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Old 1st March 2013, 01:57 AM   #221
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I did reread the article. Two things. I did not see a scantilly clad woman anywhere. If the sweet spot is like a good wine, im screwed.
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Old 1st March 2013, 02:00 AM   #222
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Quote:
Originally Posted by Nelson Pass View Post
I have a question about the cubic law component. Like everyone else, I
assume that fets are square law devices, and this is supported by the
clean second harmonics we get when we are only varying the Ids current
with the Vds voltage remaining constant. When Vds is allowed to vary with
signal, we start seeing some 3rd harmonic creeping in, and when the gain
variation due to Drain resistance matches against variation due to
transconductance, we see that "sweet spot" has clean 3rd harmonic with
the second having been cancelled.

Is this related to what you are talking about? Otherwise I'm somehow missing
that in the distortion readings of these devices in ordinary analog use.

Well, Nelson, in the end, you have far more experience with how these guys will work in the amplifier, so I won't pretend to know the answer.

But there is plenty of theory about what we can expect. The cubic law is not a "law" in my mind because there is not a physics-based reason for preferring that relationship. But the same is true for the so-called square law unless we are talking about MOSFETs. An idealized but common derivation for the long-channel MOSFET transfer characteristic is a square law. What happens in real devices is another matter. According to practically every text book, the rectangular channel JFET is not expected to have a square law, except as an approximation to the actual physics. That approximation is often observed empirically, at least for the low power variety of JFET, so you are not necessarily wrong for assuming that. Nevertheless, I did work out the Taylor series approximation to third order as recommended by Sze, and it is clear from the form of the derivatives that all terms above linear are non-zero. The square term is dominant, but the cubic term is there. No surprise that faint cubic harmonics are reported in RF mixers. I'm assuming we got em too.

A SemiSouth JFET channel is trapezoidal (an artifact of manufacturing) and heaven knows what the actual transconductance shape should be because of manufacturing variation. Thus, whatever you see has to be assumed to be device dependent. This is really what I am reporting in the article. As you get closer to threshold, the experimental data clearly deviates from a square law. The Spice model doesn't and thus gets it wrong. But up in Id where you usually bias them, the curve fits pretty well with a quadratic polynomial. It's all in the bias point and how far you modulate away from it.

But that's the transconductance. The modulation of drain current by drain-source voltage involves potential barrier modulation caused by field penetration into the channel. The shape of the output curves vary from part to part because the geometry and doping vary from part to part. Incidentally, I think this is part of the fun of playing with your amps with different JFETs and bias points. Lots of possibilities exist for optimization.

My conclusion from all of this is that we need to take quality data of the output curves and then analyze them simultaneously for Vgs and Vds variation, because that is what the amplifier will do. I have a curve fit process for doing that that I will post in the next article. I've been using that to help me understand calculated residual distortion in an amplifier. I have not experimentally checked the analysis yet, so frankly I'm not sure what will be the outcome. But I'm basing the analysis on experimental data taken from actual parts, which must be relevant. Stay tuned!
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Old 1st March 2013, 02:04 AM   #223
49 - for the 18th time
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Quote:
Originally Posted by buzzforb View Post
I did reread the article. Two things. I did not see a scantilly clad woman anywhere. If the sweet spot is like a good wine, im screwed.
We save that BS for discussion over in the Pass Pub....

This stuff here iz the real deal and I won't describe my degree of excitement about reading it....

Wonderful article Dr. Mike - thank you very much for taking the time to write it and share it with us!!!
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Old 1st March 2013, 02:05 AM   #224
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I did reread the article. Two things. I did not see a scantilly clad woman anywhere. If the sweet spot is like a good wine, im screwed.
Hmmm, two of my favorite topics, women and wine. I'm now old enough that I can't remember which comes first.
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Old 1st March 2013, 02:14 AM   #225
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Hmmm, two of my favorite topics, women and wine. I'm now old enough that I can't remember which comes first.
Then you have arrived
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Old 1st March 2013, 11:26 AM   #226
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Old 1st March 2013, 11:32 AM   #227
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Originally Posted by Semisouthfan View Post
... BTW, a "fat cab" is heading your way.
fancy door-stops , I presume ?

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Old 1st March 2013, 01:32 PM   #228
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Dr. Mazzola, the knowledge in your current article and that in The Sweet Spot article by Mr. Pass go hand in hand. But; a few audio DIYers have the equipment to test and characterize semiconductors per your teaching and Mr. Pass. May I suggest that SiC JFETs etc.. be characterized for their sweet spots. This impotant performance attribute will simplify our lives and speed up our activities.
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Old 1st March 2013, 02:26 PM   #229
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I will be starting on extensive F6 amp thingie and will be doing a lot of variation with the r100 and its operating points. I would argue that Nelson's points of operation are probably a pretty safe bet to work with. I have 8 transistors and will test them all for Yfs and leakage before putting them in the circuit and playing with rail voltage and bias.
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Old 1st March 2013, 02:38 PM   #230
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Quote:
Originally Posted by Semisouthfan View Post
Hmmm, two of my favorite topics, women and wine. I'm now old enough that I can't remember which comes first.
There is a nice little lounge tune called "Fast Cars, Naked Women", which sums
it up nicely.

Thanks for the explanation. Its been 22 years since I operated at the
Threshold (that's a joke, son!) it explains why I don't see much cubic.

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