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how about using J111 J175 Jfets in F5
how about using J111 J175 Jfets in F5
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Old 14th March 2018, 07:29 PM   #81
llwhtt is offline llwhtt  United States
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Isn't this class of Jfets primarily used for switching and not amplification?

Craig
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Old 14th March 2018, 09:46 PM   #82
Nelson Pass is offline Nelson Pass  United States
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how about using J111 J175 Jfets in F5
I believe that was their intended use.
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Old 14th March 2018, 11:08 PM   #83
Mark Johnson is offline Mark Johnson  United States
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how about using J111 J175 Jfets in F5
Although have a look at The Art Of Electronics, 3rd edition, in section 8.6.5 Selecting low-noise JFETs
[for low noise amplifiers] we favor JFETs with high transconductance and low gate-cutoff voltages, but sadly many of the old favorites with superior performance have been discontinued. NXP's BF862 has become a "go-to" low-noise part, both quiet and with attractively low capacitance. Toshiba's 2SK170 has been single-handedly kept in stock by large factory purchases by Mouser [no longer true it seems], and the second-source LSK170 is available directly from Linear Integrated Systems, who also offer a matched dual version of the venerable '170, the LSK389. The excellent IFN146 and IFN147 are available directly from InterFET. Also consider the Fairchild J107, a low-cost switch that can moonlight as a low-noise JFET amplifier; with luck it will remain available.
I measured some Fairchild J1xx devices over the weekend, and found that

J113, J112, J111 all have the same transconductance; where they differ is in Vpinchoff (which results in different Idss too). Probably they all use the same die layout, and get different implant doses to create different Vp's.

J109 has a higher transconductance than J111-113. Probably has a bigger die with larger (W/L) ratio of the conductive channel.

J107 has a yet-higher transconductance than J109. Probably has a yet bigger die with yet larger (W/L). J106 gm is same as J107

J105 is too macho for my curve tracer (Idss > 1 ampere!!)
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Old 16th March 2018, 03:13 PM   #84
Mark Johnson is offline Mark Johnson  United States
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how about using J111 J175 Jfets in F5
Quote:
Originally Posted by Nelson Pass View Post
Having gone through this exercise previously, I found that the P's and
N's of the Fairchilds are not similar enough to easily give really low
distortion as complementary parts. If you don't mind a little 2nd harmonic,
then you're OK.
Here is an example which illustrates what Nelson is talking about. This is an Idss-matched pair of Fairchild J113 + J175. Although the Idss values are the same (21.2 mA), the transconductance ("gm" == slope == dI/dV) values are NOT the same. Pew.

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File Type: png Winner_J113.PNG (39.4 KB, 220 views)
File Type: png Winner_J175.PNG (39.1 KB, 222 views)
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Old 16th March 2018, 03:33 PM   #85
DPH is offline DPH  United States
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Use 3:2 and be even closer.
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Old 16th March 2018, 03:51 PM   #86
sottomano is offline sottomano
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Quote:
Originally Posted by Nelson Pass View Post
[...]
You can tweak around with them to reduce that 2nd, but it's pretty tedious
and cumbersome. [...]
Or build a balanced gain stage...
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Old 17th March 2018, 11:19 AM   #87
marconi118 is offline marconi118  Germany
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What about using a j175 with 1mA more idss than the j113 and degenerating it with 10r resistor?
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Old 17th March 2018, 06:17 PM   #88
Nelson Pass is offline Nelson Pass  United States
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how about using J111 J175 Jfets in F5
I think you will find the J176 to be a better match to the J113. Push-pull
degenerated down to 10mA bias, I get about .016% thd driving 10 Kohm
at 1 volt.
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Old 17th March 2018, 06:55 PM   #89
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by DPH View Post
Use 3:2 and be even closer.
What I've been saying get the beta (gm parameter) to match not Idss. I tried to make this clear to the guys at Linear Systems, the first samples of J74's had low beta (gm parameter) and trying to change things to increase Idss might only increase Vp also. Last I looked they were more on the right track.
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Last edited by scott wurcer; 17th March 2018 at 07:10 PM.
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Old 17th March 2018, 07:06 PM   #90
scott wurcer is offline scott wurcer  United States
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Quote:
Originally Posted by Mark Johnson View Post
Here is an example which illustrates what Nelson is talking about. This is an Idss-matched pair of Fairchild J113 + J175.
More importantly the process on the P seems hopelessly noisy.
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