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Buffer with Dual N-Ch Jfets and Dual Power Supply
Buffer with Dual N-Ch Jfets and Dual Power Supply
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Old 6th April 2019, 08:09 PM   #41
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Buffer with Dual N-Ch Jfets and Dual Power Supply
Quote:
Originally Posted by vdi_nenna View Post
Ok, I understand. How do I find yfs?
Datasheet gives a range or typical.
work first on Idss , think later of Yfs
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Old 6th April 2019, 08:12 PM   #42
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Datasheet gives you a graph of Yfs vs Id.
You already have all the hints you need from my 2 links to build a high performance source follower with active parts.
No need to use J113 or whatever else.

There are no better N-JFETs than Toshiba, for me at least.


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Old 7th April 2019, 03:30 AM   #43
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Two very close matched 2sk170 at 9.76 mA gave a dc offset of .3v.
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Old 7th April 2019, 06:38 AM   #44
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Assume that you have shorted the input gate to Gnd, and your rail voltages are symmetrical to say 0.1V.
Then something is likely to be wrong with your matching.

If you have matched to 10A under the same temperature, I expect your offset to be < 1mV.
That is when you use Fig. 15c with Rs = 0.


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Old 7th April 2019, 10:42 AM   #45
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exact schematic needed

before that , I can only presume that upper source resistor need to be trimmed for offset
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Old 7th April 2019, 12:11 PM   #46
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I meant .3mV. Typing on phone late at night...

I tried the formula for Rs, but getting values under 1 ohm. Not sure I'm doing it correctly. Maybe it's because the idss is under required Id?
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Old 7th April 2019, 12:13 PM   #47
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ok , that's good

checked cold/hot difference?
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Old 7th April 2019, 03:21 PM   #48
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Started at .4 mV offset. After a couple of hours on, it's at around .9 to 1 mV dc offset.
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Old 7th April 2019, 10:05 PM   #49
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After 4 hours, and turning on the DMM, still settles at .9mA. Goes from 1.1mA down to .9 after a few seconds.

I'm using an 18 Ohm source resistor, because that's close to what the article use and thought it would be safe since I'm using 15v+/- source. I'm having trouble calculating the Rs value.
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Old 7th April 2019, 10:07 PM   #50
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what's voltage across 18R
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