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?
The Fets are driven from the collector of a CE BJT.
Is current choice different for a low Cob device?
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.
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.
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)
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)
"back of the envelope" estimate needs adjustment to exact circuit
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
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
Henry8,
anyway, you have the chance to build a nice sounding amp with those transistors. Could you share the schematic please?
anyway, you have the chance to build a nice sounding amp with those transistors. Could you share the schematic please?
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.
No real question, no real help. You haven't specified anything--what's the suppy voltage, power level, power bandwidth, exact VAS topology? Where's the schematic of your amp?
My short answer is: It's unclever to drive any power device directly from the VAS. Distortion will be unnecessarily high and there will very likely be some sort of large-signal limitation. Add an emitter follower in between.
Samuel
My short answer is: It's unclever to drive any power device directly from the VAS. Distortion will be unnecessarily high and there will very likely be some sort of large-signal limitation. Add an emitter follower in between.
Samuel
Henry8,
the driving current can be as high as you can go without thermally abusing the driving stage.
the driving current can be as high as you can go without thermally abusing the driving stage.
Amp topology is ltp/vas/output
psu -/+ 50V
100W/8R
VAS is a bjt in CE mode fed by a CCS
With this statement your disrespecting many amp designs.
If there s enough NFB ratio , driving 3 pairs of the discussed
devices directly from the VAS is fairly possible and works quite well.
As low as a 3 to 5 mA current is generally enough.
Although adding a pair of CE coupled drivers reduce THD in a
significant ratio , it also add another pole in the transfer function,
requiring a heavier compensation, but all in all , it worth it.
devices directly from the VAS is fairly possible and works quite well.
As low as a 3 to 5 mA current is generally enough.
Although adding a pair of CE coupled drivers reduce THD in a
significant ratio , it also add another pole in the transfer function,
requiring a heavier compensation, but all in all , it worth it.
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