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Old 25th February 2010, 11:50 AM   #1
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Default Upscaling Aleph-J SRPP tp 100W

By using high-current j-FETs at Aleph-J input one can upscale this schematic to 100W. Is it reasonable at a price of using p-channel output MOSFETs ?
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Old 25th February 2010, 01:08 PM   #2
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This version is more correct. Are there futher corrections?
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Old 25th February 2010, 06:48 PM   #3
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Ordinarily I let the availability of good cheap N channel output
Mosfets dictate the use of P ch Jfets on the input. It is not
difficult to parallel several of them to get the current up.

Also, on your schematic you definitely want some Gate stopper
resistors on your cascode transistors, say 100-221 ohms.

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Old 25th February 2010, 11:13 PM   #4
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I'm a novice when it comes to JFETs but I wonder if some source degeneration on the LTP would help alleviate the device matching requirements.

Also I wonder what the PSRR looks like, there are several resistors from the +35V rail that could be split and turned into RC filters ?
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Last edited by Bigun; 25th February 2010 at 11:17 PM.
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Old 26th February 2010, 03:31 AM   #5
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Quote:
Originally Posted by Nelson Pass View Post
Ordinarily I let the availability of good cheap N channel output Mosfets dictate the use of P ch Jfets on the input. It is not
difficult to parallel several of them to get the current up.

Also, on your schematic you definitely want some Gate stopper
resistors on your cascode transistors, say 100-221 ohms.

Dear Nelson, thanks for your input, the gate stopper must be added. I know opinions that in schematics with NFB, p-chann devices affect sound being used in either output or input stages. Also, in two-stage schematics the input stage theoretically needs more current, more current ... It would be interesting to check which combination is better, under 2 x 50mA input stage conditions, p-ch - n-ch or reverse, IMHO. As for prices, I get both 2SK1529 or 2SJ200 equally for 4 USD/piece. Questions of capacitors are more sensitive. But in USA, you are right, easier to parallel several 2SJ74 than to pay 18 USD for CP650 per piece. Since i already have a lot of j-fets similar to CP650, I am looking for possibilities of using them.
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Old 26th February 2010, 03:46 AM   #6
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Quote:
Originally Posted by Bigun View Post
I'm a novice when it comes to JFETs but I wonder if some source degeneration on the LTP would help alleviate the device matching requirements.
Yes, one might want to consider some source degeneration. This schematics is very draft. But in my case I have matched pairs of j-fets.
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Old 26th February 2010, 08:23 AM   #7
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One also might want to compare data for CP650 and 2SJ74.
CP650: gm = 0,150 S, Ciss = 20 pF, Idss = 500 mA
2SJ74: gm = 0,022 S, Ciss = 105 pF, Idss = 10 mA
For total current of 50mA, a 5-piece set of 2SJ74 results in Ciss=525pF,
to be compared with 20pF of CP650. So, it seems that CP650 might open a new quality in respective schematics. I know the one and only guru capable of schematics fine tuning and simplification. Hope he could get some interest to such mid power j-fets.
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