Spice simulation

[Cal Weldon]

Originally posted by Bob Cordell
Hi Edmund,

It's Edmond.

[Edmond Stuart]

Originally posted by Bob Cordell
Hi Edmund,

First, no offense taken.

Second, I realize that when I say SPICE simulation results don't count, there are many caveats. You are pointing out situations where they DO count, and I agree with that. They especially count in moving one in the desired direction and giving perspective on what works better.

Originally posted by Bob Cordell However, I just stop short of believing a bottom-line result on distortion in an absolute sense because there are so many factors that can influence that bottom line that may not be adequately modeled. SPICE is a great tool, and I love it, but most would agree that we recognize its limitations and don't depend on it exclusively. Those who throw it out completely because it doesn't do everything are no wiser than those who depend on it exclusively. I'm sure you agree that the middle ground is where it is at.

Cheers,
Bob

Cheers, Edmond.

[Edmond Stuart]

Originally posted by G.Kleinschmidt

Care to eleborate a bit? The non linearity of Vgs vs Id causes significant large signal distortion.

"The use of power FETs in output stages is often advocated. However, after much investigation, I have found the conclusion inescapable that FETs suffer not only from poor basic linearity, due to low gm, but also a crossover region that is inherently more jagged than BJTs. It is not possible to explore this in detail here, but see [7],[8] "............

7] Self, D "Audio Power Amplifier Design Handbook." Newnes 1996, p231. ISBN 0-7506-2788-3 (poor FET linearity)
8] Self, D "FETs vs BJTs- the linearity competition." Electronics & Wireless World, May 1995 p387. (poor FET linearity)

Above from here:
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm#R


Any charitable soul out there with a copy of ref #8 to send to Bob Cordell? He's a poor misguided FET user who needs educating.

Cheers,
Glen

Hi Glen,

This time and only this time, the honorable Douglas Self was talking through his hat.
His conclusions about MOSFET's were only base on stone aged SPICE models, which totally ignores the exponential gm of the so called weak inversion. Therefore his jagged graphs are quite misleading.
Besides, gm alone is not a good figure of merit, what counts is the product of gm and the effective impedance.
As these two parameters are quite different in nature, this also implies that you cannot blindly replace BJT's by MOSFET's, if the design is optimized for BJT's, and then complaining about the lesser performance. Designing a good MOSFET output stage demands a different approach.

Cheers, Edmond.

PS: I do have that article in EW+WW, but I don't send it to Bob, as it will certainly damage my reputation.

[GK]

Originally posted by estuart


Hi Glen,

This time and only this time, the honorable Douglas Self was talking through his hat.
His conclusions about MOSFET's were only base on stone aged SPICE models, which totally ignores the exponential gm of the so called weak inversion. Therefore his jagged graphs are quite misleading.
Besides, gm alone is not a good figure of merit, what counts is the product of gm and the effective impedance.
As these two parameters are quite different in nature, this also implies that you cannot blindly replace BJT's by MOSFET's, if the design is optimized for BJT's, and then complaining about the lesser performance. Designing a good MOSFET output stage demands a different approach.

Cheers, Edmond.

PS: I do have that article in EW+WW, but I don't send it to Bob, as it will certainly damage my reputation.

Originally posted by Bob Cordell



Self was at least partially full of baloney on this one. At the very least, he had blinders on and did not tell the whole story. MOSFETs have about 10 times less gm at a given operating current than bipolars. If you get past this, they are every bit as linear. As a very rough approximation, the dynamic source resistance (1/gm) of a MOSFET is about 250/Id in mA.

Let's build a 100 W Class A amplifier from MOSFETS. It must support 40V pk and 5A pk into an 8 ohm load. Lets give it 44V main rails. The Class A idle bias must be 2.5 A. Lets use 5 pairs of MOSFET output devices, each carrying 500 mA of idle bias and each dissipating 22W at idle. The dynamic source impedance of each MOSFET is about 250/500 = 0.5 ohms. The five upper devices in parallel will yield a net upper Rs of 0.5/5 = 0.1 ohms. The bottom five devices will do the same, bringing the total open-loop output impedance at idle to 0.05 ohms.

