Everything You Wanted to Know About JFETs (but Were Afraid to Ask)

peccary

Well-known member
Oh, man... I spent the last few days trying to decipher the Rod Elliot "designing with JFETs" piece and now this!

I feel like a blind man trying to put a puzzle together with work gloves on 🤣
 

jubal81

Well-known member
Screen-Shot-2022-02-04-at-6-35-07-PM.png
 

bifurcation

Well-known member
See, what I come to expect from Chuck when I see a title like "Everything You Wanted to Know About JFETs (but Were Afraid to Ask)" is Chuck to write a very clear and concise explainer written in simple language that my limited intelligence can parse.

Not a link to a BOOK fergodssake! What do you expect me to do, read it???
 

Chuck D. Bones

Circuit Wizard
...a BOOK fergodssake! What do you expect me to do, read it???

Yes, the same way you would read an encyclopedia. You look up what you need, not read it cover to cover. The best parts are some of the application circuits in section 6. Check out the phase shifter at the top of page 6-33. Replace the 1M pots with optos. Reminiscent of the UniVibe, only with JFET stages it takes fewer parts. Or maybe the preamp circuit at the bottom of page 6-36 could be adapted to pedal use.
 

jubal81

Well-known member
Yes, the same way you would read an encyclopedia. You look up what you need, not read it cover to cover. The best parts are some of the application circuits in section 6. Check out the phase shifter at the top of page 6-33. Replace the 1M pots with optos. Reminiscent of the UniVibe, only with JFET stages it takes fewer parts. Or maybe the preamp circuit at the bottom of page 6-36 could be adapted to pedal use.
The sample circuits are awesome. I always knew them as the AN-32 document.
 

lrgaraujo

Member
Yes, the same way you would read an encyclopedia. You look up what you need, not read it cover to cover. The best parts are some of the application circuits in section 6. Check out the phase shifter at the top of page 6-33. Replace the 1M pots with optos. Reminiscent of the UniVibe, only with JFET stages it takes fewer parts. Or maybe the preamp circuit at the bottom of page 6-36 could be adapted to pedal use.
Hey Chuck, is there any particular reason for the different source and drain resistors in the two stages of the phase shifter? I'm willing to give it a try when I find the time
 

mybud

Active member
Thanks, this is super-useful. They (JFETs and their relatives) are quite quirky beasts, I have found. I’m not sure I understand entirely how they work their magic, but if I don’t after this …
 

Chuck D. Bones

Circuit Wizard
Hey Chuck, is there any particular reason for the different source and drain resistors in the two stages of the phase shifter? I'm willing to give it a try when I find the time
I think it's because of two things: 1) the stages are DC-coupled, 2) the gate and source are not at the same DC voltage. That means the bias is different for every stage. Add more stages and you run out of headroom. The right way to do it is AC-couple the stages like Mieda-san did with the UniVibe.
 
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