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Old 12th July 2008, 09:58 PM   #611
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Quote:
Originally posted by unclejed613
i've used various versions of SPICE, and although LTSpice doesn't have some of the "whistles and bells" some of the other SPICE software (such as the "real time" oscilloscope in TINA), LTSpice has a good level of universality (you can directly import most text based models and libraries, often without modification) as well as a more stable set of solvers (often other spice software would dump during a simulation with only vague clues of the reason).

also simulating saves a lot of time with selecting a good starting point for parts values for a circuit. instead of building an oscillator and rummaging through a box of capacitors, i can just try different capacitances by typing them in and watching the results. once i get the proper capacitor in SPICE, i only have to rummage for one cap. SPICE will also measure device currents without the hassle of connecting an inline milliameter or expensive oscope current probe, so i can see how a particular current source or current mirror will behave. SPICE may not have 100% accurate results all of the time, but usually the results will be close enough to know whether a circuit is going to work or not, and be at least in the ballpark of how well it will work.

You have hit the nail on the head regarding the greatest value-added of performing SPICE simulation. In particular, I just love to be able to probe around a circuit effortlessly (and without disturbing the circuit) to see exactly what is going on. Not just currents flowing through elements, but also probing differential voltages effortlessly.

Cheers,
Bob
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Old 12th July 2008, 11:45 PM   #612
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You have a point, Bob.
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Old 12th July 2008, 11:50 PM   #613
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I have said this many times before, but don't get fooled that Spice is only useful for predicting the behaviour of actual cirucuits to be built. If you think so, you have a lack of imagination. It can be very useful to study circuits with more or less ideal models to pinpoint certain effects. For instance, one might start with the basic NPN model, which is just the basic transfer function. Then one can add various effects and study separately. For instance, add Cob and see how that affects distorsion. Then remove it and add the Early voltage, etc. And those are just straightforward simple ideas. You can do much more interesting things. Partially ideal models can tell a lot of interesting things about a circuit by factoring out various contributions in a way that is impossible to test in a real circuit. Then of course, those factors will interact both in reality and in the sim, when you throw them all in, but you might get a better idea of what causes what, and what factors are most important.

Spice is also a great tool for learning. If you try to understand theory by deriving formulas yourself (which I find a much better way to understan than just read them in a book), then you can do some Spice simulations to see if what you derived seems to be correct.

This is still no argument that John and others with very long experience in managing without Spice should necessarily start using it now. On the other hand, if trying to get outsde the mental block about how to use Spice that I point to here, they might be surprised, perhaps.
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Old 12th July 2008, 11:58 PM   #614
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I would agree, IF only I could make Spice work for ME! I keep trying, but I need an expert (as I used to be, long ago) on the subtleties of computer operation I was a WIZ on the IBM 7094!
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Old 13th July 2008, 12:05 AM   #615
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Quote:
Originally posted by john curl
I would agree, IF only I could make Spice work for ME! I keep trying, but I need an expert (as I used to be, long ago) on the subtleties of computer operation I was a WIZ on the IBM 7094!
Still using a Mac, are you? I don't think you should find much of a problem to get LTSpice running on a PC. Then, adding new models might perhaps be a bit tricky if you are not friendly with computers, but it is not so difficult if somebody shows you how.

LTSpice also seems to run fine under Linux, if installing Wine first, which I think works also for the MacOSX. I tried it under Linux and it worked without any fuzz, except possibly som ugly rescaling of text fonts sometimes.

Do you have any friendly forum members nearby who could give you a crash course?
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Old 13th July 2008, 02:40 AM   #616
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No.
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Old 13th July 2008, 02:59 AM   #617
PHEONIX is offline PHEONIX  Australia
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Quote:
Originally posted by john curl
No.

Hello John ,

What simulator package are you trying to learn to use.

Regards
Arthur
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Old 13th July 2008, 05:34 AM   #618
1audio is offline 1audio  United States
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Quote:
Originally posted by john curl
No.
John:
I'll give you a crash course (which probably will be all I know) to get you up the next time I'm nearby. Assuming you have the computer.

Getting models and making them work is the bigger pain. We all need sources for models for Jfets, MosFets and who knows what else that aren't in the default LTspice libraries. And a quick writeup on how to include them (I have to search the web every time I do that).

I personally don't think Spice is the way to -130 dB distortion products. Its a start but the devices themselves vary too much for the models to have that much accuracy. For the incredibly low distortion numbers to be real it needs to be built and verified. Its great for Monte Carlo analysis to see if you can make more than one of something. Its also great to test an off the wall idea to see if it will work.
-Demian
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Old 13th July 2008, 05:58 AM   #619
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Thanks Demian, it was good meeting up with you today. I do need to talk to you also about the QT, as 10Hz is 10db too low for some reason, maybe a setting. Let's talk tomorrow.
Spicewise, I am trying to get PMA's circuit sims into Microcap 9, and can't quite do it. That is my biggest hassle at the moment.
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Old 13th July 2008, 10:17 AM   #620
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Quote:
Originally posted by 1audio
[snip]
I personally don't think Spice is the way to -130 dB distortion products. Its a start but the devices themselves vary too much for the models to have that much accuracy. For the incredibly low distortion numbers to be real it needs to be built and verified. Its great for Monte Carlo analysis to see if you can make more than one of something. Its also great to test an off the wall idea to see if it will work.
Demian
High Demian,

Although I agree with most of your comments, I don't see a really good reason why you put the limit on -130dB. Sounds rather arbitrary. A good amplifier design is (more or less) insensitive to the tolerances of the active components. So the accuracy or inaccuracy of the models doesn't matter that much, that is, as long as the sources of distortion are quantitatively modeled between reasonable limits.

Cheers,
Edmond.
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