[sdiy] jfet driver

David G Dixon dixon at mail.ubc.ca
Mon Feb 25 21:15:39 CET 2019

Hi Tim,

Here's how I think about it -- I'm sure the "know-it-alls" will chime in and
tell me how wrong I am, but I know what works for me:

N-JFETs are ON (like a short circuit from Source to Drain) when the Gate is
at ground.

N-JFETs are OFF (like an open circuit from Source to Drain) when the Gate is
held LOW.

The question is, HOW LOW?  That depends on the JFET.  Some, like the 2N4391,
are fairly unforgiving and will only reliably shut off when the Gate is held
at about -10V or less.  It is best to take these right to the negative rail.
Others, like the 2N5485 (which I see is now "obsolete" on Digikey -- too
bad, it's a useful one) are more forgiving, and will "usually" shut off at a
Gate voltage of about -4V.

This assumes that one side of the JFET is held either at ground or at
virtual ground -- in my experience, it doesn't matter which direction a JFET
is installed (S and D are interchangeable -- some datasheets will make this
explicit, others won't -- YMMV).  The shutoff is actually driven by the
Gate-Source voltage difference, but in most of the circuits where I use
JFETs, the Source is connected to ground or virtual ground anyway.

I use 4391 to short-circuit VCO integrators for hard sync.  I drive them
with a comparator which is wired to generate GND when up and -V when down.
This works.

I use 5485 all over the place, but mostly for switching inverter/followers.
If an opamp is wired up as a differential amp, but with a JFET off of the +
input to ground, then when the JFET is on, the opamp is an inverter, and
when off, the opamp is a follower.  This is an extremely useful circuit
element.  Often, the thing driving this is a square wave which is nominally
10Vpp.  For this, I put a diode in front of the gate to remove the positive
half of the square wave.  The -5V part of the square wave is enough to turn
off the 5485, but typically not enough to turn off the 4391.

Hope this helps!

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On 
> Behalf Of Tim Ressel
> Sent: Monday, February 25, 2019 9:20 AM
> Subject: [sdiy] jfet driver
> Hi,
> It is time to admit: I don't really know transistors that 
> well, much to my shame. I grew up in an op-amp world, and 
> other than driving relays I just don't grok the transistor.
> I am trying to drive an n-channel jfet with a 5 volt ttl 
> signal. I know this is easily done with a few components. Can 
> anyone point me in the right direction?
> thanks!
> --
> --Tim Ressel
> Circuit Abbey
> timr at circuitabbey.com
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