I need to check the efficacy of a new FET biasing scheme to see how tolerant it is to the wide parameter variations one gets with real-world FETs. Hence, the usual spice model for a FET with its invariable Vgs(off) and Idss isn't going to be of any use. If I change the parameter "Vto" (which I believe is the same as Vgs(off) in the real world from between say -2 to-6 volts, will that alone suffice to emulate real world manufacturing variations? I mean, will tweaking Vto alone also change the Idss to the appropriately meaningful value for a FET's characteristic quadratic transconductance curve?
THanks,
p.
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"What is now proved was once only imagin\'d." - William Blake, 1793.
No. You need to change Beta (or Kp), the transconductance as well. SS already has built guestimates if you use its WC run facility, so you don't have to ask these questions, usually...
For frequency response you would also have to change the capacitance parameters, again, WC are already defaulted in SS:-)
Kevin Aylward snipped-for-privacy@anasoft.co.uk
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SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.
I've just had a look, Kev. Would I be right in thinking one selects the component/properties and then chooses between "weak, nominal or strong" to get the MC parameter variations for a FET? That's what seems to be happening, as I note the values change according to which of the three selections I make? Does this accurately represent the manufacturing spread one would expect to see in real-world devices? It would save me having to cobble up my own probably inaccurate guesstimates, you see.
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"What is now proved was once only imagin\'d." - William Blake, 1793.
Yes. To do a forced run. In most cases, you just run with the nominal, and use the Worst Case or MC runs to do the actual variations automatically (blur "SS" button, WC reruns tab)
It will look for a specific model variations in the library (basename_XN, basename_XW, basename_XS). If it cant find one, it will use default multipliers that are set in \\defaults\\SuperSpice.lib to create one. Once it creates one it will use that one always afterwards. If you reset the default multipliers you will need to manually change the auto created one, or delete it so that it will be re-created.
Well, "accurately" is open to debate. I just checked what I had SuperSpice.lib, and for the jfets the default variation for Vto is probably a bit low. Its set to 1.25 and 0.75 scale factors. A better guesstimate is probably about 2 and 1/2. I must have rushed this a bit.
If you press the help on the WC setup page, you wil get a description of what happens.
I'll go back over my defaults to make sure that they are all reasonable.
Kevin Aylward snipped-for-privacy@anasoft.co.uk
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SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.
Well, I must admit I am rather embarrassed by this. There was another discrepancy. The Vt multipliers were the wrong way for jfets. That is, a strong model should have a low vt, weak a high vt. Not sure how this crept in but I did have:
*these demo models may not be accurate
*worst case models only illustrate functionality
In the WC setup file (SuperSpice.lib, so I did had a cop out:-)
I have now updated the download with a corrected version that has better defaults for the jfets.
Kevin Aylward snipped-for-privacy@anasoft.co.uk
formatting link
SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.
Thanks, Kev. I'm grateful to you for looking into this for me before I carried out any runs. It's reassuring to see that you've been open about this problem and dealt with it swiftly instead of sweeping it under the carpet as others might have been tempted to do. Respect is due!
Many thanks indeed. I'll give it a whirl this afternoon. I had noticed the spreads of your hi-med-low parameters were quite close together - pity the manufacturers can't make the real thing to your tolerances! :-)
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"What is now proved was once only imagin\'d." - William Blake, 1793.
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