[sdiy] Weird JFET in Korg MS-10/20/50 style VCO

[Juergen Haible]

It's the upper part of a cascode stage.
Probably for increasing the output impedance of the expo converter, i.e. 
making it closer to a current source.

JH.


----- Original Message ----- 
From: <lanterma at ece.gatech.edu>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Saturday, April 16, 2011 9:53 AM
Subject: [sdiy] Weird JFET in Korg MS-10/20/50 style VCO


> Howdy y'all,
>
> Consider the VCOs in the middle of this page:
>
> http://www.korganalogue.net/korgms/images/service/ms20/circ1.gif
>
> C2 (and the equivalent cap in the similar VCO circuit below it) appears to 
> be an integrating cap for the VCO, fed by the current sinked from a C1583 
> NPN pair through a 10K resistor.
>
> But there's also a JFET there (Q5). The current flows through the source 
> and drain of the JFET, and the gate of the JFET is grounded.
>
> Anyone by any chance know what the purpose of that JFET is?
>
> - Aaron
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