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.
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.
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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.
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?
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...
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.
Hi,
On the supply rail maybe ? Ok, if you have multiple pairs you won't know bias for each of them separately unless you have individual supplies, but you will know the total bias current.
And if you match the mosfets on Vgs you will have pretty balanced bias between the pairs - my opinion.
Regards,
Tibi
This excellent application note from National Semiconductor will explain it:
http://www.national.com/an/AN/AN-1645.pdf
The "current sharing" resistor can serve to flatten out the transconductance of the devices. Bob Cordell discusses in Chapter 11 of his book.
http://www.national.com/an/AN/AN-1645.pdf
The "current sharing" resistor can serve to flatten out the transconductance of the devices. Bob Cordell discusses in Chapter 11 of his book.
They usually replace a fuse with a multimeter on mA setting and adjust it that way.
Thats how Maplin recomended for their lateral mosfet amp.
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