At a peak current of 5 A, each of the upper transistors is at 1 amp with an Rs of 0.25 ohms. We have five in parallel, so we get an effective net Rs of 0.05 ohms. To first order, this is the same as at idle bias. Of course, there will be wiggles in between, and this approximation is only that, but the point is that the output stage net transconductance, variations of which will cause distortion, will actually vary very little with signal in this Class-A arrangement.

In any case, the variation in net 1/gm is certainly less than 0.04 ohms over signal. This is less than 0.5% of the 8 ohm load, suggesting in very rough terms open loop distortion on the order of less than 0.5%. Now we put 40 dB of NFB around it and we are at 0.005%, and actually probably quite a bit less.

Cheers,
Bob

Yes, I know the D.Self treatise on MOSFET linearity wasn't as good as it could have been and raises many issues for debate(that's why I cited it in the cheeky manner I which I did ). However, I still think that his point that the intrinsic linearity of MOSFET's as source followers is generally worse than that of BJT's as emitter followers is valid.
0.5% THD is still quite high, especially @ 100W into an 8 ohm load with 5 parallel MOSFET pairs in class A.

Bob, your 50W MOSFET design, without the EC, I believe, produced a THD-20 of 0.02% with about 40dB NFB.
As a comparison, D. Self's EF output 'blameless' does about 0.01% THD-20 with less NFB (miller compensation), using a single pair of 2MHz fT BJT's in a 30W design.


Cheers,
Glen

[Bob Cordell]

Originally posted by estuart


Hi Bob,

I made a small mistake. 7ppm refers to a slightly different version of your amp, i.e. 2 pair of MOSFETs (they were superimposed on each other, so I overlooked it). Furthermore, the main power supply was +/- 40V in stead of 35V and Po was 85W in stead of 50W.

Here are more correct figures, although with 2SK1530/2SJ201 in stead of IRFxxx:
THD @ 20kHz and a measuring bandwidth of 200kHz
Po = 0.5W -> 1.45ppm
Po = 5W -> 4.16ppm
Po = 50W -> 6.24ppm
As you can see, my simulator is quite in his element when sniffing at your amp.

Cheers, Edmond

Nice simulation job! The changes you mentioned should not make a really big change in the behvior of the amplifier, so these simulations should be reasonably relevant.

Where did you get the SPICE models for the 2sk1530 and the 2sj201. If you can, please email me a copy of them at bob@cordellaudio.com.

Thanks,
Bob

[lineup]

Originally posted by Nelson Pass

I only recall that the Harris and Fairchild had a flat transconductance
across the audio band - I failed to note the absolute values.
------------------
I conclude that we are probably looking at a drop off from spec.

Nelson & Bob ( what a team!work )

In this proposed HEXFET amp output Class A of Bob (5 parallel MOS complementary),
which actual devices are everybody using.
To produce these test records?

Nelson says IRF9xxx ... is this the usual IRF9610 ??
... I do not see any other actual transistors

... besides Bob asking to get SPICE of 2sk1530 and the 2sj201, in his private Email


Why not publish SPICE Model
for all members of www.diyaudio.com

I usually have put my models inside a
<Quote>
spice model
spice modle
</Quote>
this is what we other do on regular basis
what is the hook here - Me Lineup, see no hook ...

Hope to get a few answers to inform me better.

Thanks & Regards to Our masters, sharing their 'in dept discussion'
lineup - stands for, I hope, Good-Will & Frank-ness

- out with it, in the open
- the only thing the will hold, in the long run
- is to be as open as you can
- with what knowledge you have earned



------------------------------------------
------------------------------------------
APPENDIX A.
Basic Lesson #1 of Marketing Symbols & Good will.

