Transfer function

What are you trying to do? You can't choke transfer functions out of Spice, but if you look at it from the right angle you can often get loads of useful information.

Most circuits are either simple enough that someone has already cataloged their transfer functions (or you can derive them easily enough yourself), or complicated enough that if you ignore all of the parasitics and nonlinearities you're doing yourself no favors -- yet factoring in the parasitics makes the transfer functions exceedingly opaque, while factoring in the nonlinearities renders the whole concept of transfer function analysis invalid.

--
www.wescottdesign.com

A symbolic algebra program is probably your best bet, or else using something like Octave to fit some suitable curve to the output of a SPICE AC analysis.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
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hobbs at electrooptical dot net
http://electrooptical.net

Depends on whether you want to plug in numbers or keep the component values as symbolic (which gets _really_ messy _really_ fast).

Something like Scilab (and I think Octave) will let you fairly easily reduce a circuit to a transfer function with numeric coefficients (or a state-space representation as polynomials are insanely sensitive to numerical accuracy issues). I've used this when I want to include a filter or something in a larger system model.

--
www.wescottdesign.com

PSpice has Laplace behavioral sources. Should be fairly easy to parameterize and fit to a Bode plot.

I'd guess that LTspice also has that capability?? ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
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SSpice (symbolic SPICE) is what you are looking for.

Pere

Where? I can't find a URL that doesn't roll-over to a book store or similar. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I played with an evaluation copy of SSpice some 15 years ago...

After a quick search I have found this link

which brings up something that vaguely matches what I remember (not very user friendly though). But there are certainly few links on this in the internet!

Another option, as someone already suggested, is to make use of the symbolic capabilities of Matlab (or Octave) and compute the relevant determinants from the (hand-written) system of equations. When I was a student I even coded a symbolic determinant function from scratch for exactly this purpose.

Pere

That's it, thanks!!!

Quote:

Late at night, by candle light, Jim Thompson penned this immortal opus:

It does, and very nicely. Use it in the gain statement with dependent voltage or current sources. V=laplace(1/(s-1)) and so on.

- YD.

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Remove HAT if replying by mail.

Mathematica in itself also allows this very easily. But it has a very nice addon package (analoginsydes) that allows a lot more, like direct netlist input, symbolic TF evaluation... and simplification (your circuit matrix is numerically evaluated WRT your design point, then its symbolic form is returned while neglecting the terms up to some given accuracy, allowing deep insight of complex circuits), and much much more... All that coupled with the mathematica power makes this a very valuable analysis/design tool... But this comes at the price of a somewhat steep learning curve, plus a few grands... Worth it, imo

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Thanks,
Fred.

Something

"computation".

of

loads

cataloged

yourself),

the

transfer

Bloody cool. With some scripts and some applied thinking you could make really nice filter optimization tools this way.

I use them for quick and (not-so)dirty system filters around my chip designs because they simulate significantly faster then "lumps" and are very accurate. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         You can never be too prepared for the REPRESSION!