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Answers for what I know and technical comment on PIM

1. I make the assumption that they measured a fully loaded amplifier. What load did they use?

Well, I had the paper, its reference was in my database, but no way to find it out... So, we'll have to guess (which is funnier than just reading somebody's else paper, with his supposed agenda of making him a name in his community ;-)
The key for further reasoning is how can we get P.I.M?
For that, we can expect the phase vs frequency (and therefore also the amplitude vs frequency) to be in some way "modulated" by part of the audio signal.
Which first means that this modulation phenomenon will only be significant for the highest audio frequency (since the low frequency response is flat or so).
Which incidently answers to your question 6: the term "carrier" was misleading, I meant that you're looking for phase modulation @ 6KHz, not at 60Hz...


So, now, how could you build quite non-existent P.I.M in a power amp ?

Let us first recall how an amplifier is built.
Below, the block diagram

The differential input stage has mutual conductance gm. Assuming the second stage has sufficient open-loop gain over the audio frequency range, the amplifier open loop gain is given by gm/(j ω C).
The closed loop gain is defined by the feedback resistors Rfb1 and Rfb2. We get the following closed-loop gain vs freq and phase angle vs frequency:


The relationship between the actual value of fc and the PIM is well obvious on this figure.
Since the -3dB closed-loop frequency fc is proportional to gm/C, we now know the two sources of PIM: variations in gm and/or variation in C

  1. Variation in gm vs input voltage is related to the first stage, as seen on the block diagram above. Since this variation is also a source of intermodulation distorsion, we can expect that good IIM figure implies good gm behaviour. I won't comment further on this topic of gm variation, since tons of paper have been written on the subject.
  2. Variation in C vs input voltage comes from the collector-base capacitance of the 2nd stage which is highly non-linear with voltage. To get rid of it, the best solution is to connect in serial with this 2nd stage transistor a common base transistor. This way, the 2nd stage transitor "sees" a constant voltage across it, and so, its capacitance won't move. This is a cascode.


Note that the source of PIM is related to gm and C, and not to the current buffer stage, as long as it is really a current buffer stage. So, we can expect that, since imperfections in the current buffer can be flush out by different tests, like IIM and TIM (Interface Intermodulation Distorsion, see a discussion thread about this topic here, and Transient Intermodulation Distorsion), so, PIM afterwards won't be very useful for optimizing the output current buffer.


So, now, we have better knowledge of P.I.M, and I can try to answer your questions.


2. Was their load capable of pulling current at (inverted) nanosecond speeds, or was the amp coasting at the hf?
Answer is just above: no care... (some people are yet preparing their flamethrower...)

3. If a reactive load is substituted, are the measurements the same?
you bet the answer: no care...

4. If the test signal amplitude is changed, do the results change?
YES: obvious after the explanations above. Which is why a strong and slowing varying 60Hz is added to the 6,000Hz

5. If the amp is pushed to output limits that mess damping factor up, does that alter the results?
I shouldn't bet my shirt on this one. Again, current buffer odd behaviour is better flushed out by different tests

6. What about measurement of the 6K signal, not the carrier. we're not very sensitive to ITD at 60 Hz..essentially, will the "carrier" time modulate the hf?
Answered above in the first paragraphs

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