[sdiy] Help with sawtooth convertor

[David G Dixon]

This is a quote from the first page of the 5484/5485/5486 datasheet:

"NOTE: Source and Drain are interchangeable."

The 5457/5458/5459 datasheet says the same thing.

The 4391 and 3819 datasheets do not say this, so they must not be
symmetrical.

Those are all the JFET datasheets I have on my computer.

Here is what Horowitz and Hill have to say about it in The Art of
Electronics:

"FETs (both JFET and MOSFET) are nearly symmetrical, but the gate-drain
capacitance is usually designed to be less than the gate-source
capacitance."

Note that gate-drain and gate-source capacitances are not typically
published in datasheets, so it would be impossible to use that spec to tell
whether the source and drain are interchangeable or not.

According to Rod Elliott at ESP:

"All FETs are rather unique as semiconductor devices, in that they conduct
more-or-less equally in both directions (i.e. both 'normally' and with drain
and source interchanged)."

I suspect (and this seems to be confirmed by other commentary that I've seen
around the interwebs) that the source and drain of all JFETs are essentially
interchangeable, and that no performance differences will be detected at
frequencies lower than RF, if at all.  For the applications we synth-nerds
tend to use JFETs for, you can swap the source and drain with confidence.


> -----Original Message-----
> From: mskala at ansuz.sooke.bc.ca [mailto:mskala at ansuz.sooke.bc.ca] 
> Sent: Thursday, January 10, 2019 4:44 PM
> To: David G Dixon
> Cc: synth-diy at synth-diy.org
> Subject: Re: [sdiy] Help with sawtooth convertor
> 
> On Thu, 10 Jan 2019, David G Dixon wrote:
> > Also note that the source and drain of JFETs are completely 
> interchangeable.
> > I connect them based entirely on layout convenience.
> 
> I don't think that's true of all JFETs.  Many are constructed 
> symmetrically, but there's no reason that the gate needs to 
> be the same distance from both ends of the channel, which is 
> what it takes for the electrical behaviour to be symmetric.
> 
> --
> Matthew Skala
> mskala at ansuz.sooke.bc.ca                 People before tribes.
> https://ansuz.sooke.bc.ca/