[sdiy] jfet driver

Ben Bradley ben.pi.bradley at gmail.com
Wed Feb 27 19:01:42 CET 2019


TAOE 3rd is a great reference, and I'm glad I got it, partly (and I
didn't even know this when I bought it) to help learn more about FETs.
There's a diagram labeled the "FET Family Tree" that describes all
types made. I'm tempted to call that the "FFT table."  :-)
Here's a similar diagram:
https://www.globalspec.com/reference/34760/203279/chapter-6-using-jfets-and-crds-to-create-current-sources

As I recall the book saying, one of the advantages of JFETs over
(small signal, a small minority of the types of) MOSFETs is that JFETS
have lower noise. One of the lowest noise was Toshiba 2SK170 at 50
cents each until they went obsolete, but a small manufacturer now
makes the equivalent LSK170 for several dollars each.

I've noticed that more and more old FETS are no longer made (even in
SMD). Some of the reason is the older parts had such great parameter
variation, especially the most popular and cheap ones. I found a lot
of MPF102 based circuits in guitar pedal and other schematics from
searching for "jfet preamp" and more googling showed me it's the
prototypical part for this, as described here:
https://www.eham.net/ehamforum/smf/index.php?topic=114252.0

The Art of Electronics 3rd Edition is barely four years old, and
several of the "current" JFETS listed in it are already obsolete!

On Wed, Feb 27, 2019 at 4:58 AM Guy McCusker <guy.mccusker at gmail.com> wrote:
>
> On Tue, Feb 26, 2019 at 6:37 AM Michael E Caloroso
> <mec.forumreader at gmail.com> wrote:
> >
> > Yes, JFETs can be depletion or enhancement mode.  My EE textbooks
> > don't lie, and neither do supplier datasheets.
>
> I'm looking at Horowitz and Hill, Art of Electronics Third Edition,
> p135, which says "JFETs, with their gate-channel diode, permit only
> reverse gate bias, and therefore are made only in depletion mode".
> Do you have a textbook reference or a datasheet to contradict this?
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