I am wondering if anyone hear has used the Compensation Pin of the AD-744 Opamp as an output for driving another active Stage sutch as a Buffer? What Sonic Results have you obtained with this method?
Interesting question that I am also very interested.
In this mode the internal push-pull output buffer is without function - only the single ended buffer together with the associated current source comming from the positive rail is in the signal path, as it is Mr. Nelson Pass it always favors.
BTW - this topic was incorrectly filed - the right folder is "Chip Amp" - so I think.
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You can do that same trick with pin5 @ NE5534. Sounds good; dosent drive as hard as the full output.
I had a chance to talk with the guy who designed the AD744 at Analog Devices, Scott Wurcer. You can see his initials on the die picture which appears on page 3 of the AD744 datasheet. Scott is also a member of diyAudio (!).
Apparently quite a few AD744 customers noticed that all three terminals of the output PNP device, come out to external I/O pins. Namely, pins 4,5,6. So these folks connected a really excellent discrete PNP on the board, to pins 4,5,6. And they got a much MUCH better opamp with a much better output stage and much better high frequency behavior. (At the time, PNPs on integrated circuits were pretty terrible, with low beta and very low fT). Here's a schematic:
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Apparently quite a few AD744 customers noticed that all three terminals of the output PNP device, come out to external I/O pins. Namely, pins 4,5,6. So these folks connected a really excellent discrete PNP on the board, to pins 4,5,6. And they got a much MUCH better opamp with a much better output stage and much better high frequency behavior. (At the time, PNPs on integrated circuits were pretty terrible, with low beta and very low fT). Here's a schematic:
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Minor caveat - PN4258 is only rated to 12V so the AD744 can't be run too far beyond its minimum rail-to-rail supply (9V).
Just as today's DIYers are quite willing to run Toshiba 2SJ74 JFETs far above their absolute maximum rating of Vds (25V), so too were people willing to run PN4258's far above their maximum rated Vce, back in the day. I imagine some of them put every device through a curve tracer test before stuffing and soldering.
BTW PN4258 is unobtanium today. It's a high speed PNP tailored for fast digital circuits made with discretes.... a market which no longer exists.
BTW PN4258 is unobtanium today. It's a high speed PNP tailored for fast digital circuits made with discretes.... a market which no longer exists.
Nearest equivalent in Digikey's inventory looks to be MMBT3640 - a tad slower and SMT package, but otherwise rather similar.
MMBT3640 ON Semiconductor | Discrete Semiconductor Products | DigiKey
MMBT3640 ON Semiconductor | Discrete Semiconductor Products | DigiKey
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