Hi Paul, hi all,
Sorry for the necro post
Last entry in here happened even before Corona I reckon.
I tried the measurement circuit for the JFET you posted (2 x 10k, 1 x 1M, JFET,
DC@9.18V). I tested the two JFETs I used in my ADHD breadboard circuit. All Vp and Idss measurements done on my T7 tester. It's cheap, but I did some measurements on a breadboard earlier, using the Fetzer Valve circuit for measurement. The T7 is not exact, but in the ballbark.
Q5 2N5458
Vd 7,60
Vs 1,56
Vp 1,86V
Idss 4,8mA
Q6 2N5458
Vd 7,50
Vs 1,66
Vp 2,04V
Idss 4.1mA
You wrote: "~2.5V on the source, ~6.5V drain, which is ideal". My two JFETs are a bit away from ideal then, it seems. But these two fets in the ADHD can handle 2.5V peak-to-peak on the input with _very_ little clipping. The output is around 2.7V ptp (with the tiniest bit of clipping).
I have some 2n5858s with Vp around 3V. So I put them into the small test circuit as well:
a) 2N5458 #1
Vd 6,70
Vs 2,47
Vp 2,99V
Idss 5,5mA
b) 2N5458 #2
Vd 6,85
Vs 2,32
Vp 2,82V
Idss 5,3mA
These measure closer to the voltages you wrote were ideal. But when I tested these in the ADHD circuit I saw them actually clipping the 2.5V ptp input pretty hard. I also have some 2n5457 with Vp of ~1.5V. They clip this signal as well.
Chuck wrote above that drain current at 225uA has most headroom. Not sure if below formula applies for the ADHD, but here it goes:
Vp/Rs=Id
Vp=Id*Rs=0,000225A*10000Ohm=2.25V
So for drain current of 225uA you'd need a Vp of 2.25V. If that's optimal headroom then the two 2n5858s I have with Vp~2V seem to be my best shot. And my scope seems to agree, as these clip the signal the least.
But down the rabbit hole I go and try, like suggested, different drain and source resistors (always pairs!). I try this with a 2n5857 to see how the changed resistors influence the voltages. I also measure drain current with the same multimeter.
Vp 1,50V
Idss 3,8mA
@18k
Vd 7,86
Vs 1,21
Id 72,6uA
@10k
Vd 7,92
Vs 1,31
Id 125,1uA
@7k5
Vd 7,96
Vs 1,21
Id 162,1uA
@5k6
Vd 8,03
Vs 1,17
Id 209,2uA
@4k7
Id 243,4uA
@3k3
Id 329uA
So, varying the resistors doesn't really change the voltages in any big way.
But anyway, at least the drain current changes. The resistors at 5k6 get me pretty close to Chuck's 225uA. So I put them into my ADHD circuit and change all four drain/source resistors that go to VCC or ground to 5k6. First thing to note is that the drain and source voltages are different to the ones in the small measurement circuit. Then I measured drain current, alas far away from 225uA, more like 150uA.
Then I connect the scope. I see no clipping. Yay! But I see the signal has been attenuated from 2.5V at the input down to 2.1V at the output of the ADHD's buffered bypass. Not good.
Sorry for the long post. But maybe it saves somebody else some time.