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Old 18th March 2013, 06:54 PM   #241
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by Edmond Stuart View Post
Well, the truth is that at low currents, that is, in the transition region (sorry for using the T-word again ) MOSFETs show a similar exponential behavior. So the X-over between the two output devices isn't that abrupt. Admittedly, WRT to BJTs, the gain 'wobble' is higher, but equally important, it is also wider. That means that the spectrum of harmonics from this wobble is smaller (i.e. less HF). Consequently, it's easier to linearize it by means of feedback.

PS: You really don't like MOSFEts, do you?
That is correct, and I would agree that Self's discussion on mosfets is lacking, however I am sure you also know there's much more in the mosfet vs. bipolar debate than the spectral distribution of distortions.

PS: You really don't like power bipolars, do you?
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Old 18th March 2013, 07:48 PM   #242
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Originally Posted by Edmond Stuart View Post
Hi Douglas,

Forgive me if I say it a bit bluntly, but you for ask additions and/or improvement on the 5th edition: Chapter 14 on MOSFET output stages should be rewritten, completely.
The simulations of the (vertical) MOSFETs doesn't take into account the so called weak-inversion (or sub-threshold conduction). This has far reaching consequences, not just regarding the the graphs, but also with respect to your conclusions about MOSFETs. The point is that they perform much better then you and your simulator might think.

In this chapter, you stated:
"However, the most important difference may be that the bipolar gain variations are gentle wobbles, while all FET plots seem to have abrupt changes that are much harder to linearize with NFB that must decline with rising frequency. The basically exponential Ic/Vbe characteristics of two BJTs approach much more closely the ideal of conjugate (i.e. always adding up to 1) mathematical functions, and this is the root cause of the much lower crossover distortion.

A close-up examination of the way in which the two types of device begin conducting as their input voltages increase shows that FETs move abruptly into the square-law part of their characteristic, while the exponential behavior of bipolars actually gives a much slower and smoother start to conduction."


Well, the truth is that at low currents, that is, in the transition region (sorry for using the T-word again ) MOSFETs show a similar exponential behavior. So the X-over between the two output devices isn't that abrupt. Admittedly, WRT to BJTs, the gain 'wobble' is higher, but equally important, it is also wider. That means that the spectrum of harmonics from this wobble is smaller (i.e. less HF). Consequently, it's easier to linearize it by means of feedback.

Actually, I'm surprised that you didn't revise this chapter already in the 5th edition, as Marcel van de Gevel pointed this already out in 1996, see below.

Cheers,
E.

PS: You really don't like MOSFEts, do you?
Hi Edmond,

You are spot-on in your observations here, and they agree with what I pointed out in my book in the MOSFET chapter. Indeed, this is why I worked so hard to come up with EKV models for some of the power MOSFETs that I posted on my web site. The square-law model for MOSFETs is prehistorically crude. In my real-world designs, the MOSFETs also show a smooth, well-behaved, non-abrupt transition as they turn on.

In some ways, the MOSFETs are the opposite of BJTs. In my 1983 paper, I coined the term "transconductance droop" to describe the class-AB crossover shortcoming of MOSFETs. Later, I believe Doug coined the term "gm-doubling" to describe the crossover problem one runs into when over-biasing a BJT output stage. The silver lining for the MOSFET is that it loves to be biased at a higher current to achieve higher gm. I usually run them at 150-200mA per device. You virtually cannot get a MOSFET stage into gm doubling. This means that you can bias them as hot as you are willing to do, at the same time increasing the class A region of the class AB output stage. Of course, if you are looking for an amplifier that runs cool at idle with small heat sinks, you are better off with a BJT output stage.

The beauty of the MOSFET with healthy bias is that the bias can move around quite a bit and you will not get into dangerous waters with crossover distortion. You cannot say that about BJTs. I believe that many people grossly underestimate the sonic deficiencies brought on by BJT output stages due to dynamic bias shifts with real program material. This problem is not generally revealed by sinewave THD testing, which is rather static thermally. ThermalTrak output transistors have mitigated this problem in BJT output stages. Once you have mastered ThermalTrak biasing techniques (ignore the OnSemi app note, please), you will use ThermalTraks and never look back.

This is not to say that MOSFETs are universally better than BJTs. Indeed, the ThermalTrak BJTs and high ft have made BJTs much better than they used to be. Although I lean toward using vertical MOSFETs, much of what I have said here applies to both vertical and lateral MOSFETs.

Cheers,
Bob
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Old 18th March 2013, 11:50 PM   #243
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Default Response to DS and question for both authors.

I am curious about a topic that involves both DS's query and Bob's enthusiasm for output triples and MOSFETs.

