Borbely Fet Follower SPICE modeling
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 30th January 2004, 06:04 PM #1 pjacobi   diyAudio Member   Join Date: Jan 2004 Location: Hamburg Borbely Fet Follower SPICE modeling This is for the SPICE modeling related discussions, which started in the "Borbely Fet Follower" thread. Let's avoid annoying the thread starter and others, who want to see practical issues, real circuits, discussed in that thread. Regards, Peter Jacobi __________________ -- YMMV
pjacobi
diyAudio Member

Join Date: Jan 2004
Location: Hamburg
Hi Frederico, All,

Quote:
 Originally posted by pjacobi .model 2sk170 NJF(Beta=51.76m Rs=8.008 Rd=8.008 Betatce=-.5 Lambda=11.22m + Vto=-.5275 Vtotc=-2.5m Cgd=18.28p M=.3367 Pb=.3905 Fc=.5 + Cgs=20.07p Isr=112.8p Nr=2 Is=11.28p N=1 Xti=3 Alpha=10u Vk=100 + Kf=92.85E-18 Af=1)
Quote:
 Originally posted by fscarpa58 using Micro-Cap 7 you can obtain the 3 curves of post #60 with any generic njfet model by putting IS= 1n , 15 p, 10F respectively for curves 1,2 and 3. I think gate current is not well modelled here I am disappointed
I've cross-checked with AIMSpice. Results are in agreement with Micro-Cap 7 results, i.e. not satisfactory.

Actually looking at the diagnostics, I can see that about half of the parameters are rejected as unknown. So there's no big guess, what 's happening.

Seems LTSpice is state-of-the-art regarding JFET modeling.

Regards,
Peter Jacobi
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pjacobi
diyAudio Member

Join Date: Jan 2004
Location: Hamburg
Hi Christer,

Quote:
 Originally posted by Christer For instance, with 10kHz 5V pk input, I got -94dB 2nd order and -117dB 3rd order. With 100mV pk input I got basically no visible distorsion above the -155dB noise floor of the FFT. Those are just simulation figures, but they seem promising.
If you operate a x1 stage at +-24V, and you don't do something really strange, it's nearly impossible to get worse than -150dB for 100mV pk signals.

O.K. There may be a bareley visible K2 spike. But everything above starts at least with a bonus factor of 1/240 * 1/240 before even taking anything into account that makes the circuit well-behaved.

Regards,
Peter Jacobi
__________________
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YMMV

 30th January 2004, 07:01 PM #4 Christer   diyAudio Member   Join Date: Sep 2002 Location: Sweden Peter, You started this thread before I made my recent post in the original thread. As I pointed out there, Spice modelling was not really the issue, but other people seemed to make it the issue, or assume it was the issue. As I also said there, the distorsion figures should be taken with a very large grain of salt. I think we can expect a real circuit to have at least as much distorsion, but otherwise I am not sure if we know much. I do think distorsion analyses in Spice can serve a purpose for comparative purposes of the kind we discussed in the Diamond buffer thread, but that is an assumption. Just for the record, I see you put "Borbely FET follower" in the thread title. Actually, the circuit wasn't one of Borbelys' but rather what I understand to be John Curls own variant of the White follower.
 31st January 2004, 08:00 AM #5 pjacobi   diyAudio Member   Join Date: Jan 2004 Location: Hamburg The riddle of the gate current simulation differences is solved. It's the JFET impact ionization current, see: http://www.google.com/search?hl=en&l...=Google+Search Its modeling appeared first in PSPICE and didn't migrate to all other SPICES. Reading the name alone, it should best be avoided, I assume. Which correlates with the practitioners' advice seen in the mother thread. Regards, Peter Jacobi __________________ -- YMMV
fscarpa58
diyAudio Member

Join Date: Nov 2003
Location: Genova, Italy
Hi

Thanks Peter for the search

However I think that static curves of
ID versus VDS are more important in
distortion analysis. look at the difference
between M-Cap and the Toshiba sheets

Amazing

Federico

Toshiba curves
Attached Images
 fet2.jpg (31.2 KB, 1361 views)

fscarpa58
diyAudio Member

Join Date: Nov 2003
Location: Genova, Italy
MicroCap curves

I think it will be better to use
pentode models

Federico
Attached Images
 fet1.jpg (21.9 KB, 1335 views)

pjacobi
diyAudio Member

Join Date: Jan 2004
Location: Hamburg
Hi Federico, All,

Quote:
 Originally posted by fscarpa58 [...] However I think that static curves of ID versus VDS are more important in distortion analysis. look at the difference between M-Cap and the Toshiba sheets [...]
JFETs, the neglected stepchild of SPICE modeling?

