Not too sure of your objective, whether it is to understand JFETs or high side switches.
Both. I came along JFETs by searching for a low latency high side switch solution for small signals, eventually driven by a push pull output to be used as a (not really precise) buffer or impedance changer. There will be a load in parallel to the R/LED, that is why I require high side switching.
Preferably a high side switch, that works, as one would expect: A "high" on the base/gate turns on, a low off and the voltage drop being more or less constant with variying load (within in a certain range, 1-30mA in this very case). Kind of like a relay really, but for energy reasons, I would like to stay clear of a dedicated IC switch for this presumably simple task, I've once thought, this would be.
But with this final bastion of hope has fallen now, I've learned, this is not possible with silicium. Unless you build more complex units like a npn driving a pnp or p-mos or bootstrap/pump an n-mos - as your graphics show as well: The p-mos is plain in the wrong state with regards to the gate. Back in the days I was hoping to fix this with symmetrical JFETs.
Or of course by accepting a higher voltage drop I could use a npn alone. All quite unelegant solutions to the simple task of switching, but thats our choice. So my next thread will be about mosfets. And push-pull outputs.
As you've written, mosfets are more easily found for larger currents.
* But then they may need a driver and have way slower turn on/switching times due to larger gate capacities (though I do not really have a need for that extra high speed, except enjoying a clean square on the scope and maybe the learning effect by taking propagation into account from the beginning). On the first glance, JFETs looked quite simple and less demanding than mosfets. I may have squinted here quite a bit.
However, I have a couple of p-mos on my list that could fit my bill to test next.
the other thing is that the voltage necessary to turn a JFET off varies wildly,
Yes, I've noted that too. None of the JFETs I've tested would really turn off at 0V. The 2N3819s were the best/most consistent, but still did not turn off completely. But that was not as mysterical to me as the forward gate current, but more a matter of wrong reading of specs.
For switching things like an LED MOSFETs are usually better suited.
My next step. Though quite surprising, as integrated switches and lots of other chips are made of JFETs. And a lot of those seem to be able to work fine without a negative voltage. Where I was not even able to fully turn of my few devices with 0V. But then again, I am way less complex.
Yes, precisely: the reference terminal is the lower of the two channel terminals.
Thank for confirming this again. Though bad news.
*[Edit]: And it is not that easy to find those that actually do work properly with logic levels, especially for p-mos. But that did not really work out well for my JFETs either.