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Old 17th May 2007, 02:07 PM   #21
johnrtd is offline johnrtd  Belgium
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Default op amps

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Old 17th May 2007, 03:00 PM   #22
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janneman: " ... the whole thing with a closed loop gain of say 10x which is 20dB. You'll see that the higher the closed loop gain, the less your stability problems. That's logical: less closed loop gain is less [out of phase] feedback. ... "

Pardon my edit above, but this is not really "logical" or intuitive. At first blush one would assume that increasing feedback would or should exaserbate the problem ... unless one were totally familiar with all of the factors of feedback (gain, phase shift, distortion, slew rates, etc). It does explain why a unity gain op-amp with, say, 10k input resistors and 10k (-) feedback resistor is more stable than one with no resistors in the circuit ... ?? Not ??

Needed: an extended book list of revised op-amp cookbooks ala Walt Jung, Don Lancaster, Bob Pease, Forest Mims ... ( http://www.amazon.com/Op-Amp-Cookboo.../dp/0138896011 [1986], http://www.amazon.com/Active-Filter-.../dp/075062986X [1996], http://www.amazon.com/Timer-Amp-Opto.../dp/0945053290 [2000], http://www.amazon.com/Troubleshootin.../dp/0750694998 [1991] ) ... plenty here, but cookbooks targeted strictly for analog / audio would be nice. Got links? Got a favorite ??
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Old 17th May 2007, 03:44 PM   #23
peufeu is offline peufeu  France
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You really need to read The Art Of Electronics


Basically an opamp has an open loop gain which is V(out) / ( V(in+) - V(in-) ), being a complex number with magnitude and phase.

When you define the closed loop gain with a feedback network, you use part of this OL gain as your closed loop gain, and the rest as fedback factor.

Suppose you have an OL gain of 60dB at 1 kHz ; you want closed loop gain of 10 (20dB), then you'll have 40 dB of feedback.

The more OL gain you use as closed loop gain, the less you have available for feedback.

More intuitive ?
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Old 17th May 2007, 04:21 PM   #24
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" ... Basically an opamp has an open loop gain which is V(out) / ( V(in+) - V(in-) ..." thus the term "differential amplifier", and the difference between V(in+) and V(in-) being the "gain factor".

" ... you use part of this OL gain as your closed loop gain ... you want closed loop gain of 10 (20dB), then you'll have 40 dB of feedback. ... The more OL gain you use as closed loop gain, the less you have available for feedback. ... More intuitive ? ..." ... Not.

We may have a "lost in translation" question here as well. I think I understand what you are trying to say ... but the " V(out) / ( V(in+) - V(in-)" part is easy to understand and is "intuitive".

Now ... getting back to the more detailed questions ... the devil is in the details like phase shift reduction, roll off approaching unity gain bandwidth, improving slew rate, reducing output noise, tweakin tips (plastic caps on the power pins, etc.) ... theories are nice, but lab bench experiences are what we are looking for here, no?
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Old 17th May 2007, 05:20 PM   #25
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Default Re: OP AMP EXPERIENCES and HOW TO

Quote:
Originally posted by tawn10
Snip - I have implemented the new LM4562 into my ARCAM DV89 which used to have 2x OPA2134, to be honest I think they are far better than any I have heard before, tonally a lot more balanced its like someone has gone accross the sound stage with an iron! I found that the 2134 in this app was fatiguing in the upper mid, sometimes drowned out and sometimes aggressive in a small bandwith. - Snip

Quote:
Originally posted by analog_sa
Snip - Wouldn't we all want to know? But why start with such a complicated device? Why not first ask how capacitors, resistors and wire influence the sound?
Are you sure those components actually influence the sound? Are you sure it’s not just the big, squishy grey lump between your ears making the components sound better? When I ask are you sure, I mean facts, graphs, curves, measurements and not anecdotal fluff-words like open, airy, tight, spatial, etc. Not to throw a wrench into the warm-and-fuzzies of DIY audio, but for new builders, would be beneficial to learn some solid science and methodologies along with electronics.

