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Old 3rd July 2008, 01:25 PM   #1
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Default Bob Pease on op amp loading

Application Note posted on Nat Semi's website in May --

http://www.national.com/an/AN/AN-1485.pdf
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Old 3rd July 2008, 07:01 PM   #2
Gordy is offline Gordy  United Kingdom
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Many thanks for the pointer.

G.
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Old 3rd July 2008, 07:27 PM   #3
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Default Re: Bob Pease on op amp loading

Quote:
Originally posted by jackinnj
Application Note posted on Nat Semi's website in May --

http://www.national.com/an/AN/AN-1485.pdf

Excellent! Thanks,

Jan Didden
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Old 4th July 2008, 01:35 AM   #4
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interesting...... i'm doing some power amp output impedance experiments. looks like i've got some reading material on the subject. output impedance is one of the least written about subjects, both for opamps and power amps.
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Old 4th July 2008, 06:23 PM   #5
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About time you folks got updated about thermal feedback.
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Old 4th July 2008, 07:45 PM   #6
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Quote:
Originally posted by john curl
About time you folks got updated about thermal feedback.
Yes. I remember many years ago trying to explain to some otherwise bright people how an early JFET op-amp (LF256?) could appear to have negative gain in a standard test circuit.
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Old 4th July 2008, 08:04 PM   #7
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That was worth reading.

w
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Old 4th July 2008, 08:43 PM   #8
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Quote:
Originally posted by john curl
About time you folks got updated about thermal feedback.

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
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Old 4th July 2008, 08:58 PM   #9
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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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Old 4th July 2008, 10:02 PM   #10
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Default 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
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