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Old 30th March 2005, 05:10 PM   #11
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You will get better thermal tracking and a more appropriate
voltage with a Vgs multiplier.

Depending on the Source resistance, the regulation through
the VAS, the amount of sinking, size of the output stage and
the bias current, you might be able to use a resistor for the bias -
increasing any or all of these increases bias stability.
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Old 31st March 2005, 11:12 AM   #12
ilimzn is offline ilimzn  Croatia
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(Obviously) I would have to concur with Nelson, he is THE guru after all
In fact, chosing the correct MOSFET in the Vgs multiplier can even enable the use of a single pair MOSFET output entirely without source resistances (Better Gm, but read on), plus for best results you want to carefully look at the (measured) data on your MOSFETs. Once again, IRFPXXX/9XXX (XXX being the same number for both N and P) are not by far the ideal complements, but if you take the unorthodox path, you can find ones that are very well matched.
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Old 1st April 2005, 01:58 AM   #13
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A design using IRFP240/9240pairs x4 with no degen in a nested FB with IRF510 as a Vgs multiplier was published in Electronics Now Oct'95. It's an interesting circuit getting around the Vgs losses but uses no degen for sharing or thermal reasons. I'd want to see it tracking/sharing
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Old 1st April 2005, 03:43 AM   #14
zox2003 is offline zox2003  United States
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Default Schematic???

Can you post that schematic please?
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Old 1st April 2005, 05:23 AM   #15
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I have a hard copy but no scanner. Must buy one.
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Old 1st April 2005, 07:50 AM   #16
ilimzn is offline ilimzn  Croatia
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Heh, I would not go as far as using a Vgs multiplier and multiple parallel pairs without degeneration. But I have used a Vgs multiplier and no degeneration with single pair outputs so many times I've lost count
Choosing a small MOSFET (2N7000, BS107, BS170) with this approach will even overcompensate a pair of IRFP outputs...
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Old 1st April 2005, 09:36 AM   #17
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Default Re: HIgher voltage rails???

Quote:
Originally posted by zox2003
Mu power supply is +/- 42V .I do not have option to increase rails for input stage and Vbe.
Why not add a small transformer in series with your 42v power supply.
This costs very little, takes up little space, with regulation it works as good as independantly from the main PS.
I've used that on a few amplifiers, currently using this approach for the Leach amplifier with discrete +/- 75 vdc regulators.
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Old 1st April 2005, 10:21 AM   #18
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Don't you mean a higher voltage transformer that has it centre tap connected to the main centre tap? So kind of parallel not secondary.
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Old 1st April 2005, 11:45 AM   #19
ilimzn is offline ilimzn  Croatia
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It's not too difficult to get high voltage rails at lower current, enough for the front end + driver of an amp, using a charge pump off the main transformer. In theory, the extra rails are 2x original rails, but using appropriate regulation and capacitors you can tailor that as you need it.
Look for the A75 under amplifiers -> legacy projects at passdiy.com for an example how to do this.
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Old 1st April 2005, 01:16 PM   #20
zox2003 is offline zox2003  United States
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Default Hey,ilimzn

Mozes li mi poslati kakvu semicu?NEsto sa IRF ali da radi 100%,ali da nije komplikovano.Treba mi za prijatelja.
Sta ima novo u Zagrebu?


Pozdrav



I need schematic.Anybody has schematic?
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