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Old 3rd February 2002, 11:59 PM   #1
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Question Biasing output stage FETs

Hi Guys,
Can anyone point me to an online tutorial, or give me a breif overview of how to bias output stage fets for a class AB amp?
This is my first attempt at a fet amp, and although I could just copy an output stage from someone else's schematic, I'd like to get an understanding of what's going on...I don't want to blow up those expensive devices!!! (2sk1058 & 2sk162)
I'm planning on driving them with an SRPP valve stage, via a capacitor, not direct coupled, and using a single ended power supply taking the speaker out via a cap., (this amp will be for guitar, you audiophiles can relax...)
so the biasing arrangement should be fairly simple, I'm just not sure how to work the numbers.

any help for this newbie would be appreciated,
cheers, Pete McK
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Old 4th February 2002, 01:44 AM   #2
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Default Re: Biasing output stage FETs

It would be much easier to help if you post a schematic of what you have so far. I have no idea how you are configuring the OPS.

Quote:
Originally posted by PeteMcK
Hi Guys,
Can anyone point me to an online tutorial, or give me a breif overview of how to bias output stage fets for a class AB amp?
This is my first attempt at a fet amp, and although I could just copy an output stage from someone else's schematic, I'd like to get an understanding of what's going on...I don't want to blow up those expensive devices!!! (2sk1058 & 2sk162)
I'm planning on driving them with an SRPP valve stage, via a capacitor, not direct coupled, and using a single ended power supply taking the speaker out via a cap., (this amp will be for guitar, you audiophiles can relax...)
so the biasing arrangement should be fairly simple, I'm just not sure how to work the numbers.

any help for this newbie would be appreciated,
cheers, Pete McK
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Old 4th February 2002, 03:49 AM   #3
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Default Rough Shem

Hi Randy, sorry for the crudity, but paint's all I've got at the moment:
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Old 4th February 2002, 04:01 AM   #4
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Default Rough Schem

Hi Randy, sorry for the crudity, but paint's all I've got...
(I hope this comes out, first time I've tried to attach a pic)
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Old 4th February 2002, 03:20 PM   #5
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Default Biasing Output Mosfets

Here is a link that may help a bit:
http://www.homestead.com/whaan/files/index.html
and this:
http://www.tubecad.com/april_may2001...%20Technic.gif
And this:
http://www.tubecad.com/april_may2001/page9.html

dshortt9
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Old 4th February 2002, 09:55 PM   #6
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Default that's it!

Hi DShort,

Thanks for the links
The second link you provided is pretty much the amp i'm building, just with lower voltages; 40v for the FEts and 80v for the valve, which will be a 12ax7.
I suppose I'm just trying to get an understanding of what the proper gate to source voltages should be.

Cheers, Pete McK
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Old 5th February 2002, 03:35 PM   #7
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Default Biasing Fets

Doing the math shows the mosfets in the schematic being biased at 1.27 volts each. This should give about 500ma of idle current according to the data sheet.
I am concerned with using a 12AX7 as you will only have a couple of ma. of current to drive the mosfets.
The math can be found at this link http://www.tubecad.com/november99/page3.html
This shows that you need a minimum slew rate of 5v/us. and 7.5 ma. to accomplish this at 20khz, using the combined capacitance of the mosfets. Keep in mind that this will give you a waveform resembling a triangle at 20khz, and phase beginning to fall off at 2khz.
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Old 7th February 2002, 09:37 PM   #8
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Default ...um, yeah...

...thanks, that's an aspect I didn't know about.
I might use a mosfet as a driver then, it would simplify the power supply too. Cheers, Pete McK
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