Bob Pease on op amp loading

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#21
janneman said:
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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Yes we can, but so can we choose to go for the ancient greek language and culture instead of e.g. french. It might be interesting and entertaining, but to expensive and ultimately useless for any practical purposes. Discretes are soon going to be only a die hard DIYer and simulation afficionados thing.
 
#22
pioneer, denon, yamaha, harmon kardon, and others still use discretes. granted, some of them look like an amp design from the early 70's (resistors instead of current sources, no current mirrors, darlington output transistors) but discretes are still in use on a massive industrial scale. class D makes for nice "drop-in" amps, and many manufacturers are using large chip amps (STK430-xxx), but there are still plenty of discrete amps being rolled out every year.
 
#23
Re: better op amps in better topologies

jcx said:
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
Click to expand...

You might check out Bateman's capacitor distortion articles.
he plays with a few combinations and looks a thd around -120db
 
#25
john curl said:
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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Pease writes an article with references to the uA709 you ask about the LM725. You folks are going to have to let go of some of this 40yr old stuff and move forward. Paul Brokaw and Mitch Madique presented cross quaded input devices and outputs on a thermal center line with NO thermal feedback (fractional ppm) more than thirty years ago, it was then end of story.
 
#27
Sorry for my lack of precision.


A high precision monolithic super-beta operational amplifier
Maidique, M.A.
Solid-State Circuits, IEEE Journal of
Volume 7, Issue 6, Dec 1972 Page(s): 480 - 487

Summary: A single-chip (67/spl times/90 mil) integrated-circuit operational amplifier using thin-film resistors and super-gain transistors has been designed to achieve dc follower accuracies of 0.001 percent with 100-k/spl Omega/ source resistance. The circuit achieves gains of 140 dB using thermally balanced layout designs for both input and output stages, nulled drifts of 0.3 /spl mu/V//spl deg/C, and offset currents well under 1 nA. All other dc specifications including power-supply variation error (PSRR), common-mode gain error (CMRR), etc., are in the 1-10 ppm error range; and a procedure is given by which long-term drifts of less than 10 /spl mu/V/month can be assured. AC performance is comparable to general-purpose integrated-circuit operational amplifiers, i.e., f/SUB t/=300 kHz and slew rate of 1.2 V//spl mu/s at gain of ten. The circuit is externally compensated for unity gain with a single 390-pF capacitor and is fully input and output protected

EDIT - Excuse the special character snafus, but the abstract gets across the point Solomon pointed out a problem, it was easily solved, end of story.
 
#34
Re: Re: better op amps in better topologies

myhrrhleine said:


You might check out Bateman's capacitor distortion articles.
he plays with a few combinations and looks a thd around -120db
Click to expand...


zapnspark said:


Here is the link to bateman's capacitor articles:

http://uk.geocities.com/cyrilb2@btinternet.com/downloads_4.html

ZAP
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My recollection is that Bateman used a "active feedback" Reddy style composite amplifier in his oscillator, these are entirely dependant on op amp matching for any improvement - I think Soliman had articles on active op amp compensation in Wireless World in the 80's

the topology can work to some degree as Bateman shows, more ususally this topology is chosen for reduced phase errors in active filters

I'd claim using "active compensation" topologies for output load distortion reduction is less reliable - for best results you'd have to be sure that you loaded the feedback path op amp exactly the same as the forward path op amp

op amp parameter matching in these circuits usally is assummed when using dual op amps, but finding data on the degree of parameter matching of the 2 op amps in a monolithic dual is rare

for more fun you could try to predict thermal crosstalk effects when using both sides of a dual in a single loop


Jung style multiloops should give more robust reduction of output loading distortion reduction
as I mentioned in my thread on composite op amp circuits I've built high loop gain multiloop circuits and couldn't see 1 KHz 2nd order IMD product in a setup with ~ -160 dB resolution limit
 
#36
syn08 said:


ND was the first semiconductor fab for AD, isn't it?
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Yes, though we had some pans in the fire at Micropower. I don't remember if Nova got seed money from ADI. My high school chemistry teacher's son was Nova's first "knob tweaker" in the fab (I found out years later that they were related).

Jackinnj - that second plot is what I expect these days.
 
#40
Looks like I must have misremembered, need to find my "Capacitor Sound" folder

I still have a strong recollection of seeing the Reddy/Soliman acitve feedback circuit recommended for low distortion amplification - where?
 
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