simple JFET Buffer, what is the minimum Source resistor/current required ?

[antonis]

Current draw is easily calculated:



or place a series resistor of 1k between Vcc and circuit and measure voltage drop across it..
(120μA will result into 120mV drop..)

[MoruyaGuitarist]

or place a series resistor of 1k between Vcc and circuit and measure voltage drop across it..
(120μA will result into 120mV drop..)

Thanks, I thought about that but the resistor also changes the the current draw of the circuit.
Hang on,  Doh !
When it's only a small voltage drop it really is just like the battery getting flat a tiny bit. My bad. 

[Steben]

http://www.lynx.net/~jc/transferCurvature-TubeSimulation.html

as you increase Rs the transfer curve gets stretched out to the right indefinitely
relative to a give signal strength you get a more linear transfer (ie., less added harmonics per pound of signal)

a bit off topic, but that is exactly what is needed to mimic a triode.
Pure jFET full non-linear produces way more harmonics.

[Eb7+9]

^^^ that's right, why I referred to the idea as an exaggerator ...
(the other "bird" in my partially revealed invention is doing away with capacitor distortion and shadowing in a most important section of the circuit)

you'll notice people who casually throw around this 3/2 power idea for Triode circuit transfer are generally mistaking the forest for the trees here

as if the device's terminal transfer order magically translates to the same order manifesting in the large signal transfer curvature of
the gain stage employing that same device

not to mention the fact that Triode device transfer is anything but a fixed power function, let alone 3/2 ...

---

one of the few papers that addresses the dilemma of large-signal transfer equivalence (or lack of) and offers a
viewable solution when applied to real world AC-coupled hardware is this one:

https://www.dafx.de/papers/DAFX02_Moeller_Gromowski_Zoelzer_measurement_nonlinear.pdf

to confirm what I'm saying you'd have to apply their technique to a single stage common-source/cathode arrangement employing different device types ...
with "typical" biasing, same/similar gain levels etc ...

and then A/B the fixed source/cathode voltage biasing technique against the typical resistor/AC-bypass
approach to demonstrate how dynamic transfer in both these cases is the same, or nearly

and then, lastly, you could then compare (take difference) between cap-bypass case against DC-bias case to
extract dynamic capacitor distortion profile - ie., way more realistically than what you get with steady state (fixed sine-wave) testing

lots of unexplored fun ...

[Steben]

That paper indirectly shows that an amp like an AC30 is - besides having a non feedback output stage with typical smooth transfer curve - very asymmetric in its preamp and phase splitter. On top of that there is cathode shift strangling the signal into compression and cross-over.
What an amp. From clean low harmonics to soft clipped rather symmetrically into more and more asymmetrically (and more IM) and with cross-over.
And now I have gone from a bit to completely off topic....