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Old 18th June 2007, 04:22 PM   #161
syn08 is offline syn08  Canada
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Default Re: MOSFET Cgs

Quote:
Originally posted by Bob Cordell
Based on some earlier discussions, I was under the impression that the gate-source capacitance, Cgs, of a MOSFET would rise as the forward gate voltage was increased, particularly as the device passed threshold and entered conduction.
<snip>

I understand the possible issues beyond a strong Cgs(Vgs) dependency. A nonlinear Cgs characteristic can generate intermodulation distortion at the input of the circuit. The distorted signal is amplified by the active device, affecting the overall linearity of the circuit.

I'm not sure where you got the assumption on the Ggs variation, but the basic Meyer model for MOSFETs (also included in SPICE) is not predicting such a strong dependency. See http://nina.ecse.rpi.edu/shur/Ch5/sld020.htm and the following slides. The equivalent Cgs is the sum of a gate-source overlap capacitance (which is basically independent of the bias conditions Vgs and Vds) and a periphery diffusion capacitance which is nonlinear, it's dependency on Vgs and Vds changes with the MOSFET operation region (subthreshold, linear, saturation).

The situation is much better than for bipolars; usually the periphery capacitance is much smaller than the overlap cpacitance, even in strong inversion. Though, this nonlinear Cgs dependency is a major concern for microwave MOSFETs build on compound semiconductors; such devices have extremely small overlap capacitances (partly due to very shallow junctions) and therefore the nonlinear part of Cgs is a major contributor to the overall capacity.

For power devices, the nonlinear Cgs contribution would be significant for high voltage devices; due to a ligthly doped substrate and deep junctions, the nonlinear Cgs contribution would increase. But then nobody is using such MOSFETs for linear applications.

I don't think that the nonlinear Cgs and Cgd should be a major concern in audio applications.
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Old 19th June 2007, 12:27 AM   #162
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Default Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: I repeat my Request

Quote:
Originally posted by Bob Cordell



Just what I thought. You should save your bluster and bragging until you actually build and measure the real thing. Hopefully it will not oscillate.

Bob

And this is exactly the reply I was expecting.

I never said that the thing was completelly up and running, and I never said that I had a complete EC + bootstrapped 500W class A stage operational. If I did, I would be posting photos and measured results instead of revealing my schematics as I develop the final circuit blocks. This is a big project and it takes time. I have however (way back when we discussed this last) built and tested both an EC and and an opamp-linearised class A/AB output stage with bootstrapped rails and they worked fine, remarkably without oscillating.
I have experimented with these circuit blocks and topologies to know what I am doing (regardless of presumptions) and to have enough confidence in the final design to blow about 3k building it.

If anyone thinks my claims for the potential performance of the scheme I have outlined thus far are off the mark, then I'd welcome some good technical arguments as to why or what I am doing wrong.

Cheers,
Glen

PS.
Since this is deviating from the BJT vs MOSFET debate, I'll start a new thread for this later in the week. This will make a good excuse for developing some Sanken BJT SPICE models.
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Old 19th June 2007, 01:45 PM   #163
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Default Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: I repeat my Request

Quote:
Originally posted by G.Kleinschmidt



And this is exactly the reply I was expecting.

I never said that the thing was completelly up and running, and I never said that I had a complete EC + bootstrapped 500W class A stage operational. If I did, I would be posting photos and measured results instead of revealing my schematics as I develop the final circuit blocks. This is a big project and it takes time. I have however (way back when we discussed this last) built and tested both an EC and and an opamp-linearised class A/AB output stage with bootstrapped rails and they worked fine, remarkably without oscillating.
I have experimented with these circuit blocks and topologies to know what I am doing (regardless of presumptions) and to have enough confidence in the final design to blow about 3k building it.

If anyone thinks my claims for the potential performance of the scheme I have outlined thus far are off the mark, then I'd welcome some good technical arguments as to why or what I am doing wrong.

Cheers,
Glen

PS.
Since this is deviating from the BJT vs MOSFET debate, I'll start a new thread for this later in the week. This will make a good excuse for developing some Sanken BJT SPICE models.

Hi Glen,

In post # 1652, you stated, "From my experience in building/prototyping the various blocks of my rail-tracking 500W class A with 60 MHz/35 MHz Sanken BJT's, the output impedance and distortion figures quoted by you and Nelson look lame, and that is before I apply EC."

That is a pretty bold claim.

You failed to be forthcoming about the conditions of the claim and caveats after being questioned. Even now, you have still not said what distortion number you achieved that made Nelson's number look lame. This is why you have lost a lot of credibility with me.

Your amplifier looks like a very interesting and ambitious project, and I agree that it deserves its own thread. I hope you will not feel the need to raise up that accomplishment by putting down the accomplishments of others. I'm sure it will stand on its own.

It would be great if you develop some good SPICE models for the Sanken devices. Check with Andy_c, as he has done a very good job of putting together some good SPICE models of the ThermalTrak transistors, which will likely have some similar behavior and issues.

Bob
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Old 21st June 2007, 03:59 AM   #164
GK is offline GK  Australia
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Default Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: Re: I repeat my Request

Quote:
Originally posted by Bob Cordell



Hi Glen,

In post # 1652, you stated, "From my experience in building/prototyping the various blocks of my rail-tracking 500W class A with 60 MHz/35 MHz Sanken BJT's, the output impedance and distortion figures quoted by you and Nelson look lame, and that is before I apply EC."

