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Old 23rd October 2012, 11:23 PM   #1851
fas42 is offline fas42  Australia
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I've found that at least one key thing is include all relevant parasitics: if you assume that a drawn ground is a real ground, and that voltage supplies are perfect, then you're likely to have problems, for starters. The point is to model what will actually be there in the real world, not skate on top of a set of assumptions ...

Frank
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Old 24th October 2012, 12:54 AM   #1852
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Originally Posted by RNMarsh View Post
So, I go buy this MicroCap sim software for thousands and cant trust results for anything except designs which do not use transistors!
I used to use Ansoft's Serenade for RF design, including that of SAW and other oscillators. IIRC the transistors were characterized in terms of their S parameters using a network analyzer. This isn't necessarily helpful other than that it tends to indicate that useful simulations including active devices are possible. I haven't had any call to do any of that stuff for years tho'.
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Old 24th October 2012, 01:09 AM   #1853
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Scott's remarks about FET models are most apt. My simulator accepts the same parameters as others, but what it winds up doing with them isn't pretty. It makes an abrupt transition from the so-called triode region to an output conductance region, the latter which is modeled as a constant slope!

At some point I may concoct a model using the available math functions which will be cumbersome, but work adequately at low frequencies.
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Old 24th October 2012, 01:25 AM   #1854
RNMarsh is offline RNMarsh  United States
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Originally Posted by scott wurcer View Post
Essentially all the industry standard circuits were done that way until the 80's. We built discrete breadboards, and frequently had to revise the chip. You can't afford to work that way now, frequently parts from the very first wafer go right to the customer.
For those not in the business, breadboarding is still practical for once in awhile projects. How about we identify the parameters that are most important to audio - those which affect noise, distortion, gain linearity etc and describe how to extract what is needed for the sim model that is typically missing? A short tutorial for the most important - missing-parameters. For bipolar and for jFET. Low freq audio isnt all that complex that DIY'er need extreame modelling accuracy or precision. -Thx
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Old 24th October 2012, 01:32 AM   #1855
RNMarsh is offline RNMarsh  United States
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BTW -- modelling has replaced many cut-n-try approaches to designing; One is nuclear weapons design. Each time a live 'event' (as it was called) was done to test the design, the data gathered via piggy-backed detectors onto the 'device'. would help refine the model. Since, the under ground nuclear testing was stopped, design can continue due to the models. Its is Still a dangerous world.... maybe more so due to the speed of highly developed modelling.
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Old 24th October 2012, 01:37 AM   #1856
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Quote:
Originally Posted by bcarso View Post
Scott's remarks about FET models are most apt. My simulator accepts the same parameters as others, but what it winds up doing with them isn't pretty. It makes an abrupt transition from the so-called triode region to an output conductance region, the latter which is modeled as a constant slope!

At some point I may concoct a model using the available math functions which will be cumbersome, but work adequately at low frequencies.
That's the Level 1 model basicly circa 1952, the discontinuous derivative sucks. You want to search for Curtice and his refined JFET models.


EDIT - W. R. Curtice, "A MESFET Model for Use in Design of GaAs Integrated Circuits," IEEE Trans. Microwave Theory Tech., vol. MTT-28, pp. 448-456, May 1980.
C. T. M. Chang, et al., "A Subthreshold Current Model for GaAs MESFET's," IEEE Electron Device Letters, vol. EDL-8, No. 2, Feb. 1987.
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Last edited by scott wurcer; 24th October 2012 at 01:42 AM.
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Old 24th October 2012, 01:48 AM   #1857
fas42 is offline fas42  Australia
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Quote:
Originally Posted by bcarso View Post
Scott's remarks about FET models are most apt. My simulator accepts the same parameters as others, but what it winds up doing with them isn't pretty. It makes an abrupt transition from the so-called triode region to an output conductance region, the latter which is modeled as a constant slope!

At some point I may concoct a model using the available math functions which will be cumbersome, but work adequately at low frequencies.
My understanding is that the transconductance is proportional to the gate voltage in the saturation region, or am I misunderstanding you, or are you talking about more subtle aspects of behaviour?

Thanks,
Frank
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Old 24th October 2012, 02:00 AM   #1858
bcarso is offline bcarso  United States
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My understanding is that the transconductance is proportional to the gate voltage in the saturation region, or am I misunderstanding you, or are you talking about more subtle aspects of behaviour?

Thanks,
Frank
Problem is the output conductance (partial derivative of drain current with respect to drain voltage). It's a continuous function with continuous derivatives (or equal-to-zero derivatives), and the dumb model says you have a triode region with a certain curvature, then suddenly you have a linear function beyond a certain drain voltage (the so-called saturation region). Not the way it works, and an especially lousy approximation to short-channel devices like the lately-favored BF862, and the old mainstay the 2SK170.
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Old 24th October 2012, 02:02 AM   #1859
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or am I misunderstanding you, or are you talking about more subtle aspects of behaviour?

Thanks,
Frank
Yes, these low noise JFET's are short channel devices and the behavior with Vds is not linear but fits better to a hyperbolic tangent function. This is a fine point that lies on top of the basic behavior that you mention. The distortion characteristics due to Vds are all wrong.
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Old 24th October 2012, 02:16 AM   #1860
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Anyone know if LTspice accepts Level 3 FET models whatever these are? I note it complains of

* Unrecognized parameter "jd" -- ignored
* Unrecognized parameter "js" -- ignored

In Scott's j305 & bf862 models
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