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 Forum index » DIY Hardware and Software
Voltage Controlled JFET Filter?
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alkopop79



Joined: Aug 21, 2008
Posts: 44
Location: London

PostPosted: Thu May 09, 2013 3:54 am    Post subject: Voltage Controlled JFET Filter? Reply with quote  Mark this post and the followings unread

I wonder, since JFETs are capable of creating varying current, can they be used as the heart of a VCF? The JFET could replace the potentiometer or OTA, the gate theoretically could control the cutoff. I know JFETs have a rather non-liner response but is there such thing as JFET filter?
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JovianPyx



Joined: Nov 20, 2007
Posts: 1249
Location: West Red Spot, Jupiter
Audio files: 161

PostPosted: Thu May 09, 2013 9:51 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm sure I've seen a schematic or two, I have no idea where though. I did a cursory search and found stuff related to using a FET as a voltage control element and found:

http://users.ece.gatech.edu/~lanterma/sdiy/datasheets/transistors/vishay_fet_cvr_an.pdf
http://www.experimentalistsanonymous.com/diy/Schematics/Filters%20Wahs%20and%20VCFs/4007%20Based%20VCF.pdf

You can probably find others with google "fet voltage controlled filter".

Part of the difficulty is the way FETs are biased (you can find this information by searching).

One way to eliminate the biasing difficulties is to use a "floating" FET, or MOSFET. H11F1, H11F2 and H11F3 are parts that might help. They are opto-isolators with a MOSFET and a LED inside. This allows using the LED to control the transconductance of the MOSFET so that it can be used as a resistor to control current in a circuit.

The LED should be controlled by a current source.

In the long run though, I see no real advantage to using a FET or MOSFET for controlling a filter. Not being in the audio signal path, it won't add any "special character" to the sound (in my opinion). It's nonlinearity might be fun though, depending on the nature of the nonlinear response curve.

HTH.

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