JFETs as clipping diodes?

[earthtonesaudio]

...Using the constant-current trick of gate to drain/source with a resistor in between.  Anyone do this?  I can't think of why it wouldn't work, but I've never seen it in a schematic.

[R.G.]

Think about it.

Diodes of all stripes clip signal voltages by having some voltage above which an unlimited current can flow. The JFET set up as a constant current diode conducts resistively at Rdson until it hits Vpinchoff, then the current is fixed and no greater current will flow. It's the opposite of a clipping diode. I'm guessing that is why you haven't seen it in a schematic.

The gate-channel diode does act like a normal diode when forward biased, and so you can use JFETs for clipping by tying the drain and source together and using them as one terminal of the clipping diode, the gate as the other one. But that's just using the junction forward voltage for clipping, like any other diode. That one has been done many times, starting long ago. It's mildly different, but then so is every other diode.

[brett]

Hi
IMO there's probably mileage in more experimenting with JFETs as active loads, as in the "mu" amp, and done so well for regular transistors in the Jordan Bosstone. 

RG's "Foolin' with FETs" is an excellent article, and explains the principles, but few of us seem to have taken up the challenge.

(I found those (Russian?) "valve distortion from FETs" schematics a bit too complex and I wasn't really convinced they would be effective.)

cheers

[sfx1999]

You can use BJT transistors as clipping diodes, though. For an NPN, you can tie the base to the collector and use the collector as P and the emitter as N. It might be a good idea to use a resistor between the base and collector just in case your transistor cannot handle the base current.

[slideman82]

I've used JFETs as diodes... just find its P side and N side, in any combination of the 3 legs. Then solder the unused leg to one of the others, always respecting you have just one diode (for example, in a Si NPN bipolar, if you solder both N legs, you'll have two Si diodes in parallel).

[R.G.]

I've used JFETs as diodes... just find its P side and N side, in any combination of the 3 legs. Then solder the unused leg to one of the others, always respecting you have just one diode (for example, in a Si NPN bipolar, if you solder both N legs, you'll have two Si diodes in parallel).

Yep:

...
The gate-channel diode does act like a normal diode when forward biased, and so you can use JFETs for clipping by tying the drain and source together and using them as one terminal of the clipping diode, the gate as the other one. But that's just using the junction forward voltage for clipping, like any other diode. That one has been done many times, starting long ago. It's mildly different, but then so is every other diode.

[slideman82]

Sorry RG, I didn't ignore you... just put it in some easy words, even for me! These times English requires almost all my RAM memory, and I'm really sleepy today... Sleepy Time Time.

[earthtonesaudio]

Thanks everyone.  I think it would be fun to make a minibooster with JFET "diodes" to ground.  3 ways of using one component... you could call it something cute like "JJJFET."

[WGTP]

Time to drag out one of my old favorites.   

http://www.diystompboxes.com/smfforum/index.php?topic=38581.0

[gez]

I've messed around with a circuit that uses JFETs as diodes in the feedback loop of an op-amp.  I actually used the FETs' channels to do the clipping, not their gate-source junctions.  A couple of trim pots set the thresholds for clipping. 

I posted a MOSFET version, but it was flawed (forgot about the substrate).  The JFET version definitely worked, and was relatively simple, but was more trouble than it was worth as the JFETs had to be tested.  It was also +ve ground (those little plastic jacks weren't commonly available in the UK at the time, so I used to use metal jacks and design all my circuits that way).

[aron]

Also in simple mods.

http://www.diystompboxes.com/wiki/index.php?title=Simplemods#Simple.2C_Easy_Mods.2FTips_.26_Techniques

[DougH]

Here's another example. Something I whipped up on the breadboard a couple years ago:

http://www.aronnelson.com/gallery/main.php/v/DougH/OddsNEnds/820_.gif.html?g2_imageViewsIndex=1