[sdiy] Dual Matched N JFET from Linear

[René Schmitz]

On 20.06.2025 23:15, david walker via Synth-diy wrote:
> hello taking about JFET,s can any point out any were on line that show 
> you how to use JFET,s in oscillator circuits ?


A prominent example is the SEM1a from Oberheim:


JFET as high-Z buffer in the VCO

https://synthfool.com/docs/Oberheim/Oberheim_SEM1A/Oberheim_SEM_1A_Schematics.pdf

Two JFETs (Q2/Q3) are used in a source follower configuration to buffer 
the main integration cap with high impedance.

Today we would probably find a jfet opamp instead.

(JFETs are also used in the SEM filter.)


Another example:

JFETs as discharge switch

https://schmitzbits.de/vco3.html

This one used two FETs, but they don't need to be matched. And you can 
do this with a single FET as well.


I've only used once a dual JFET where good matching was required (in 
fact this was a 2SK389), but this circuit was never published.


FETs have large unit to unit variations, good for one-off hobbyist or 
space mission work, but

annoying to show-stopper for mass production. So you'll not see them 
that often in the wild.


Cheers,

  René




> On Friday 20 June 2025 at 21:58:12 BST, Mike Bryant 
> <mbryant at futurehorizons.com> wrote:
>
>
> LSK389 is hardly new.  Been around for a long time, although 
> availability varies.
> But the price never seems to vary.
>
> ------------------------------------------------------------------------
> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Tim 
> Parkhurst via Synth-diy <synth-diy at synth-diy.org>
> *Sent:* 20 June 2025 21:34
> *To:* synth-diy at synth-diy.org DIY <synth-diy at synth-diy.org>
> *Subject:* [sdiy] Dual Matched N JFET from Linear
> Hey, Analog Hounds,
>
> Linear Systems is promoting a new Matched Dual JFET, and you might 
> even be able to get a sample or two. Useful? Not?
>
> https://www.linearsystems.com/post/why-engineers-choose-the-lsk389-for-high-precision-analog-design?utm_campaign=d2b0395f-db24-487d-8e02-adcff714e7a8&utm_source=so&utm_medium=mail&cid=a05ef50a-a9c5-473d-950b-37f3f1a2d83d 
> <https://www.linearsystems.com/post/why-engineers-choose-the-lsk389-for-high-precision-analog-design?utm_campaign=d2b0395f-db24-487d-8e02-adcff714e7a8&utm_source=so&utm_medium=mail&cid=a05ef50a-a9c5-473d-950b-37f3f1a2d83d>
>
> Tim (define "useful") Servo
> ---
> "Imagination is more important than knowledge." - Albert Einstein
>
>
>
>
> ------------------------------------------------------------------------
> *From:* Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Tim 
> Parkhurst via Synth-diy <synth-diy at synth-diy.org>
> *Sent:* 20 June 2025 21:34
> *To:* synth-diy at synth-diy.org DIY <synth-diy at synth-diy.org>
> *Subject:* [sdiy] Dual Matched N JFET from Linear
> Hey, Analog Hounds,
>
> Linear Systems is promoting a new Matched Dual JFET, and you might 
> even be able to get a sample or two. Useful? Not?
>
> https://www.linearsystems.com/post/why-engineers-choose-the-lsk389-for-high-precision-analog-design?utm_campaign=d2b0395f-db24-487d-8e02-adcff714e7a8&utm_source=so&utm_medium=mail&cid=a05ef50a-a9c5-473d-950b-37f3f1a2d83d 
> <https://www.linearsystems.com/post/why-engineers-choose-the-lsk389-for-high-precision-analog-design?utm_campaign=d2b0395f-db24-487d-8e02-adcff714e7a8&utm_source=so&utm_medium=mail&cid=a05ef50a-a9c5-473d-950b-37f3f1a2d83d>
>
> Tim (define "useful") Servo
> ---
> "Imagination is more important than knowledge." - Albert Einstein
>
>
>
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-- 
--
synth at schmitzbits.de
http://schmitzbits.de
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