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Old 29th February 2008, 01:22 AM   #511
fotios is offline fotios  Greece
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Default Re: Re: Re: Re: off topic

Quote:
Originally posted by syn08


Simulating what?

Don't let the number of trannies fool you.
Hi again syn08

What means "trannies"? Transistors? Please i ask from you when reply to me or to other person which does not have lived in your countries (included USA) don't use "slang" words because we don't understand and we only suppose many times wrong the sense of phrase.
Anyway, i am not crazy with the number of "trannies". I am crazy only with a safe margin of 20% further from that gives the semiconductor constructors in their datasheets, and most crazy to lowering the output impedance of the amplifier. Moreover if i had placed this project in production for sale, this would be only for small quantity. How much it is the cost for 4 trannies more and for a heatsink further by 3 cm? Only 5 Euros. So, why not?

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Old 29th February 2008, 01:49 AM   #512
syn08 is offline syn08  Canada
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Default Re: Re: Re: Re: Re: off topic

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Originally posted by fotios


I ask your forgiveness if i misinterpreted your phrase "already analyzed to death". I translated it as "already analyzed FOR death".
"Analyzed to death" = Analyzed and re-analyzed all over until nothing new is to be revealed and basic information (like the bias) can be quickly calculated without any Spice help.

Example: Assume beta very large, so that IB~0 and IC~IE. The IQ3=Vbe/R3~3mA. IC4~Vcc/R11=3.7mA. Assuming the circuit is balanced, then IQ1=IQ2=IQ3/2~1.5mA IQ5=IQ1, IQ2=IQ6, etc... Same as above, IQ8~3.7mA and IQ~Vbe/R18~20mA. IQ9~IQ12 and IQ11=(Vbe+R22*IQ12)/R21~2mA Q7 is normally off. It would take much more time to draw the schematic in the capture software than to directly write this bias estimate. The same with the AC analysis, C13 defines the dominant pole, and the impedance at C13 terminals (giving the time constant, which is related to the pole frequency by f=1/(2*PI*Tau) is a textbook exercise as well.

Trannies = transistors in jargon (not "slang"). The number of trannies in parallel do not influence the amp architecture, it is only an implementation detail.

I am having fun with MOSFETs but I'm not necessary a MOSFET fan.

Anyway, your english is much better than my greek. Good luck.
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Old 29th February 2008, 02:01 AM   #513
fotios is offline fotios  Greece
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Default Re: Re: Re: Re: Re: Re: off topic

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Originally posted by syn08
I am having fun with MOSFETs but I'm not necessary a MOSFET fan.
Sorry for the spelling error "fun". Sincerelly i mean "fan". I made the same error for that i wrote before for you.

Fotios
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Old 29th February 2008, 03:27 AM   #514
fotios is offline fotios  Greece
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Default Re: Re: Re: Re: Re: Re: off topic

Quote:
Originally posted by syn08


"Analyzed to death" = Analyzed and re-analyzed all over until nothing new is to be revealed and basic information (like the bias) can be quickly calculated without any Spice help.

Example: Assume beta very large, so that IB~0 and IC~IE. The IQ3=Vbe/R3~3mA. IC4~Vcc/R11=3.7mA. Assuming the circuit is balanced, then IQ1=IQ2=IQ3/2~1.5mA IQ5=IQ1, IQ2=IQ6, etc... Same as above, IQ8~3.7mA and IQ~Vbe/R18~20mA. IQ9~IQ12 and IQ11=(Vbe+R22*IQ12)/R21~2mA Q7 is normally off. It would take much more time to draw the schematic in the capture software than to directly write this bias estimate. The same with the AC analysis, C13 defines the dominant pole, and the impedance at C13 terminals (giving the time constant, which is related to the pole frequency by f=1/(2*PI*Tau) is a textbook exercise as well.

Trannies = transistors in jargon (not "slang"). The number of trannies in parallel do not influence the amp architecture, it is only an implementation detail.

I am having fun with MOSFETs but I'm not necessary a MOSFET fan.

Anyway, your english is much better than my greek. Good luck.
You may don't believe that in reality i use the Spice engine for making those calculations. Of course and i make the same as you. Moreover the two currents of IcQ3 and IcQ9 defines the rise time and this can be done only by experimenting. For this architecture, the IcQ9 must be about 20mA for uniformity of signal during the negative swing in comparisson with the positive swing. This is in effect with any type of VAS transistor and any Supply level. It is de facto. For the IcQ3 there is the need of experimenting when the supply level is to be changed in conjuction with the type of transistors used in the small stages. Also may be notted that for the experiments used only square signal for a clear viewing of the variations of rise time and thus the bandwidth (frequency response). In the given circuit with the 2N5401 and the supply of +/-82V it is enough a current of 3mA to achived an acceptable average rise time of 1,8ěsec (with the Miller pole fully compensated wich is also the upper frequency pole of whole the amplifier). More current it does not offers nothing furthermore except the overheating of the LTP transistors and thus the increasing of Johnson noise. In the second case with the MPSA56 in the small signal stage and with a supply of +/-60V from my experiments proved that the corresponding current of IcQ3 must increased in 6mA to achived the same rise time as previous. Sincerelly i tried many times to simulate this condition, but the results of simulator was very different from the measurements with real instruments. From this event and after i understood the problem of Spice which is that it supposes the transistors as ideal according to the models that includes. In the real world we are not in position to find two transistors with the same beta without enough search. And the beta it is only one of the endless parameters of a spice model of a semiconductor. And be it so we can find transistors with the same parameters as the models included in the spice engine; there is any simulator with so much accuracy to calculate differences between 1,8 to 2,0 ěsec?
Anyway, i hope to have explained my position that i expressed in my first post. Furthermore thank you very much for the lesson about the jargon language. Indeed there is in the dictionary.
And again sory for the misinterpreting and the spelling errors.
Glad to meet you and good luck also to you.

