. The lateral parts have about half the capacitance, so will take about half the current. I use a rule that the source should have approx 10 times the current it is required to deliver, so in that case I would run the VAS at about 15mA. This is probably a fair bit higher than what a lot of published schematics use.

[jcx]

the IRFP240 VMOSFET have way more nonlinear input C - especially near saturation so high VAS current is a good ieda

Lateral MOSFET are better behaved so aproximations based on ( output current slew demand / gm ) to get gate V slew requirement gives gate charging current - much smaller than your estimate only ~ 0.5mA required for "audio" so 5 mA VAS bias likely fine (Crss in Laterals is small enough to be ignored)

[WuYit]

jcx,
the input capacitance includes Cgs as well, that is 900pF for 2SJ162.

[jcx]

only makes a ~25% difference in Class A - in AB you might need 2x the current unless you use a "non-switching" bias which wouldn't discharge the off Q gate C

another topological difference would be if push-pull VAS was used

[WuYit]

Henry8,
anyway, you have the chance to build a nice sounding amp with those transistors. Could you share the schematic please?

[nigelwright7557]

Originally Posted by jcx

the IRFP240 VMOSFET have way more nonlinear input C - especially near saturation so high VAS current is a good ieda

With source follower outputs the input C becomes less significant because the source pretty much follows the gate voltage.

I use 15mA VAS current with 3 pairs of IRFP240/9240 and get a good bandwidth.

[jcx]

its Crss that becomes huge when Vds drops below 2*VT in Vertical MOSFET

[Zero Cool]

You know this is a really interesting question and thank you to richie00boy for his explanation. I have had my own theory that Mosfets need a large VAS driver stage. All of the mosfet output stage amps I like have very large current capacity driver stages. and far too many of the early designs had wimpy driver stages and i think this had a lot to do with the early descriptions of "mosfet mist" and other anomalies . this has always just been a guess on my part. But I do find it interesting that all of the amps i like have overbuilt driver stages. either by design or error, there is something about a large driver stage ahead of mosfets that I like.

[Henry8]

not much real help so far........................