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Old 19th May 2007, 09:08 AM   #131
peufeu is offline peufeu  France
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I downloaded the Microcap 9 free evaluation version. It looks a lot better than the version 6 demo I had !

However Mextram self-heating model is only available in the professional version... argh.
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Old 19th May 2007, 03:44 PM   #132
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Default noise reductiom

Hi Grey and A.M.

I'll send the article to you, probably tomorrow.

Cheers,
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Old 19th May 2007, 03:50 PM   #133
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Instead of doing Spice, yesterday I bought a book on transistor theory: 'The Bipolar Junction Transistor' Vol3 By Gerold W Neudeck from the modular series on solid state devices, Addison Westley Publishing, once used at a course at Purdue, just to get back to understanding how transistors REALLY WORK.
I hope to cast further aspersion on Spice models in the near future by reading this text. vampire2:
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Old 19th May 2007, 04:57 PM   #134
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Hi John,
That would be illuminating.

cheers,
Ashok.
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Old 19th May 2007, 06:35 PM   #135
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Quote:
Originally posted by andy_c
With a little work, you can adapt them. I was recently working on a sim of an amp whose small-signal loop gain showed a phase margin of nearly 90 degrees. But I looked at the error current in the input diff amp when simulating a square wave input that drove the output almost to clipping. There was some really bad ringing in the error current. The ringing was at a frequency near the unity loop gain frequency of the global feedback loop. It was worst when the output voltage was near the negative rail.

So I set up the sim so the amp had a closed-loop gain that was flat to DC. I set the input signal to a simple DC value that put the amp's output almost to the negative rail where the ringing in the error current was worst in the transient sim. Then I did a loop gain test. Sure enough, the phase margin wasn't much better than zero. I modified the compensation so the phase margin was about 80 degrees. When I re-ran the square wave transient test, the error current looked nearly perfect, like a first-order system.

So yes, the transient response test is necessary, and tells you there's something wrong, but doesn't tell you the best way to fix it. A loop gain test with DC offset can be a good tool to determine how to fix it without overcompensating.

Another sim, suggested by mikeks, is to have a low-frequency sine wave input whose amplitude almost drives the amp into clipping. Then you can add a small-amplitude square wave of a frequency about 10-20x higher than the sine wave. You look at ringing on each little pulse on the output. In practice, it seems to be worst when the output is near the rail.

Very good point and approach, Andy. I've also seen lots of cases where the phase margin went south under certain voltage or current conditions.

Bob
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Old 19th May 2007, 06:38 PM   #136
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Quote:
Originally posted by john curl
Instead of doing Spice, yesterday I bought a book on transistor theory: 'The Bipolar Junction Transistor' Vol3 By Gerold W Neudeck from the modular series on solid state devices, Addison Westley Publishing, once used at a course at Purdue, just to get back to understanding how transistors REALLY WORK.
I hope to cast further aspersion on Spice models in the near future by reading this text. vampire2:

Hi John,

I think I remember that series of books. Maybe six fairly small volumes. I think I had the series in one of my courses in college.

You're right that SPICE models are certainly not perfect, but the simulation results can still be very valuable.

Bob
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Old 19th May 2007, 07:11 PM   #137
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Quote:
Originally posted by john curl
Instead of doing Spice, yesterday I bought a book on transistor theory: 'The Bipolar Junction Transistor' Vol3 By Gerold W Neudeck from the modular series on solid state devices, Addison Westley Publishing, once used at a course at Purdue, just to get back to understanding how transistors REALLY WORK.
I hope to cast further aspersion on Spice models in the near future by reading this text. vampire2:
John,

I had Professor Neudeck for a course in transistor circuit design, EE255 at Purdue, around 1976 or so, when he wasn't all that much older than I was. He was a great guy, and also sometimes quite entertaining.

-------

Simulations, and also hand calculations, obviously depend on how good the models are, for the usefulness of their results.

But, given that the models are not perfect, the usefulness of simulations depends at least as much on the assumptions of the person doing them.

Bob Pease's famous "issues" with the use of Spice seem to have more to do with some people's idiotic "blind faith" assumptions about the models and simulation results, than with simulation per se.

One of the main reasons we have "engineering" is to try to be able to design things and then "know" that a design will work, before it is built. That's usually an imperfect process, of course (sometimes "by design"), and often requires iteration, after prototypes are built. However, the basic idea implies that we should consider using whatever design tools are available and helpful, incuding computerized tools, of which Spice is but one. Of course, our brains are our most-important tools. If they are not used properly, tools like Spice can be worse than un-helpful.

