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Old 21st May 2005, 06:02 PM   #1
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Default input stage current required to drive output mosfets?

my output stage is 2 paralleled mosfets with 1400pF of input cap each.

they are operated in source follower config.

how much current should be previous stage operate at to allow decent HF response and minimal distortion?

My input stage is a hybrid u-follower and trying to judge at what current to run it at....

Thanks!
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Old 21st May 2005, 06:09 PM   #2
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It's important to know have much gm your mosfets have. If gm is much the 1400 pF gets less but we must also know the load, 4-8 ohms I suppose?

Cgd is also something to count in.

We must also know max frequency and max voltage => desired slew rate.

SR = I/C
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Old 21st May 2005, 06:26 PM   #3
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Default ok ...

the output stage is 2 mosfets... maybe irfp140n's.

run at 25v and 1.5A each... source follower

connected to 8 ohm load thru 1:1 output transformer.

so max frequency is probably 30-40Khz with swing up to PS voltage.

gfs = 11

input cap is 1400pF
output cap is 330pF
reverse transfer cap is 170pF

does this help?

wondering if a 18mA input stage will cut it..
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Old 21st May 2005, 06:50 PM   #4
mikeks is offline mikeks  United Kingdom
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Use a complementary emitter-follower driver with cross-coupled emitters....(preferably two pairs in series)

....and you wont have to worry about how much input capacitance your MOSFETs posses....


Select R<=511Ohms for the cross coupling resistor...
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Old 21st May 2005, 07:06 PM   #5
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Default well...

actually wanted to use a ECC99 tube with mosfet source follower as load... i.e. u-follower.

18mA through tube... can run u-follower higher if wanted too... see alan kimmels stuff.

question is what current is good.. is 18mA good or should I run it at 50mA?

my 300v PS is good to 100mA.
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Old 22nd May 2005, 06:12 AM   #6
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Default Output x-former

Hey Audiogeek,

What are you using for output iron? I would interested in the specifications and source for the output transformer.

BTW. I used to live near Saint Paul. I am now in Indiana and working my way south as the cold is less appealing every year.

mike
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Old 22nd May 2005, 04:05 PM   #7
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Default output tranny

probably custom made by Jack at Electra-Print
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