Beginning FETs

[newfish]

Can anyone reccomend a good beginner FET project?

I've got Vero layouts for Fetzer and SHO (thanks Renegadrian!) and have draw schematics of these.

Next step will be to breadboard them and get my hands good and dirty with 'what happens when I change this bit'-itis.

The FET theory at the minute is not sinking in, despite having read the GEOFEX articles and a couple of other texts.

I'm hoping things will sink into place when I have a circuit in front of me I can measure etc.

Would there be any mileage in a FET-based project in the 'Beginners' section of the board to complement the NPN Boost and Overdrive projects?

[mdh]

Have you seen the rewrite of the Fetzer Valve article on Runoffgroove?  I haven't really been through it, but it looks like they put a lot of effort into explaining things, and giving kind of a study guide for just your situation.

[PerroGrande]

One of the issues encountered with many JFET-based projects (apart from simple followers acting as buffers) is the wide variance in JFET specifications within a given part number.  This is simply the nature of the beast and why their role in commercial applications is somewhat limited -- typically as followers, switches, and voltage-controlled resistors.  It isn't cost-effective for large-scale manufacturers to individually test Idss and Vp for large quantities of JFETs, and trim pots are an accountant's nightmare...

The Fetzer valve article, as well as the great stuff over at GEOFEX talks about this a lot, and it is a very real issue.  From a beginning project point of view, the JFET would introduce some unique challenges.  Beginning projects, in my opinion, should have a reasonably high chance of success *and* provide something useful to the builder when all is said and done. A JFET buffer would fall into the "high chance of success" category, but may be underwhelming to a first-time builder in terms of "exciting results"...

On the other hand, a simple common-source amplifier could end up as a nice boost, or even overdrive circuit, but now is highly dependent on not only the JFET part number selected, but the actual JFET itself.   It presents the realistic chance that a perfectly wired, well-constructed circuit might not bias properly.  Compare this to the BJT beginner project which is workable with literally thousands of transistor variants...

Jack Orman's excellent "Mini Booster" project is a fun build and has a high success rate, but the explanation of its operation is not as straightforward as the beginning BJT circuit.

That said, the FET (and MOSFET) are not unassailable. 


Check out the WIKI article:  http://www.diystompboxes.com/smfforum/index.php?topic=57219.0

and

http://www.colorado.edu/physics/phys3330/phys3330_fa05/pdfdocs/AN102FETbiasing.pdf

[Renegadrian]

Well, I'd say...BUIlD THEm boTH!!! eheheh
there's plenty of FETs circuits...I'd suggest you the Sparkle Drive, it's one of my favs...Otherwise, I made a vero layout for the Fullclone fat boost- it's in the gallery, in my album - easy to build but good sound!!!

[PerroGrande]

I didn't want to make it sound like one shouldn't try building JFET projects!  Sorry if it came off that way.

I think Jack Orman's Mini Booster would be an excellent and useful build for a beginner.  It is a fairly straightforward build, and would be useful to have in one's arsenal of pedals.

Mu-Amp-based builds (like the mini booster) might be better candidates for a beginning project because they offer a larger degree of independence from device variations.

[newfish]

Thanks for the advice folks.

I realise the FET world is infinitely more complicated than a 'normal' BJT in terms of variance, biasing etc.

Having breadboarded both the Fetzer and SHO this weekend - using an MPF102 since that was all I had to hand - some of the theory is beginning to sink in.

I realised that the gain available from this FET would not be anything like the gain available from J201s, for example, but you have to start somewhere.

Incidentally, since I had a voltage divider going on for the SHO, I dropped an OC44 in for kicks - and it wasn't half bad...

Here's to the learning of new things...

I'll get there.

[PerroGrande]

Awesome!  Keep at it!

I enjoy the various FET circuits that circulate around here, and some of my favorite builds ever are FET based (ROG's 18, and the BSIAB2).  Slapping a JFET follower onto something with less-than-desirable input impedance is a neat trick to have.  My "audio probe" is actually an external computer speaker that has been fitted with a nice, high-impedance front end - a JFET follower.