Bottom line: Blame the user, not the tool.


- Tom Gootee

http://www.fullnet.com/~tomg/index.html

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Old 19th May 2007, 10:58 PM   #138
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You must come to understand that I have been professionally involved with circuit simulation since 1966. That is a long time! I just don't DEPEND on Spice for my circuit designs. It's OK, and it gets better all the time. Still, I don't need it to make a successful product. Neither did the designers who made the ua741 possible.
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Old 31st May 2007, 04:58 PM   #139
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Default Re: sophisticated amplifiers

Quote:
Originally posted by estuart
BTW, Bob, there is a difference between sophisticated amplifiers and real sophisticated amplifiers.
Let's say 20dB less THD and IMD, or, if you like, anything equal or better than Halcro.

Cheers. Edmond

Hi Edmund, I must have missed a post you made (maybe it was not in this thread).

I don't want to get into semantics between sophisticated and real sophisticated, but I think my definition above was adequate, and of course I think that the MOSFET power amplifier with error correction that I did 25 years ago lies in that realm. It achieved 6 ppm (0.0006%) at 20 kHz THD in a 200 khz measurement bandwidth.

With regard to Halcro, are you referring to Halcro's stated performance or actual performance? I've seen no evidence that they meet their stated performance (even after Stereophile reviewed the DM38 again), but I allow for the fact that properly measuring performance in the range that they state can be difficult.

Forgive me if I missed a post, but have you actually built and measured an amplifier that does 1 ppm or below ( e.g., <120 dB) across the full spectrum from 20 Hz to 20 kHz on both THD and CCIF IM at any power from 1 watt to full power? If so, that is very impressive, and please refresh my memory and/or tell us more.

Simulations don't count. Although I am a firm believer in SPICE, I have always been reluctant to depend on it for estimates of distortion achievable in practice.

Cheers,
Bob
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Old 31st May 2007, 06:44 PM   #140
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Default Re: Re: sophisticated amplifiers

Quote:
Originally posted by Bob Cordell
Hi Edmund, I must have missed a post you made (maybe it was not in this thread).

Hi Bob,

Yes, probably you did. In my post#1552, I was just a little bit joking, as somebody was referring to "sophisticated" amplifiers, which shows quite a lot of distortion. Therefore I stated: "Real sophistication starts at thd<1ppm, not at 0.02% or 0.25% in case of tubes."


Quote:
Originally posted by Bob Cordell
I don't want to get into semantics between sophisticated and real sophisticated, but I think my definition above was adequate, and of course I think that the MOSFET power amplifier with error correction that I did 25 years ago lies in that realm. It achieved 6 ppm (0.0006%) at 20 kHz THD in a 200 khz measurement bandwidth.
Sure, you definition is adequate, but also 25 years in the future?
6ppm? NO, 7 ppm, according my simulation.

Quote:
Originally posted by Bob Cordell
With regard to Halcro, are you referring to Halcro's stated performance or actual performance? I've seen no evidence that they meet their stated performance (even after Stereophile reviewed the DM38 again), but I allow for the fact that properly measuring performance in the range that they state can be difficult.
Maybe Candy is lying, but I never trust these measurements performed by such magazines. Besides, as you said, it's difficult to check the performance of a Halcro by means of an AP, for example. They should consult you to get reliable numbers.

Quote:
Originally posted by Bob Cordell
Forgive me if I missed a post, but have you actually built and measured an amplifier that does 1 ppm or below ( e.g., <120 dB) across the full spectrum from 20 Hz to 20 kHz on both THD and CCIF IM at any power from 1 watt to full power? If so, that is very impressive, and please refresh my memory and/or tell us more.
Built? No, I'm retired and don't have the facilities any longer to build such amp. But I have designed amp's on paper with a thd (at 20kHz) around 20ppb. If some body give me the money and facilities to build such thing, I should really love it.

Quote:
Originally posted by Bob Cordell
Simulations don't count. Although I am a firm believer in SPICE, I have always been reluctant to depend on it for estimates of distortion achievable in practice.

Cheers,
Bob
Well, it depends on the tools and models, and of course who is doing the simulation, it's really an art, just as building the real thing, to understand the power and limitations of a simulator.
Anyhow, my latest design shows, a thd (20kHz) of 20ppb. Maybe my sim is one order of magnitude wrong, that still leaves a 0.2ppm of distortion.

Cheers, Edmond.

edit: if you are really interested in my design, I'll send you the schematic (confidentially, of course).
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