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Old 17th July 2004, 11:09 PM   #1
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Default confused on mosfet biasing!!!!!

help!!

how do i bias a mosfet for say 50K input impedance but still have good high frequency response??

looking to bias a mosfet at 1.5A bias 40v drain with a output transformer on the drain (64:8 Z ratio).

BUZ900 mosfet and around 2v bias.

is this possible?

Thanks!!!!
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Old 18th July 2004, 07:29 PM   #2
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You don't indicate the mode of Mosfet operation, or whether
the input value is a resistor to ground or a resistor in series
with the Gate.

Assuming the former, you can use a 50K value to DC bias the
input and place a couple hundred ohms and a cap in series with
the Gate. Driving that Gate, you will see the 50K at low
frequencies, but eventually the capacitance of the Gate will
become part of the circuit, much the same way that the
capacitance of the connecting cable will, but usually at a higher
value. If your source impedance is very high, you will see rolloff
at audio frequencies, but if you can keep it down around 1K or
so it won't be too much of a problem.

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Old 19th July 2004, 03:08 AM   #3
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Default hmm...

well.... i just need 50K input impedance...

dunno what is the difference between those two configurations for the input value.

to bias I can connect a 50K resistor from drain to gate then another variable resistor from gate to ground???

another question:

Can I add a 1.5 ohm resistor on the source to reduce distortion??

also how do I adjust the overall gain??

btw, the mosfet has a 64:8 transformer between drain and power supply as the load.

Thinking about adding that P-channel buffer on the front end....

with adjustable CCS to allow some distortion cancellation.

hmmm.... what else... thats all for now.
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Old 19th July 2004, 04:17 AM   #4
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Default Re: hmm...

Quote:
Originally posted by AudioGeek
to bias I can connect a 50K resistor from drain to gate then another variable resistor from gate to ground???

Can I add a 1.5 ohm resistor on the source to reduce distortion??

how do I adjust the overall gain??

Thinking about adding that P-channel buffer on the front end....

with adjustable CCS to allow some distortion cancellation.
1) Yes, but then you won't get a 50K input impedance

2) Yes

3) Method of 1) and method of 2)

4) Yes, that will get the input impedance right up there.

5) Sure thing.

I hope that answers your questions.
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Old 19th July 2004, 09:50 PM   #5
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Default input impedance

so...

what is the best way to bias the mosfet.. and what resistors determine the input impedance??

Thanks!
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