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Transitional Miller Compensation (TMC) vs. Two Pole Compensation (TPC)
Transitional Miller Compensation (TMC) vs. Two Pole Compensation (TPC)
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Old 10th September 2019, 08:47 PM   #11
MarsBravo is offline MarsBravo  Netherlands
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The Baxandall papers show clearly the relation of parasitic capacitances, high (output-) impedances and high amplification of the given and mostly adopted amplifier topology, that of the input stage, voltage ampifier stage and current amplifier stage.

The aim to become less dependent of the somewhat conflicting properties of the voltage gain stage to eliminate the frequency-related problems caused by the miller capacitance and high output impedance ('resistance') to compensate these unwanted frequency responses or oscillations, by making a connection to a next low (lower) output impedence stage, one arrives at an even more complex circuit which has to be calculated with accuracy and possible adjustments and tuning afterwards before a reliable amplifier has been realised.

Studying the circuits in the various references, I noticed high complexity, countless components and hypnotising plots and graphs.

Not exactly preferred I guess by commercial enterprises (save the niche market) or down to earth experimenting diy-enthusiasts by the simple argument of the wide tolerances of most components other than precision 1% resistors. Capacitances +/- 20% are an usual yield when buying them, bjt's ranges of beta with a factor of two, jfet's spreading is notorious.

The results however reached by this subtle architecture are impressive anyway. But it turns my thoughts towards a different direction, to an approach which differs considerably from this classic (conservative) topology and is in its fundamental 'design-nature' less or not susceptible to millercaps and it burdens.

One might think of triode tubes, which do amplify (significant less) but with an output impedance in the kilo-ohms instead of mega-ohms. No millercap-compensating circuitry known, overall feedback modest and sounding awesome though less 'precise' as laboratory equipment is.

What would happen with these TMC and TPC solutions if we were able to transform the usual voltage amplifier stage with a high-beta bjt to a modified stage with less amplification but also with a substantial lower output impedance. Would all problems being solved then and would these TMC/TPC additions disappear?

I am curious if someone would dare to simulate such a variation.
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Old 10th September 2019, 09:07 PM   #12
PB2 is online now PB2  United States
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The authors are quite frank about the issues with an average home builder building the PGP, yes it is overly complex but it is just an example of what can be done. I believe that they learned a lot and have come a long way so that a simpler topology might come close enough to the PGP to make the complexity unnecessary. In fact they call their older designs outdated, still we can learn from their work.

Bob Cordell simulated a slightly modified "Blameless" design with TMC and obtained excellent, good enough IMO, results. This is what is of more interest to me. Bob's old MOSFET design with a bipolar output stage triple and TMC is also of interest to me - not too complicated.

Should have included a link to Bob's Blameless simulation comparing TMC and TPC: Bob Cordell's Power amplifier book

Link to Bob's revision of his old MOSFET amp to employ TMC rather than error correction in the output stage and obtain similar results with less complexity: Bob Cordell's Power amplifier book

I thought that I included these links in the first post of the thread.
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Old 10th September 2019, 09:41 PM   #13
MarsBravo is offline MarsBravo  Netherlands
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As I am biased myself: I prefer simple designs such as by Hiraga. Millercaps are present there too, but only affecting local, openloop and closed loop performance when it is poorly build (eg 'long' twisted wires to the output stage, increasing capacative load on the voltage driver causing oscillations; yes, some people do this to prevent external noise interfering).

But in overall, low gain stage impedance (load resistance of 1-2 kohms), no bias circuit at all, no protection needed (no pops, no dc lability). But of low power output as well. Sufficient for home use, others prefer beefier menus.

After my current design exercise, a challange awaits with the classical, more powerfull design. Then this and referring topics are very usefull indeed! I might end up with some strewn capacitances in obscure but yet unknown design corners.

Hats off for Baxandall.
And thanks PB2.
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Old 18th September 2019, 02:56 AM   #14
bimo is offline bimo  Indonesia
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Usually, simple design can have very good result if using almost perfect components. But I know, there are none.
No transistor have zero capacitance, very high Early Voltage, very high hFE, no beta drop, no temperature dependent , etc.

Every design compromised.
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Old 18th September 2019, 06:46 AM   #15
MarsBravo is offline MarsBravo  Netherlands
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Quote:
Originally Posted by bimo View Post
Every design compromised.

Totally agreed.
All variables are set in compromise values.
However, balancing and refining these might end in different topologies.
Lots of fun though!
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Old 19th September 2019, 03:47 AM   #16
bimo is offline bimo  Indonesia
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This is another two pole compensation which is TPC + cherry or OITPC by Dadod:

Anistardi Peletuk

Distortion is H2 dominant at all audio frequency.
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