Cheers
Fotios
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Old 29th February 2008, 05:25 AM   #515
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Interesting input, Fotios. For the record, I have used circuit simulation professionally for the last 42 years, but I just don't rely on it for my linear circuit designs. Filters, yes!
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Old 29th February 2008, 06:13 AM   #516
PMA is offline PMA  Europe
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I use the attached circuit to test opamps for noise, non-linearity and PSRR. It unmasks behavior masked by high NFB. In simulator (MC) the results are nonsense, in orders of magnitude more optimistic than the real world.
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Old 29th February 2008, 09:19 AM   #517
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It seems to me that most of the people "contributing" to this thread are forgetting what a simulation means.
Me (I’m considering myself as being “pro” that methodology) I don’t consider the world that comes out from a simulation as being a faithful representation of the reality, so I keep the results with (some) suspicion, but I keep and I use them as a mean to get the result in a faster, safer, less costly way. My professors at University (Theoretical Physics) taught me how to evaluate first the results by hand, providing an order of magnitude, but then let me find more trustworthy results using calculators, or even computer programs.
So, please, stop discussing about the validity of the (simulation) approach !
Simulators are as good as the models they use (garbage in, garbage out).
I remember when I first tried to use Circuit Maker: it was impossible to get the correct biasing. Then I discovered why: the embedded 1N4148 model was fake, since it gave 850mV of voltage drop (this remind me what happened to me, in the real world, many, many years ago, when I used germanium diodes instead of silicium diodes ...).
So, please stop asking for and then using models whose validity is suspicious ! and then criticise the results ...
Me I’d liked to start some cooperation leading to some “machine” that, starting from experimental data, could give reliable models. Me, I could provide the mathematics, you, the community, could contribute with better knowledge (than me) about the model parameters, better measurements and so on.
Is it possible to turn all that noise into music ?
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Old 29th February 2008, 10:22 AM   #518
PMA is offline PMA  Europe
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You know opamp models that are valid enough? No one doubts about the fact, that simulation is a helping tool. We speak about limits of available models and tools.
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Old 29th February 2008, 12:23 PM   #519
syn08 is offline syn08  Canada
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Quote:
Originally posted by john curl
Interesting input, Fotios. For the record, I have used circuit simulation professionally for the last 42 years, but I just don't rely on it for my linear circuit designs. Filters, yes!
There are circuit topologies for which using some Spice help in design and analysis is saving time and effort. Some people prefer not to use such topologies, precisely because it's difficult to predict the circuit behaviour by simply inspecting the topology - and that's, to me, flat wrong. Spice is like a microscope, it shouldn't be used it as a hammer to hit nails, or to inspect the Golden Gate bridge. but for what it was designed for: inspecting bacteria and viruses.

You raised the filter design problem; indeed, some sort of CAD help for filter design is very helpful. However, I am not sure if Spice is the best tool for that. Reason is that Spice won't tell you a thing about (e.g.) filter sensistivity, unless you have the time and patience to start sweeping component values and their combinations. Filter design may benefit from using specialized software (e.g. Linear Technology's FilterCAD, really good stuff). But even for filters, some good understanding of architectures and principles is mandatory to succeed. Here's a good example of what Spice won't tell or teach you about filters: http://www.maxim-ic.com/appnotes.cfm...te_number/738/

My problem with simulations starts when one attempts to replace the brain with the computer screen. It is very easy to get addicted to any kind of simulations - because they show a simplified and glossy projection of reality, deceiving you with the illusion of being in control.

Good/ideal models for devices are an illusion as well. Poor models are not necessary only the result of a poor extraction job, but also because the model templates (which are mostly hardcoded in Spice, e.g. Gummel Poon for bipolars) may not be good enough for certain applications. The art is to understand the models and their limitations! Example: noise simulation. Spice has no provisions for noise analysis other than thermal noise, where it automatically inserts a voltage source in series with each resistor in the circuit. Is this good enough? Sometimes yes but mostly, to stay accurate, no: http://www.utdallas.edu/~hellums/doc...oiseModels.pdf
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Old 29th February 2008, 01:07 PM   #520
KSTR is offline KSTR  Germany
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Quote:
Originally posted by syn08
You raised the filter design problem; indeed, some sort of CAD help for filter design is very helpful. However, I am not sure if Spice is the best tool for that. Reason is that Spice won't tell you a thing about (e.g.) filter sensistivity, unless you have the time and patience to start sweeping component values and their combinations.
Hhm, what other option do we have? We can try to do the math (not at all trivial in a multi-component sensivity analysis) and get lost there for a day or two (and need to reality check the results anyway somehow), or we can build the actual circuit and swap components, record tons of values and do thousands of measurements and then do sensivity analysis "by hand", or we can burden most of the tedious work (not all, though) to SPICE and let it do the variations and measurements, and later back the accuracy of the results with some sampled test cases. IMHO, SPICEing is the only way to complete tolerance sensivity analysis with reasonable time and effort, for the engineer.


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My problem with simulations starts when one attempts to replace the brain with the computer screen. It is very easy to get addicted to any kind of simulations - because they show a simplified and glossy projection of reality, deceiving you with the illusion of being in control.
That's, of course, the key. SPICE cannot replace knowledge (it's a "stupid" tool, not a full blown expert system -- as of yet), but it can steepen the learning curve to get that knowledge.

- Klaus
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