Bob Pease on op amp loading

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john curl

john curl

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Joined 2003
It doesn't really happen with discrete circuits in the same way, and these sorts of measurements don't show much, BUT Scott Wurcer has assured me that his parts are virtually perfect in this respect (at least, that is the impression I have gotten). Also, look carefully at Fig. 23 and the loaded transfer function of the LM725. This is what I have repeatedly questioned Scott Wurcer about regarding his AD797. Has he fixed the xover glitch as well?
Please Scott, tell me specifically if the thermal and xover problem is completely fixed? If not, how much different from the LM725 example? I am very serious, and somewhat frustrated that I might have to measure it myself, if I can't get any response.
 
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jcx

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Joined 2003
better op amps in better topologies

Good article but it still doesn't explain why people wanting excellent performance don't just use Walt Jung's advocated buffered multiloop circuits when they want to avoid these probelms

remove the load, remove the heat, remove the thermal feedback from the input op amp altogether

the output device can be Class A for no crossover distortion

the output device can be really fast, have low output impedance to MHz (better RF ingress immunity?)

and for those who know how to design with it the output amp can add loop gain to the overall feedback loop at audio frequencies - even further reducing input op amp distortions by reducing differential signal at its input and Vswing at its (practically unloaded) output
 
syn08

syn08

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J

jcx

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Joined 2003
jcx said:
LT appnotes 18,21:
http://www.linear.com/designtools/app_notes.jsp

Walt Jung's Classic articles, lower half of page for multiloop/buffererd op amps:
http://waltjung.org/Classic_Articles.html
Click to expand...

also my thread:

http://www.diyaudio.com/forums/showthread.php?s=&postid=512806&highlight=#post512806


discrete buffers can be a lot simpler than that!

cheapest would be resistive biased ef

for low parts count I'd look at the dual njm/jrc4556 with a pull down R for Class A bias if the 8 MHz gbw worked in the particular app without extra compensation components for the input op amp - wouldn't this 40 cent op amp outrage audio snobs paired in a multiloop with a AD797 or OPA627 input op amp

Walt usually shows the AD811 cfa which was good (but more probably because it was available in DIP)
more modern cfa op amp performance simply can't be touched in hobbyist level discrete designs - not too many can close feedback loops around 4 GHz transistors - but we can buy cfa op amps with -90 dB distortion driving 25 Ohms at MHz - I'm pretty sure we wouldn't have to worry about crossover distortion at audio frequencies
 
C

Christer

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Joined 2002
John, in case you missed my point. I assumed your point was that IC op amps usually do have thermal coupling problems and wanted to point that out to forum member who haven't realized it. However, I found the use of the term "updated" somewhat amusing, since there is nothing to be updated about. If people don't know about this problem they have simply missed what has been well known for many decades, which is why I linked to that app note. I just made a joke about your your phrasing (with no malicious intention). :)

Then, as you point out, progress seems to have been made in decreasing the thermal coupling problem and perhaps some modern op amps don't suffer from it at all.
 
john curl

john curl

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Joined 2003
I work to point out such factors in electronic design. This is one that I didn't get around to discuss recently, but I read Solomon's article in 1974, when it was in the 'IEEE' transactions. I also had the test equipment made by Tektronix at the time to directly test for the effect of thermal feedback and xover distortion, and I ran lots of tests. However, my own designs did not have this problem, and I found the measurement was not useful for discrete circuits.
 
C

Christer

Member
Joined 2002
john curl said:
I work to point out such factors in electronic design. This is one that I didn't get around to discuss recently, but I read Solomon's article in 1974 or so when it was in the 'IEEE' transactions.
Click to expand...

And you should point it out, since most forum members probably aren't aware of it. As I said, I just made a benign joke about the way you phrased it.

The effect is hardly surprising since all amp stages share one small single piece of silicon.
 
unclejed613

unclejed613

Member
Joined 2006
Christer said:



Updated? Why? Have they finally eliminated the problem? :)


Edit: The topic was discussed quite extensively in this classic app note almost 35 years ago:
http://www.national.com/an/AN/AN-A.pdf
Click to expand...


a "must-read", definitely. good app note. i have one of the "orange and blue" NS books. even tells how to minimize thermal distortion in op amps. it seems not a lot of designers have read it... a good app note with information useful in power amp design as well.

yet there are still op amps and power amp chips with the same thermal problems, and they're relatively recent designs.....


too bad those of us working on discrete amps dont have the availability of multi-collector and multi-emitter transistors.......
 
jan.didden

jan.didden

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linearaudio.net
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Then again, with opamps like the LM4562 at 0.025ppm non-linearity available, can't we say we licked it for practical purposes?

Discrete circuits also have thermal non-linearities; even a few degrees temp difference inside an amp case (and often its more than that) makes the bias drift here and yonder. Admittedly, it's a lower (much) time constant, but again, are we supposed to go to discrete to overcome 0.025ppm? And can we at all?

Jan Didden
 
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