In the chapter on OutputS there is a CFP + EF triple shown (p 156 pic. 6.17b of 5th Ed.) and the comment is made that it looks to have "promise".
It looks so to me too, that it combines the best points of a CFP that DS advocates with the triple that Bob likes.
So a follow up on this promise would be of interest.
The CFP stability is of course a concern so more material here would help too.
It occurs that a MOSFET as the second transistor could provide some extra bandwidth that would allow more flexibility to compensate the CFP internal loop. Would the BJT buffer the problematic MOSFET capacitance variation?
In view of Bob's comments above and DS's observation that "it is... wise to consider whether BJTs or FETs are the best devices for the job" do either of you have any comments?

Best wishes
David

Last edited by Dave Zan; 18th March 2013 at 11:55 PM.
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Old 19th March 2013, 05:53 AM   #244
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Originally Posted by DouglasSelf View Post
I think the more accepted name for TMC is Output-Inclusive Compensation. May not roll off the tongue, but it does at least describe what it does.
I propose this be called Cherry compensation.

He may not have been the first to do this but he was certainly the first to analyse it in detail and show its advantages.

I shall resist chiming in on the pros & cons of TPC vs Cherry etc except to suggest the various pundits look at the open loop operating conditions 'generated' by the various schemes. Some of these go against Self & other Gospels but surprisingly result in less open loop distortion too.
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BTW, where is the thread where Edmond Stuart pontificates on these matters?

Last edited by kgrlee; 19th March 2013 at 06:16 AM.
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Old 19th March 2013, 06:25 AM   #245
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Originally Posted by kgrlee View Post
I propose this be called Cherry compensation.

He may not have been the first to do this but he was certainly the first to analyse it in detail and show its advantages.

I shall resist chiming in on the pros & cons of TPC vs Cherry etc except to suggest the various pundits look at the open loop operating conditions 'generated' by the various schemes. Some of these go against Self & other Gospels but surprisingly (??) result in less open loop distortion too.

For this reason, I spurn the bastardized TMC as proposed by Self, Edmonds, Uncle Tom Cobley & even Great Guru Baxandall as it doesn't have ALL the advantages of pure Cherry.
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Old 19th March 2013, 06:38 AM   #246
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Originally Posted by kgrlee View Post
I propose this be called Cherry compensation.

He may not have been the first to do this but he was certainly the first to analyse it in detail...

BTW, where is the thread where Edmond Stuart pontificates on these matters?
This shows a complete lack of comprehension and/or a stupid attempt to troll. In either case, best to remove the post within the edit time window then search for "TMC" and read before you post.

Best wishes
David

Last edited by Dave Zan; 19th March 2013 at 06:44 AM.
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Old 19th March 2013, 06:44 AM   #247
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Quote:
Originally Posted by kgrlee View Post
I propose this be called Cherry compensation.

He may not have been the first to do this but he was certainly the first to analyse it in detail and show its advantages.

I shall resist chiming in on the pros & cons of TPC vs Cherry etc except to suggest the various pundits look at the open loop operating conditions 'generated' by the various schemes. Some of these go against Self & other Gospels but surprisingly result in less open loop distortion too.

As to Mr. Self's assertion that pure Cherry compensation always results in oscillation, I would suggest the truth is that a

"Blameless" amplifier is difficult to get stable with pure Cherry. Other, often simpler topologies can take full advantage of pure Cherry with less aggro.

But to get back to the new book ....

I don't think there is any point in asking Mr. Self to put stuff in that he doesn't believe in. The worth of his pontificating is the care he has put into stuff that he believes.

Though I don't agree with everything he proposes, ALL of it is worth reading for this reason. This is true even for Great Gurus like Baxandall.

For the best expose of CFA, MOSFET O/Ps etc. find someone who believes in such mechanisms and beg them to write a book. It would be perfectly valid to beg this worthy fellow to 'prove' that CFAs etc are far better than the evil VFAs etc that Doug puts his faith in. But don't ask Mr. Self to do this.
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Old 19th March 2013, 08:00 AM   #248
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Well said, Dave.
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Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
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Old 19th March 2013, 08:30 AM   #249
RNMarsh is offline RNMarsh  United States
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It's ok to want to sell books primarily about one topology and using primarily one type of semicinductor.

Plenty has been written about other topologies and it is true that some excellent refinements have been shown in DIYAUDIO let alone by the IC industry. It doesnt matter that it is current-mode or voltage in disguise... the topology is interesting and has its set of pro-con. I called it complimentary push-pull before it got called current-mode feedback. And, with simple circuits obtained below .001% THD+N many decades ago. To get below .001 can be done with several topologies. Push pull is one of the family of differential amplifiers composed of differentail, push-pull, paraphase. Explore them all and use the one which best matches your performance priorities.

Thx-RNMarsh

Last edited by RNMarsh; 19th March 2013 at 08:36 AM.
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Old 19th March 2013, 08:54 AM   #250
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by Edmond Stuart View Post
Hi Douglas,

The idea was born somewhere in the mid-nineties, when I was thinking how to make Cherry's output inclusive compensation more stable.

I sent you the emails around February 2002. Perhaps this helps to find them again.
.
And after ten years, you still hold a grudge against mr. Self for ignoring you in this matter? WOW!
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