Perhaps some advanced MESFET models can be abused for better results, when it is possible to selectively disable the GaAs specific effects.

Anyway, careless parameter extraction is to blame for the huge differences seen in your posts.

Compare these Philips models with the datasheets:

.MODEL BF245A NJF(VTO=-1.7372 BETA=1.16621E-3 BETATCE=-0.5 LAMBDA=1.77211E-2
+ RD=9.01678 RS=9.01678 CGS=2.20000E-12 CGD=2.20000E-12 PB=7.80988E-1 IS=2.91797E-16 XTI=3 AF=1
+ FC=0.5 N=1 NR=2 MFG=PHILIPS)

.MODEL BF245B NJF(VTO=-2.3085 BETA=1.09045E-3 BETATCE=-0.5 LAMBDA=2.31754E-2
+ RD=7.77648 RS=7.77648 CGS=2.00000E-12 CGD=2.20000E-12 PB=9.91494E-1 IS=2.59121E-16 XTI=3 AF=1
+ FC=0.5 N=1 NR=2 MFG=PHILIPS)

.MODEL BF245C NJF(VTO=-5.0014 BETA=5.43157E-4 BETATCE=-0.5 LAMBDA=2.71505E-2
+ RD=1.20869E1 RS=1.20869E1 CGS=2.00000E-12 CGD=2.00000E-12 PB=1.24659 IS=3.64346E-16 XTI=3 AF=1
+ FC=0.5 N=1 NR=2 MFG=PHILIPS)

This almost makes sense, doesn't it? Add impact ionization current to prevent designers setting Vds too high, and the models would be good enough for most simulations.

Compare with power BJTs. Almost no models implement quasi-saturation. Didn't stop us from using them. Hexfets? If used in class B amplifiers, devices spend signifant part of the cycle in weak inversion, but you'll find models which doesn't model diffusion current at all!

Regards,
Peter Jacobi
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 1st February 2004, 09:28 PM #9 Christer   diyAudio Member   Join Date: Sep 2002 Location: Sweden Peter, Although I haven't really investigated that many manufacturers models, my impression is that also BJT model are often mediocre at best when compared to the datasheets. I spent quite some time about a year ago trying to DIY models for some BJTs I couldn't find any Spice models for. I tried to really understand the Spice equations as described in my, admittely somewhat old, semiconductor physics book, and figure out both by theoretical thinking and by epxeriments with tweaking parameters how to create a good model. It may be my due to limitations in my understanding, but I really couldn't find any way either in theory or in practice to come up with a suffciently good general-purpose model of any of these BJTs. I ended up having to make a compromise fitting neither of the most importand diagrams really well nor any of them really bad. Some improvement could be achieved if tailoring the models to the intended operating conditions, but some parameters seem still impossible to model well. I couldn't find any way to get the high injection effects (beta droop) as serious as often indicated in the datasheets. There might be further parameters in some modern Spice implementations that improve on this, but if so they seem not to be standard Spice 3.
pjacobi
diyAudio Member

Join Date: Jan 2004
Location: Hamburg
Hi Christer,

Quote:
 Originally posted by Christer [...] my impression is that also BJT model are often mediocre at best when compared to the datasheets. [...] but I really couldn't find any way either in theory or in practice to come up with a suffciently good general-purpose model of any of these BJTs. [...] I couldn't find any way to get the high injection effects (beta droop) as serious as often indicated in the datasheets.[...]
You've done something I always planned to but it never got the top of priority list. As always I'll comment anyway.

- Not perfectly modeling beta droop shouldn't be a problem, as it is easy to operate all your devices below the F(transit) peak (in DIY, as you don't have to optimize your part cost)

- You have IKF, base and collector resistances as the most straightforward ingredients to model beta droop. Want to share one your attempts? I'll have a look on it.

- OTOH you can't avoid quasi-saturation, I'll consider this much more problematic

- If you have time and a C compiler at hand, you can experiment with the Philips MEXTRAM model for BJT:
http://www.semiconductors.philips.co...polar/mextram/

Regards,
Peter Jacobi
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YMMV

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