Beginners JFET FAQ

[Dr.H]

Perhaps we can start a FAQ here?
I've got a basic question:
I've looked at the Id vs Vds charts (with varying Vgs) and it all makes sense.

But, say I have a J310 and I'd like to build a simple buffer.

How do I choose the appropriate Vds for my application? What am I trying to optimise in the choice of Vds?

 

[Nelson Pass]

Choice of Vds is simple enough, as it mostly considers

1) How much output swing do you need? (Vds has to be higher than that)

2) What is the maximum Vds of the part? (Vds has to be lower than that)

3) What is the load? (The Ids x Vds has to be less than the part can dissipate)

 

[Dr.H]

Thanks Nelson, appreciate the steer. I also came across Designing JFET Audio PreAmplifiers

 

[Bksabath]

Dr H
tanks for posting that link
Just about what I can bite and chew on.

Now I wants more.
IMO this tread

 

[Bksabath]

I think I have found more
possibly more than I can chew but..

Gogle up Horowitz and The art of electronics

 

[Nelson Pass]

Horowitz and Hill, The Art of Electronics

also known as the Bible.

 

[buzzforb]

Although the link is helpful, I find there is a gap between common electronic knowledge and what makes for good audio amplification. Too many subtle influences between good and great. Ultimately I believe the thing that separates the good from the mediocre, is sheer experience. Best teacher is just trying and possibly failing. Nelson probably has a notebook collection of info/practical experience that would baffle most people.

 

[Mark Johnson]

Pretty good book by the staff at Siliconix (later sold to Vishay) -- see attachment

Also, if you can find it, "FET Circuits" by Rufus P. Turner, ISBN 0672214393 (0-672-21439-3)

 

[Peter M.]

I just found this:

Foolin' with FETs

 

[homemodder]

I think the most helpfull info on Jfet use in audio is the articles by Boberly. I dont think they are still available on the net but if anyone is interested I could send by email.

Borbely practically only used Jfets and mosfets in his designs.

The rest you can read up on Horowitz and Hill.

 

[Dr.H]

http://www.colorado.edu/physics/phys3330/phys3330_fa10/pdfdocs/AN102FETbiasing.pdf

 

[jcx]

Siliconix app note collection on "Wayback Machine"

JFETs Application notes

 

[Dr.H]

Good animation of very basic JFET operation here:
Junction Field Effect Transistor Animation

 

[Dr.H]

Some good stuff on simulation:
Simulating a FET amplifier


Step by step via load lines into a JFET preamp design here:
Design a Depletion-Mode FET Preamp: Curve Method, Part 1

Good JFET biasing load line material:
http://ux.brookdalecc.edu/fac/engtech/andy/engi242/powerpoint/fet_dc_biasing.pdf

Several good pages here:
FET Preamplifier, Part 1

 

[Dr.H]

Another good one
http://www.beknowledge.com/wp-content/uploads/2010/09/1421.pdf

 

[Bksabath]

Tanks Dr.H

realy apreciated

Keepgoing

AL