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Old 12th July 2008, 02:34 PM   #11
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Unclejd, I don't know if you were commenting on my post, but if you were, please note that I only said there is no use for local feedback around the buffer. That is, I still intended a global feedback loop around opamp plus buffer.
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Old 30th March 2011, 03:11 AM   #12
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Quote:
Originally Posted by unclejed613 View Post
go to Linear Technology - Home Page and look up Application Note 47 for an in depth explanation of what happens with fast op amps and slow buffers.
Old thread, but I see Jed's still active. Thanks, that's a good find. Appendix C of AN47 was just what I was looking for.
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Old 31st March 2011, 02:10 AM   #13
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glad to be of help. i think if some of us could get our heads better wrapped around the concepts and principles in that app note, we'll make better amplifiers.... at the very least, amplifiers that are easier to stabilize
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Old 2nd April 2011, 06:14 PM   #14
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I've long been aware of the difficulties the need for output power creates with PSRR a linearity at more typical listening levels but this LME49724+AD8017 composite amp I'm designing is doing an excellent job of educating me on bandwidth management. The 8017 should be good for 100MHz typ in the configuration I'm operating it in and the 49724 is 50MHz typ. Should be stable in most cases but looks marginal with a fast 49724 and a slow 8017. So the board has pads for comp caps. Walt Jung's articles have touched on the topic, as do the BUF634 and LMH6321 datasheets, and Jerald Graeme's op amp book has a chapter on it. But appendix C of AN47 is the best discussion on the topic I've come across so far. I only skimmed the rest of AN47 as it's largely material I'm already familiar with or out of scope to my present projects but it seems like a good introductory reference.

Some time ago I laid out a power amp board where an LME49990 controlled Sanken STD03N and STD03P output devices without any compensation. Probably a good thing I never sent it to fab as it looks like a stability trainwreck; 110MHz GBP op amp and BJTs where the transition frequency isn't even speced. It was never more than a 10W design anyway so it'd probably be happier implemented as an LME49710 using 2SA1859+2SC4883 with a comp cap.
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Old 3rd April 2011, 01:25 AM   #15
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the discussion of Murphy's law in AN-47 is a must read.... the STD devices are the 5-pin Darlingtons, aren't they? compensation is a must when driving darlingtons, as they are notoriously slow, especially the prepackaged ones. at least with cascaded EF outputs you can select the drivers and output devices, but with the prepackaged darlingtons, you're stuck with whatever is in there..... there's a rule of thumb with japanese transistors 2SA, 2SC devices are faster than 2SB, 2SD devices. darlingtons tend to be in the 2SB, 2SD category
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Last edited by unclejed613; 3rd April 2011 at 01:28 AM.
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Old 7th April 2011, 03:37 AM   #16
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Yes, the 5 pin Darlingtons. Not sure how much compensation an op amp circuit would need, though I do know of LME49811+STD03 designs which use 33pF. The 49811's 50MHz GPBish so that's probably representative for many of the 497xx parts as well.
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Old 7th April 2011, 04:55 AM   #17
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Arent most opamp buffers typical high gain opamps with a built in connection between out and -ve input (full negative feedback)? Draw the circuit including this buffer local feedback for a better idea of the circuit. This cant be considered equivalent to a power amp output stage?
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Old 7th April 2011, 03:18 PM   #18
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Equivalent in what sense? Power stages are somewhat abstracted in this discussion so it's unclear what the question is.
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Old 10th April 2011, 04:16 AM   #19
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Quote:
Originally Posted by unclejed613 View Post
compensation is a must when driving darlingtons as they are notoriously slow
I got curious and did a trawl through Sanken's lineup. The single emitter devices all have fTs of 20MHz and the slowest device in the LAPT pairings is typically 50MHz. The Darlingtons are mostly comparable to the LAPTs but the fastest pair of TO package power output devices Sanken makes is the 80MHz 2SB1559+2SD2389.

This implies the STD01 and STD03 Darlingtons might be pretty quick---I have a bunch of STD03Ps I picked up for cheap; be interesting to measure some of them if I get around to it. In the meantime the 2SB1559 and 2SD2389 have a min hFE of 5000 so their bases are an easy load for an op amp. Likely wouldn't require too much compensation when used with something like an LME49710.
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