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Old 19th March 2013, 10:58 AM   #251
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Old 19th March 2013, 02:46 PM   #252
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Originally Posted by RNMarsh View Post
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
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Old 19th March 2013, 05:43 PM   #253
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For a start, "current feedback" amplifiers are nothing of the sort: they are, in fact, merely voltage feedback am pliers (shunt derived series applied negative feedback) with the input stage designed so that the value of the feedback network's resistor connected to ground affects the forward path gain of the amplifier.

This is possible because in a "current feedback" amplifier the feedback network is directly connected to the emitter of input stage and constitutes a significant load on the first stage, while with an ordinary voltage feedback amplifier the feedback network is buffered from the input transistor's emitter by what is effectively an emitter follower.
So it's the same but different? What's the point the acronym or name we call something? There is fundamentally different behavior for two amplifiers that are distinguished by what has been an industry accepted nomenclature. Remember there is more than audio out there and we a talking by and large about three terminal general purpose devices as well. Do you honestly think a successful product could be made out of a multi-pin VFA giving the user access to input degeneration and two or three compensation nodes so that it could be customized for constant BW at each closed-loop gain?

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Further, as demonstrated by Edward Cherry in a paper I referred to a long time ago on this forum, the notion that the bandwidth remains "constant" for different closed loop gains with "current feedback" amplifiers is illusory as the unity gain bandwidth remains virtually unchanged. The reason for this illusion is the alteration of the amplifier's forward path gain as the feedback component values are changed.
A fundamentally useful property an illusion? Better tell all the users that their performance is an illusion. What unity gain bandwidth is unchanged? The open-loop transimpedance? I thought breaking the loop included the feedback network as a load on the input. Of course buffered it hardly matters, but here the input transconductace is modified so with a constant shunt Ccomp the unity voltage gain frequency scales with closed-loop gain. Something has to give. For voltage feedback there is in general a constant GBWP, for constant bandwidth over a range of closed-loop gains this cannot hold.

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Although "current feedback" amplfiers can have very high slew rates, these are not really required in audio applications; the price paid in other parameters for this attribute is simply not worth it.
From a very narrow viewpoint (audio) maybe and even then some here will disagree. Please keep it civil.
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Old 19th March 2013, 06:03 PM   #254
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Originally Posted by Edmond Stuart View Post
Hi Douglas,
I sent you the emails around February 2002. Perhaps this helps to find them again.
Not so far, I'm afraid. I have done a bit of searching but nothing has so far emerged.


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Maybe you confuses it with another letter (see picture below, a shortened version). That was about input inclusive compensation. But also in this case we didn't have any discussion about this subject, though you know about it, as you have listed it on your website.
Anyhow, it was you who told me that TMC was already invented by Baxandall. By then, you were the only one who know this. So no one else could have told me about Baxandall. This leaves no other possibility than our emails were about TMC.
Possibly; I have to say I don't recall. Professional matters are rather pressing at the moment, but when I can, I will have another trawl through the archives and see what can be found.

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Much depends on minor circuit details and how it was built. Too long traces/wires to the output devices for example, will be devastating.
How long is too long? The Blameless amplifiers are certainly quite happy with 6 inches or so, (probably much more) and I would expect any workable design to be not very different. I don't recall all the details of construction after this length of time but I am sure I would remember something eccentric like extra-long leads to the output devices.

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Also notice that Cherry applied output inclusive compensation in conjunction with shunt compensation by putting (small) capacitors between the base of the drivers and the supply rails. I guess without the latter has amp will also be unstable.
I assume you are referring to the two 33pF capacitors in the ETI design? That is not shunt compensation- they would have to be very much larger for that. They look very much as if they were added to suppress parasitic oscillation in the output stage; a connection to ground would be better to avoid the possibility of injecting rail noise. This technique is dealt with on p222 of the Fifth edition of Audio Power Amplifier Design.
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Old 19th March 2013, 06:31 PM   #255
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Originally Posted by Dave Zan View Post
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.
Thank you for this David.

TMC appears to most clearly and usefully described (by Great Guru Baxandall) at The Baxandall Papers

I distinguish between the bastardized TMC and pure Cherry compensation. Prof. Edward Cherry analysed his compensation scheme in a number of JAES articles from about the early 80's. All are worth reading even if you don't agree with his final solutions.

