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Old 25th August 2011, 05:09 PM   #1
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Default Biasing lateral FET's, component placement

For biasing laterals FET's used for output, driving them directly from the VAS, a standard 2 resistor/1 transistor Vbe multiplier would be good enough I think? The Vbe transistor would not have to go on the heatsink, right?

Now what if you incorporate a pair of BJT drivers, would it not work well enough to have the Vbe transistor and the 2 BJT drivers mounted on the same small heatsink? Hence, no connection to the main HS either.

The BJT drivers would be biased to 10-20 mA so the thermal compensation should be rock solid once they heat up the small heatsink.

Last edited by Neutrality; 25th August 2011 at 05:13 PM.
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Old 25th August 2011, 05:14 PM   #2
wahab is offline wahab  Algeria
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If it s Hitachi/Renesas Laterals , a simple pot will be enough generaly.

With drivers , use two diodes in serial with the pot.
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Old 25th August 2011, 05:24 PM   #3
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If it s Hitachi/Renesas Laterals , a simple pot will be enough generaly.

With drivers , use two diodes in serial with the pot.
Thinking about Semelab ALFET's or some of the Exicon Latereal FET's.

You sure just 2 diodes is enough? Sounds too simple and easy.
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Old 25th August 2011, 05:55 PM   #4
MiiB is online now MiiB  Denmark
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simple resistor/trimmer biasing is sufficient....they have negative tempco and are not hard to drive...5-10 mA is normally sufficient..
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Old 25th August 2011, 05:58 PM   #5
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simple resistor/trimmer biasing is sufficient....they have negative tempco and are not hard to drive...5-10 mA is normally sufficient..
Hmm, so basically nothing needs to be done with the BJT drivers? No Vbe multiplier for those and they would not even have to be mounted on the same heatsink?
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Old 25th August 2011, 06:16 PM   #6
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simple resistor/trimmer biasing is sufficient....they have negative tempco and are not hard to drive...5-10 mA is normally sufficient..
I agree with Michael. Makes the PCB a lot simpler.
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Old 25th August 2011, 06:22 PM   #7
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So BJT drivers could just do without any compensation, due to them operating purely in class A and never switching to class AB due to the ease of driving the lateral FET's.
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Old 25th August 2011, 06:26 PM   #8
MiiB is online now MiiB  Denmark
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nope...laterals have also superior current sharing...so they need no balancing resistors either...

N and P channels have different capacitance.. so gate stoppers should be different..(like 150 and 270 ohms) or you can add some extra capacitance to the N channel...
but there can be some religion in that....some prefer to use low value gate stoppers and a carbonbeads...

for shure good practice is to have a small fast capacitance like 100nF just above the leg...
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Old 25th August 2011, 06:40 PM   #9
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nope...laterals have also superior current sharing...so they need no balancing resistors either...

N and P channels have different capacitance.. so gate stoppers should be different..(like 150 and 270 ohms) or you can add some extra capacitance to the N channel...
but there can be some religion in that....some prefer to use low value gate stoppers and a carbonbeads...

for shure good practice is to have a small fast capacitance like 100nF just above the leg...
So the drivers need no compensation whatsoever, interesting, makes it alot easier.

You sure about Laterals not needed current sharing resistors?

EDIT : How would you adjust bias then, since you cant measure across the non implemented balancing resistors. Hmm.
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Old 25th August 2011, 06:41 PM   #10
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for shure good practice is to have a small fast capacitance like 100nF just above the leg...
What do you mean? Supply decoupling?
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