Tweaking spice models to generate device parameter variation
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Posted by Paul Burridge on September 7, 2004, 8:30 pm
Please log in for more thread options 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. -- "What is now proved was once only imagin'd." - William Blake, 1793. | |||||||||||||
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Posted by Kevin Aylward on September 7, 2004, 8:59 pm
Please log in for more thread options 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 salesEXTRACT@anasoft.co.uk http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. | |||||||||||||
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Posted by Paul Burridge on September 8, 2004, 1:56 am
Please log in for more thread options On Tue, 07 Sep 2004 17:59:52 GMT, "Kevin Aylward"
>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:-) Thanks, Kev. These runs are only at a fixed LF, BTW. -- "What is now proved was once only imagin'd." - William Blake, 1793. | |||||||||||||
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Posted by Paul Burridge on September 10, 2004, 2:56 am
Please log in for more thread options On Tue, 07 Sep 2004 17:59:52 GMT, "Kevin Aylward"
>Paul Burridge wrote:
>> Hi,
>> >> 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? >> >
>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... 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. -- "What is now proved was once only imagin'd." - William Blake, 1793. | |||||||||||||
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Posted by Kevin Aylward on September 10, 2004, 10:02 am
Please log in for more thread options Paul Burridge wrote:
> On Tue, 07 Sep 2004 17:59:52 GMT, "Kevin Aylward"
> >> Paul Burridge wrote:
>>> Hi,
>>> >>> 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? >>> >>
>> 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... >
> 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? 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. >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? 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. >It
> would save me having to cobble up my own probably inaccurate > guesstimates, you see. I'll go back over my defaults to make sure that they are all reasonable. Kevin Aylward salesEXTRACT@anasoft.co.uk http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. | |||||||||||||
>
> 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?
>