I apologise for chiming in before finishing my study of ALL this thread and its links ... I'm only at about page 200 on the thread off Cordell's book. I hope to finish it before the next millenium.

Last edited by kgrlee; 19th March 2013 at 06:38 PM.
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Old 19th March 2013, 06:45 PM   #256
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Originally Posted by forr View Post
My own suggestions (refering to the 5th edition),
I would like to read some more words on :

Page 119 Figure 5.4f
Bootstapping VAS load R using an emitter follower
I rarely met this configuration but I found its simplicity rather smart.
It seems a very simple enhancement to buffer the VAS and may provide the same advantage as a triple output stage without the drawback for the bias of having six Vbe to compensate.
There is some more on this useful technique, and more on bootstrapping in general, which (surprise) is not as straightforward as it looks.

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Page 356, Figure 12.12 (circuit of the class G amplifier)
A resistor of 2.2 kOhm (R11) is insterted in the Constant Current Source (Q6) for the input differential pair.
Some amplifiers have a resistor (1 kOhm or so) in this place, most of them have not. I never saw many explanations on its role.
Here, wiht high voltage power supply rails, one aim is certainly to lower the heat dissipation in Q6 wich is about 6 mA * 50 V = 0.3 W without it.
Has it any other effect ? I think of a compensation of the not purely resistive behaviour of the CCS at high frequencies.
Its presence may also help to control the CCS output current when debugging or repairing.
That resistor was pure dim-wittedness on my part. The original idea was to protect the circuitry if the CCS failed short-circuit; actually very unlikely. The disadvantage is that the voltage drop across the resistor means that when you are variac-ing up the supply from zero the amplifier will not work until the rails reach some +/-9V. By that time damage may have been done if there is a fault. Without the resistor, you can get a visibly good sinewave with only say +/-3V and the operation is much safer. This is dealt with on p511 of the 5th edition. I hope I have by now removed all those resistors.

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Amplitude and frequency distributions
Amplitude and frequency distributions wanted
For me, it's worth of more investigations.
There will be a good deal more on the characteristics of the audio signal.
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Old 19th March 2013, 06:53 PM   #257
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I have just came across Current conveyors maybe not much to do with power amplifiers but to have this topology dissected as only Douglas Self can will be a real bonus.
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Old 19th March 2013, 07:05 PM   #258
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Originally Posted by Bob Cordell View Post
Doug,

I really hope you will stick with TMC in your upcoming edition. No three letter acronym is a perfect, unambiguous description, including TPC. Edmond deserves credit for popularizing it and giving it a pretty descriptive name, and I followed suit by using the term Transitional Miller Compensation in my text (which I believe is the first text to describe it, and where due credit is given to Edmond). To have two competing names for TMC would just add to the confusion. I urge you to stick with TMC.

Cheers,
Bob
Hi Bob

I'm sorry, but I cannot get enthusiastic about such a vague acronym. Three letters just aren't enough to convey the information.

I could settle for TOIMC, (output-inclusive) so we can also use TIIMC. (input-inclusive)
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Old 19th March 2013, 07:32 PM   #259
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Back to suggestions for the next edition ..

I'd like to make a plea that the various schematics be checked for missing details that might lead to self destruction on overload. My particular beef is the emitter follower driving the VAS.

(all references to the 4th edition

eg fig 6.16 : TR12's collector current is unlimited on overload so will kill TR4 on HF overload.

A simple 'cure' is a resistor in TR12 collector.

Some of the circuits shown are OK. eg fig 5.24 where Q9 protects both Q10 & Q11.

I feel this point needs to be stressed ad nauseum as the emitter follower is an essential part of "Blameless".
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Old 19th March 2013, 07:36 PM   #260
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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.
Edmond, I wholly agree with you. That chapter does indeed need a complete makeover. However, the structure of the next edition is gelling as we speak, and I am afraid it's not going to happen this time round.

I am painfully aware that both the FET chapter and the chapter on Class-D have nothing remotely like the depth of the rest of the topics. But if they did, consider the impact on the size of the book- it would be something like twice the length. As it is, I have pushed the limits on size as far as I could, having persuaded my publishers to increase it several times. Sometimes compromise is inevitable.

Both these chapters could easily be separate books. Anyone fancy tackling Class-D?

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PS: You really don't like MOSFEts, do you?
No. Variable Vgs, transconductance...not helpful. You know where you are with a BJT.
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