That is a pretty bold claim.

You failed to be forthcoming about the conditions of the claim and caveats after being questioned. Even now, you have still not said what distortion number you achieved that made Nelson's number look lame. This is why you have lost a lot of credibility with me.

Your amplifier looks like a very interesting and ambitious project, and I agree that it deserves its own thread. I hope you will not feel the need to raise up that accomplishment by putting down the accomplishments of others. I'm sure it will stand on its own.

It would be great if you develop some good SPICE models for the Sanken devices. Check with Andy_c, as he has done a very good job of putting together some good SPICE models of the ThermalTrak transistors, which will likely have some similar behavior and issues.

Bob


Bob,

It was also a general claim.

Your hypothetical/computed THD figure was 0.5%. For a 100W class A stage, that is lame (sorry).

Nelsons THD figures were presented without any mention of load impedance, power output, specific frequency, output stage topology (single-ended / bridged?) or number of output devices used, or bias current. All that is completely clear to me about Nelson's output stage in question is that it uses complementary P/N MOSFET's.

Yet, apparently, as demanded by you, all these things must be explicitly detailed by me, lest my credibility suffers.

I can assume, for the sake of the argument, that his Class A output stage with 0.1 and 0.06% THD is identical in every other respect to the hypothetical 100W class A stage put forth by you, but that would seem a coincidence. For this reason, I did explicitly ask Nelson if this was the case (for the purpose of making a more accurate comparison - particularly with regards to your hypothetical 0.5% THD figure) but I was ignored.

My comments on the THD figures presented were to express my opinion that they do not represent the performance limit of bipolar class A output stages.

Finally, as far as I can tell, ultra-low THD isnít one of Nelsonís design aims, and this hasn't hindered his business successes. Now I could be wrong, but I doubt that he really cares if I donít like his THD figures. We are each entitled to design to a specification that we personally see fit.

I not sure who's accomplishments exactly Iím guilty of putting down here.
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Old 12th July 2007, 12:08 PM   #165
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Default Toshiba 2SJ201 & 2SK1530

Does anybody have a good source for the Toshiba power MOSFETs 2SJ201 and 2SK150?

A source where there is a low probability of counterfeit devices?

Digikey lists them and prices them, but appears to have none on hand.

BTW, I contacted Toshiba re SPICE models for them, and of course they don't have them. Anyone got some?

In some applications these Toshiba MOSFETs may be more attractive than the IRF-like devices because they appear to have a somewhat lower turn-on voltage.

Also, they appear to spec a higher 100V, 10 ms SOA of about 5.5A than the number for the IRF devices, but this might just be a reflection of some greater conservatism on the part of IR.

Thanks!
Bob
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Old 12th July 2007, 12:10 PM   #166
AndrewT is online now AndrewT  Scotland
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Toshiba don't have Spice models for their own devices.
They should be hanged!
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Old 13th July 2007, 02:03 PM   #167
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Quote:
Originally posted by john curl
Some of use don't need Spice to design successful circuits. Apparently, the Japanese don't overdepend on it either. For the record, I use Toshiba FET's exclusively in my designs and they all seem to work, even without Spice emulation. What a concept!

Hi John,

Your comment reminds me of an old computer book I bought once: it was called "Real Men Use DOS" :-).

Seriously, very good amplifiers can be made without resort to SPICE. Indeed, I did not use SPICE in doing my MOSFET amplifier with error correction. This includes development of the compensation scheme necessary to make HEC practical. So it certainly can be done.

As I have said many times before, SPICE is an excellent tool that can play a very positive role in design exploration and in optimizing a design. It would be unfair to suggest that those who use SPICE over-depend on it.

Bob
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Old 13th July 2007, 04:34 PM   #168
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Quote:
Originally posted by john curl
Hi Scott. I still have that computer simulation that you sent me back in the early '80's, emulating DA in caps using your differential subtraction method. I sure could have used SPICE in those days, although I did get an early version of Microcap that did about the same thing about that time.
I got hammered on that one by a prof at NYU. He asserted that you can't possibly measure any difference at null between caps. I add that one to the time Dr. Lipshitz blew me off at an AES conference.
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Old 13th July 2007, 06:25 PM   #169
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Quote:
Originally posted by john curl
Personally, I'm not against SPICE, but OVERDEPENDENCE on it over real circuits.
I use SPICE to good effect, but I don't rely on it to give very
accurate results for non-linear circuits. I have to smile if someone
quotes parts-per-million distortion figures from a simulation.


Quote:
Originally posted by john curl
You might remember that Walt sent the WORST tantalum caps that he had ever found, to Dr. Lipshitz and they could not detect a difference.
You may have been there at AES when Lipshitz and Van der Kooy
presented their (masterful) analysis on the Quad current dumping
amplifier. When asked by a member of the audience how it
sounded, one of them said (and I paraphrase here), "I don't
know, we didn't listen to it."

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Old 13th July 2007, 06:35 PM   #170
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Quote:
Originally posted by Nelson Pass


I use SPICE to good effect, but I don't rely on it to give very
accurate results for non-linear circuits. I have to smile if someone
quotes parts-per-million distortion figures from a simulation.



Hi Nelson,

My sentiments exactly.

Bob
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