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Old 31st December 2005, 07:40 PM   #1
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Default JFET matching

Since people are going ahead on purchasing the Lovoltech
power JFETs, I thought I would share some of my matching
data.

It's quite simple, as seen from the circuit. A measurement of
the Vgs under identical conditions is adequate for most
applications. The value is useful for creating differential pairs,
and in cascoded single-ended applications becomes valuable
when selecting a Source resistor to ground.

In the case of selecting a Source-to-ground resistor, the value
of the resistance can be used to set up the optimal load line
for the lowest distortion.
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Old 31st December 2005, 07:41 PM   #2
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Here's a distribution of values of the population of a couple
of tubes:
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Old 31st December 2005, 07:53 PM   #3
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Depending on all the other usual factors, optimizing the
Source resistance in a cascoded single-ended system can
improve the performance quite a bit. Your results will vary
somewhat, and so for a given circuit design, it is necessary
to test to find the optimal values, but here is an example
of a distortion curve in an arbitrary system where the cascode
has a grounded Gate. One curve shows the untrimmed value
of Source resistance, and the other has been adjusted slightly.
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File Type: gif jfetm03.gif (10.3 KB, 1438 views)
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Old 31st December 2005, 07:57 PM   #4
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For this same circuit, here is a curve of optimal resistance
values for a range of Vgs values. Again, this is simply an
example of a single design, and these values should not be
applied in general. For a given design, there will likely be a curve
which resembles this in principle which will obtained by actual
testing.
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File Type: jpg optimal resistance.jpg (36.4 KB, 1408 views)
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Old 31st December 2005, 07:58 PM   #5
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Happy New Year.

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Old 31st December 2005, 09:26 PM   #6
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by Nelson Pass
Happy New Year.

same to you and your folks



and many more-spent in good health and fun


hehe
same wishes to any DIYer around
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Old 1st January 2006, 02:53 PM   #7
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Nelson, and all DIYer's

Happy New Year,

lot's of circuits and thanks so much for your
power JFETs matching advise.

Alain.
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Old 2nd January 2006, 12:56 AM   #8
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How much I have learned thru Pass Labs forum . . .
Looking back myself . . .

Happy New Year !!!

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Old 2nd January 2006, 02:16 AM   #9
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Thanks Nelson,
and Happy New Year to you and all.
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Old 6th January 2006, 04:30 PM   #10
tmblack is offline tmblack  United Kingdom
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Mr. Pass,

How close do the Jfet's need to be matched as diff pair and as cascode?

Tom
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