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Old 28th August 2012, 06:20 PM   #421
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Another strategy for temperature maintainance is to bore a hole in the ground about six or eight feet (until you get tired) and to bury the critical parts. An old ham radio trick for crystal oscillators. Not too great for repairs and mods though.

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Old 28th August 2012, 06:21 PM   #422
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Originally Posted by Wavebourn View Post
What is "The Norm"?

It is very personal definition. Is "OpAmp" topology "The Norm" for an audio amp? Not for me. But yes, for somebody else.
I was refering to what I thought was a question from you, regarding the slang term "outside the box". It is not a literal box around a circuit, but, a range of thinking considered normal amongst a high percentage of people. Like they are all in a box and think the same But some, don't go with the status quo and think "outside the box".
I would guess you think outside the box. So, I'm waisting my time...
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Last edited by flg; 28th August 2012 at 06:26 PM.
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Old 28th August 2012, 06:44 PM   #423
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Default JFET tempco with operating point

Another thing: there is a bias point for JFETs which allows almost zero drift in offset voltage. For long-channel devices it occurs about 626mV more turn-on voltage than the threshold voltage. For shorter-channel devices it shifts, and if the threshold voltage is low enough (like in our friends the BF862, 2SK170, 2SK389) it may even occur for a small forward bias on the gate, hence when biased at more than Idss.

Attached is a page from Richard S. C. Cobbold's book Theory and Applications of Field-Effect Transistors, showing three devices with drain current adjusted to give best results. One of them in particular is very good indeed, but the worst is only about 3mV shift over a wide temperature range.

If one is lucky enough to have a nice dual to begin with, it can further enhance the drift performance to operate both devices near this zero-tempco point.

When used for current generators, a similar enhancement of temperature stability can be had, and if the current be used as a reference in a regulator, some very-low-noise regulators/references result.
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Old 28th August 2012, 06:47 PM   #424
bcarso is offline bcarso  United States
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Originally Posted by Chris Hornbeck View Post
Another strategy for temperature maintainance is to bore a hole in the ground about six or eight feet (until you get tired) and to bury the critical parts. An old ham radio trick for crystal oscillators. Not too great for repairs and mods though.

All good fortune,
Chris
Especially good if you are using TrenchFETs or buried-channel devices
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Old 28th August 2012, 06:49 PM   #425
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... but not with the same old linear brute force methods.
How is a servo "brute force"? A cubic inch, no heat, less that $2 in parts, reliable, high performance. Perhaps the reason people still use it is because it's an elegant solution that solves the problem completely.
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Old 28th August 2012, 07:24 PM   #426
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Actually servo in audio amps is nothing more than additional feedback loop with additional amplification. It increases open loop gain on frequencies below audio band. You have to remember that the servo loop has own linearity problems, like any other amplification stages, so on frequencies that are attenuated by servo loop it adds own non-linearities. Compared to balance and self-compensation it is indeed a brute force. And it does not eliminate problems. It minimizes them.
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Old 28th August 2012, 07:33 PM   #427
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Quote:
Originally Posted by Wavebourn View Post
It is exactly what I proposed for JC's unstable FET drawing: to add diode sensors of temperature of output MOSFETs and control reference voltage for current source in the tail of an input differential pair. If you want "all FET" design you can use JFETs instead of diodes. And additional opamp in this "thermal servo" loop.
can you proto it and see how well it works? This is a good first attempt IMO of thinking outside the box. As this is a joint opamp design effort forum of a new opamp.... not a rehash of existing opamp designs... it will be nice to see real improvements over those existing designs. Thx-RNM
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Old 28th August 2012, 07:37 PM   #428
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Quote:
Originally Posted by RNMarsh View Post
can you proto it and see how well it works? This is a good first attempt IMO of thinking outside the box. As this is a joint opamp design effort forum of a new opamp.... not a rehash of existing opamp designs... it will be nice to see real improvements over those existing designs. Thx-RNM
I can proto it, but I have much more things to do than to prove lessons that I learned 35 years ago. Including design of frequency-stable control loops in systems. Problems of stability of control systems are the same like in opamps, for example. Thermo-stabilization as well can oscillate. I would suggest again to go from basics, instead of stochastic jumps all around "the box" without understanding of details.
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Old 28th August 2012, 07:38 PM   #429
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I put that topology up, because it was DIFFERENT, not necessarily practical.
Wavebourn, I think your approach to bias stability is very elegant.
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Old 28th August 2012, 07:39 PM   #430
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Quote:
Originally Posted by SY View Post
How is a servo "brute force"? A cubic inch, no heat, less that $2 in parts, reliable, high performance. Perhaps the reason people still use it is because it's an elegant solution that solves the problem completely.
Thanks for the compliment on my elegant solution. Brute force refers to cooling and/or heating systems etc. that are being thought about.
Time marches on... the dc servo requires another IC (usually). Now I think it can be integrated into the design. We have one good idea so far From Wavebourn.... I am sure there are others if we brain storm on this.

If you want a clue.... sim the circuit I used for a line stage/headphone amp and figure why IT is so thermally stable without the added servo parts. Then come up with a new opamp design incorp the best idea(s). -Dick Marsh

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