The Actual Bottomline here is
Good Public Relations - to all diy audio people of this world.
From Canada in west - to Shanghai, Tokyo & Dehli in the east.

A good NAME, Your good will shown = GOODWILL, is better than money.
A bad name ( BadWill, will not give good money, in the long run.

You may have heard about the value of Brand, Trademark & Logos
.. and what they can represent.


Name, your LABEL, It is the Flag of your Activity ...
'lineup' it stands for something, represents a whole bunch of matters,
like Union Jack & Star's & Stripes & The NaziFlag
... it can for people be something positive / negative

... But !!!
This is NOT a static Value !!!!!!!

... it changes dependent on What the people using this Trademark
are up to and are actually doing/saying.

It is the same with symbols like World trade Center & Statue Of Liberty.
They can represent different values and these values
as seen from a perspective of The total world's populations
can change, rather quickly, at times.

The Naziflag was actually first a symbol of success.
It was admired by many, not only Germans,
until things began to unfold.
----------------------

Nelson Pass Labs - this makes people associate.
Today, it represents HIGH Quality, Craftsmanship and great Positives to most all DiyAudio Community.
And I forsee no change at all in this specific case

----------------------

It is like with Money.
There can be an Inflation or Reflation of value.
If Nelson would start massproduction of cheap and low quality audio amps,
you should soon, as a few years passed by, see a change in people minds.

.... You better believe me.
I tell no lies & I do no unnecessary sweet talk here.

[Edmond Stuart]

Originally posted by Bob Cordell
Nice simulation job! The changes you mentioned should not make a really big change in the behvior of the amplifier, so these simulations should be reasonably relevant.

Where did you get the SPICE models for the 2sk1530 and the 2sj201. If you can, please email me a copy of them at bob@cordellaudio.com.

Thanks,
Bob

Thanks Bob.

The models are home brew and running only under Micro-Cap V8. They are partly based on the BSIM3 model (level 8) and partly on a couple of diodes representing Cgd and Cds.
Which simulator you are using? If it's LT-Spice for example, I'm afraid my models are of no use to you.
Please, let me know.


Cheers, Edmond.

[Bob Cordell]

Originally posted by estuart


Thanks Bob.

The models are home brew and running only under Micro-Cap V8. They are partly based on the BSIM3 model (level 8) and partly on a couple of diodes representing Cgd and Cds.
Which simulator you are using? If it's LT-Spice for example, I'm afraid my models are of no use to you.
Please, let me know.


Cheers, Edmond.

Hi Edmond,

Thanks. Yes, I use LTSPICE. Oh, well...

Cheers,
Bob

[andy_c]

Originally posted by estuart
The models are home brew and running only under Micro-Cap V8. They are partly based on the BSIM3 model (level 8) and partly on a couple of diodes representing Cgd and Cds.
Which simulator you are using? If it's LT-Spice for example, I'm afraid my models are of no use to you.

Hi Edmond,

LTSpice supports BSIM3 level 8. If your model is just a subcircuit containing a BSIM3 model combined with two diodes as you say, it should be able to be adapted to LTSpice quite easily. All that's needed are text files with the BSIM3 and diode parameters and of course, knowing which diode is which.

If you have these, I could hook them up to an LTSpice symbol file (.asy) so the symbol representing the device would be draggable onto the schematic and use the appropriate subcircuit automatically.

[Edmond Stuart]

Originally posted by andy_c
[snip]
In an email, Edmond mentioned that the Cgs values appeared to be low. I had a cursory look at them and this does appear to be the case.
[snip]

Hi Andy,

Meanwhile, I had a closer look at some (old) test circuits and the CGS cap between node 3 and 5 is NOT the real input capacitance, rather a correction term to make Ciss at Vds=30V exactly equal to the value in the data sheet. The bulk of Ciss is (apparently) derived from the geometric parameters of the BSIM3 model.
Anyhow, I think it's wise to make some LT-Spice test circuits too, and check if the capacitances are in accordance with the data sheet.

Cheers, Edmond