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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 6th January 2010, 05:21 PM   #3381
scott wurcer is online now scott wurcer  United States
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Originally Posted by john curl View Post
Idss is controlled by another variable, does anyone know what it is?
Do you? Come on John on these short channel JFET's there is some variance from drawn geometry and you CAN'T see it in a microscope. The Idss probably has most of its variation from implant dose. The basic equations are readily available for Vp, Idss, and Cgs,gd vs geometry, junction depth, doping, etc.
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Old 6th January 2010, 05:28 PM   #3382
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John Curl's Blowtorch preamplifier part II
Hm, let me re-phrase. Since the absolute value of said capacitance isn't what I'd be after, is there enough variability in samples of the same device, say a few 2sk170 to warrant measurement and ranking? Any easy way to gauge the capacitance of interest visually? Like SY's suggestion?
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Old 6th January 2010, 05:29 PM   #3383
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What is the mechanism for the spread in Idss? It is not geometry, is it?
Surprise, surprise, short of implant dose, it's mostly geometry. Namely, the W/L process variations, and the short channel effects (aka channel length modulation).
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Old 6th January 2010, 05:30 PM   #3384
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Well, everyone, I was told this morning by Edwin Oxner, consultant to Siliconix and Interfet, author of books on FET design, and friend to Linear Systems, that Idss is primarily caused by differences in resistivity of the silicon material. He said that if they were made with GaAs, they could all have virtually the same Idss, because the material can be controlled more easily. That's what he says, I am only the messenger.
PS. He liked the pictures of Burning Amp that Kirkwood Rough sent him. Kirkwood is next on the list to ask.
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Old 6th January 2010, 05:36 PM   #3385
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All those DIY'ers not in the industry posting her, raise your hands...
Raises hand - but that doesn't mean the other stuff doesn't apply to me .
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Old 6th January 2010, 05:37 PM   #3386
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Speaking of geometry, anything that can be derived from this picture ?

(i'm a food hoarder, the kids overhere call me a fat, dumb, old, bald guy. Dirty is somewhere in there as well, but i forgot the rank)
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Old 6th January 2010, 05:38 PM   #3387
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I do know this: If I directly replaced a 2SK146-J73 pair with a 2SK240-J75 pair, with the SAME Idss, my measured distortion with a 10K input pot, would drop. This is what I did with the JC-80 discussed here on previous pages.
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Old 6th January 2010, 05:46 PM   #3388
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the kids overhere call me a fat, dumb, old, bald guy. Dirty is somewhere in there as well, but i forgot the rank)
are we allowed to start a lottery on who can rank them in the correct order?
Sorry, I'm more than a little off topic.
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Old 6th January 2010, 05:55 PM   #3389
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Originally Posted by john curl View Post
Well, everyone, I was told this morning by Edwin Oxner, consultant to Siliconix and Interfet, author of books on FET design, and friend to Linear Systems, that Idss is primarily caused by differences in resistivity of the silicon material. He said that if they were made with GaAs, they could all have virtually the same Idss, because the material can be controlled more easily. That's what he says, I am only the messenger.
PS. He liked the pictures of Burning Amp that Kirkwood Rough sent him. Kirkwood is next on the list to ask.
Not for short channel JFETs like 2SK170, that also have the channel doping controlled by ion implantation.

What Ed is telling is a good approximation for long channel JFETs, like the 2N5458.

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Old 6th January 2010, 05:56 PM   #3390
scott wurcer is online now scott wurcer  United States
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He said that if they were made with GaAs, they could all have virtually the same Idss
John what do you think controls the channel resisivity?

NEC N channel GASFET -> (Don't drool folks it probably has 1000nV/rt-Hz @ 1kHz)
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