Those interested should also see Putzeys' article in Linear Audio for his take on having more gain at lower frequencies, and its effect on settling time.
It would be interesting to add overload behavior to the repertoire of tests in sim and in reality, including behavior under excessively high slew rate signals.
Virtually all conventional feedback amps (single-ended ones) will overload asymmetrically, the cure for which is making them faster, or bandlimiting ahead. Of course really poor slewing performance may require a lot of bandlimiting! But such performance is easy to avoid. According to Zanfino, Miller determined that Otala selected particularly poor designs to support his arguments about TIM. This does not invalidate the thesis, but it exaggerates a bit.
Once you make the bandwidth flat out to the highest frequencies of interest, does this guarantee equal distortion performance at all in-band signals? No, as there are other mechanisms besides those that the loop gain alleviates. One of the things I like about what linuxguru posts is the inclusion of the 1k resistor in series with the source, which is a reasonable start for what may be feeding the amplifiers. Those who omit any consideration of the source impedance are inviting sweet Spice lies.
You know, it depends. This example has no GNFB, and THD below 0.0005% in audioband. I suppose no one would also predict TIM problems
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Putzeys, in a later discussion in the John Curl thread here in DIYAudio, also made the point that the transfer function of the fedback signal differs above and below the open loop pole. Above has additional phase shift, of course, but this is a different transfer function.
Does this matter? And is it even possible to generalize? Dunno.
Thanks,
Chris
Yes, sorry PFET's were the only thing compatible so it's one flavor of each device.
Wilson opamp
Hope these don't get Wiley's panties in a bunch, as they say. If so I'll take them down.
The spine is a little the worse for wear, but I've gotten a lot of use out of this book.
Hope these don't get Wiley's panties in a bunch, as they say. If so I'll take them down.
The spine is a little the worse for wear, but I've gotten a lot of use out of this book.
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Most of the circuits I've seen have ample junction capacitance to act as such.
Dick you would like the Wilson current mirror Wiki page. Barrie Gilbert and George had an overnight bet on who could make the best current mirror out of only three transistors, and George won.
I find it strange since given the first two there are not many ways to connect a third.
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Such an excellent article! As you remarked, the core material has appeared elsewhere, but Bruno's exposition is masterful. And note that he also listens (see for example the remarks about high-frequency loop gain deficiencies in the third paragraph on page 13).
Good article (quite basic findings, though) , but some of the statements are at least controversial, like: "Open-loop bandwidth is no measure of how fast an amplifier is. Gain-bandwidth product is"
GBW speaks only about small signal "speed".
Instead of Wilson Type, the third could be used as "helper" to overcome the base current difference?
Here is a good book by the late Hans Camenzind, check Page 3-4 to 3-6 for these mirrors.
He died past August 8th, RIP Hans.
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I believe you should not pull this out of context. He reasons, and shows graphs, comparing rising waveforms from the two extreme positions and then concludes what he said.
I don't think you can argue with that reasoning and thus the conclusion from that reasoning.
BTW Bruno will come to Burning Amp this year so you can tell your objections to the man himself ;-)
jan
Many thanks to Jan for making this article freely readable
Doing some tests on what Bruno writes about in his article was more or less my idea when I started playing with discrete CFAs.
L.
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Nice to see that article again ...
Frank
The simple phenomenon that's occurring here, is that better performance in one part of the overall system, audio, that is, then emphasises weaknesses elsewhere, shines the spotlight more strongly on the remaining deficiencies. So, there are two obvious choices here: clean up the remaining deficiences, or lower the strength of the spotlight and mask, soften the edges of that "unpleasant stuff". Reducing feedback is an easy version of the latter appoach; it seems only a few appear to take the other tack, and with gritted teeth push through to the logical conclusion - eliminate all the subjectively audible distortions. But they're the ones who reap the ultimate rewards ...
Frank
I don't think so, fas42. Reducing feedback also reduces potential distortions that are MORE exotic than we can easily measure.
I am pleasantly surprised that Richard Marsh brought out the high open loop bandwidth. He apparently is not aware of the prejudice against this concept shown on this website in the past. However, I agree with him, that it is useful and important to have high open loop bandwidth.
I am pleasantly surprised that Richard Marsh brought out the high open loop bandwidth. He apparently is not aware of the prejudice against this concept shown on this website in the past. However, I agree with him, that it is useful and important to have high open loop bandwidth.
May be this is one of the potential distortion mechanisms you are addressing:
(P.364 from the excellent book of Bob Cordell: Designing Audio Power Amplifiers )
This may look exotic to me that I am an - non EE - amateur, but for a cautious designer, it should be a more homeland one.
Short time clipping certainly has the potential to occur in listening sessions quite frequently and get unnoticed.
The acoustic signature of clipping and subsequent sticking will be attributed to increased feedback from a person that adjusts the amount of feedback on an amplifier he is building/optimising and then evaluating by listening to music.
It is difficult to spot short term clipping while measuring, as the source material is rarely a music piece. The crest factor on some music is high and we may underestimate the overhead actually required.
Therefore, it is even more difficult to locate the stage that distorts first, then think of the underlying mechanism and decide on the remedy.
Regards
George
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