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Old 17th July 2004, 12:18 AM   #1
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Join Date: Jan 2004
Location: Minnesota
Default biasing a mosfet?

I am looking for online info on how to bias a mosfet all the different ways...

specifically how connecting a resistor from drain to gate affects operation of the mosfet?

I need to bias... say a BUZ900 at 40v 1A, but reduce gain with negative feedback...

do I do this with the resistor from drain to gate????

Thanks for any help!!
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Old 17th July 2004, 12:45 AM   #2
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Join Date: Apr 2003
Location: US
one resistor wouldn't work, as the gate (DC) current is zero.

Using a two resistor set-up like the ones used for BJTs wouldn't work either as most mosfets have a positive temp coefficient at low Ids.

I proposed a 5-resistor set-up (it can be further reduced to a 3 or 4 resistor set-up depending on what you want to do with the circuit) that is very thermally stable. It is under the thread "simple pre-amp" I think.

It is essentially one half of Nelson's latest Aleph (full symmetry). I have ran it as a pre-amp and power amp without any problem and it sounds very sweat (almost no high order harmonics).
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Old 17th July 2004, 06:21 AM   #3
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Join Date: Dec 2003
Location: Mumbai
hi

check the 3 types of biasing used in these designs

1) holtons designs (2 types) - in diy audio thread + aussieamps - site

2) Elektor Me - Elektor + diy audio - thread

3) Buz 160 W amp (Elector audio vedio sp) - if u serch for the above there are lots of sites with the schems of this amp + diy audio thread

4) Mackie Srm series powered spk amps , I had repaired one but had forgotten to draw out the schem of this - i havnt found it on the net yet

suranjan

transducer design engineer
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