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23rd October 2012, 11:23 PM  #1851 
Banned

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 
24th October 2012, 12:54 AM  #1852 
diyAudio Member
Join Date: Jun 2011

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'.

24th October 2012, 01:09 AM  #1853 
diyAudio Member RIP
Join Date: May 2005
Location: Canoga Park, California

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 socalled 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. 
24th October 2012, 01:25 AM  #1854  
diyAudio Member

Quote:


24th October 2012, 01:32 AM  #1855 
diyAudio Member

BTW  modelling has replaced many cutntry 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 piggybacked 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.

24th October 2012, 01:37 AM  #1856  
diyAudio Member

Quote:
EDIT  W. R. Curtice, "A MESFET Model for Use in Design of GaAs Integrated Circuits," IEEE Trans. Microwave Theory Tech., vol. MTT28, pp. 448456, May 1980. C. T. M. Chang, et al., "A Subthreshold Current Model for GaAs MESFET's," IEEE Electron Device Letters, vol. EDL8, No. 2, Feb. 1987.
__________________
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed allimportant." Last edited by scott wurcer; 24th October 2012 at 01:42 AM. 

24th October 2012, 01:48 AM  #1857  
Banned

Quote:
Thanks, Frank 

24th October 2012, 02:00 AM  #1858 
diyAudio Member RIP
Join Date: May 2005
Location: Canoga Park, California

Problem is the output conductance (partial derivative of drain current with respect to drain voltage). It's a continuous function with continuous derivatives (or equaltozero 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 socalled saturation region). Not the way it works, and an especially lousy approximation to shortchannel devices like the latelyfavored BF862, and the old mainstay the 2SK170.

24th October 2012, 02:02 AM  #1859 
diyAudio Member

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.
__________________
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed allimportant." 
24th October 2012, 02:16 AM  #1860 
diyAudio Member
Join Date: Nov 2011
Location: Cooktown, Oz

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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