Do some Google searches on op-amps, op-amp design, op-amp applications, audio op-amps, etc. and you will uncover a wealth of information from OEMs, other DIY’ers, and data sheets. Plus, there no better experience than getting a Radio Shack breadboard and a handful of components and building, experimenting, and troubleshooting circuits to get intimately familiar with circuits.

To the newcomers, don’t get frustrated with this site too quickly. It probably has all of the information you’re asking for, but just not all summed up and presented in a nice sequential fashion as we’d all love to see and offer. Part of the fun with this site is to search, read, explore, and learn. Before you know it, you’ll figure out the BS stuff, the good stuff, and probably find yourself thinking about new projects and new design ideas. There’s a wealth of information and ultra-educated posters on this forum, but there’s also the fluff, junk, and woowoo (audiophool’s) garbage as well.
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Old 17th May 2007, 07:44 PM   #26
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Default Re: Re: Re: OP AMP EXPERIENCES and HOW TO

Quote:
Originally posted by janneman


The thermal coupling and feedthrough from output to input is a non-issue. Opamps nowadays have routinely temp induced drifts of a few microvolt (10^-6V) per degree. In fact, due to the very smart designs, thermal effects in opamps is orders of magnitude less than in discrete systems.

Walt Jung wrote his article 20+ years ago (or was it 30?). Modern opamps don't display this effect.

Jan Didden
Thermal drift and thermal coupling between output and input are certainly related but temperature induced drift which is a function of macroscopic environmental effects does not necessarily correlate with thermal coupling from output to input due to temperature gradients across the chip. An increase in distortion below 1kHz would be an indication of thermal coupling, and this does seem to be quite low in modern op amps. Certainly thermal drift is much higher in discrete circuits but thermal coupling as seen in op amps is absent due to the much larger distance between input and output transistors. An output buffer eliminates thermal coupling in op amps as Walt Jung describes.

Walt's later articles on this and other op amp limitations are from 1998 and are available here . Still well worth reading.
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Old 17th May 2007, 08:20 PM   #27
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Quote:
Originally posted by FastEddy
janneman: " ... the whole thing with a closed loop gain of say 10x which is 20dB. You'll see that the higher the closed loop gain, the less your stability problems. That's logical: less closed loop gain is less [out of phase] feedback. ... "

Pardon my edit above, but this is not really "logical" or intuitive. At first blush one would assume that increasing feedback would or should exaserbate the problem ... unless one were totally familiar with all of the factors of feedback (gain, phase shift, distortion, slew rates, etc). It does explain why a unity gain op-amp with, say, 10k input resistors and 10k (-) feedback resistor is more stable than one with no resistors in the circuit ... ?? Not ??

Needed: an extended book list of revised op-amp cookbooks ala Walt Jung, Don Lancaster, Bob Pease, Forest Mims ... ( http://www.amazon.com/Op-Amp-Cookboo.../dp/0138896011 [1986], http://www.amazon.com/Active-Filter-.../dp/075062986X [1996], http://www.amazon.com/Timer-Amp-Opto.../dp/0945053290 [2000], http://www.amazon.com/Troubleshootin.../dp/0750694998 [1991] ) ... plenty here, but cookbooks targeted strictly for analog / audio would be nice. Got links? Got a favorite ??
Well, I still think that it is intuitive that if you have less feedback you have less problems with stability. I must assume SOME basic knowledge, like that oscillations cannot occur if there is no feedback.

The other factors you mention certainly are important, but are secondary etc effects. I was giving a very simple but correct example as was requested.

Jan Didden
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Old 17th May 2007, 08:28 PM   #28
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" ... I still think that it is intuitive that if you have less feedback you have less problems with stability...."

... and your favorite op-amp cookbook??
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Old 17th May 2007, 08:30 PM   #29
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Default Re: Re: Re: Re: OP AMP EXPERIENCES and HOW TO

Quote:
Originally posted by nuvistor


Thermal drift and thermal coupling between output and input are certainly related but temperature induced drift which is a function of macroscopic environmental effects does not necessarily correlate with thermal coupling from output to input due to temperature gradients across the chip. An increase in distortion below 1kHz would be an indication of thermal coupling, and this does seem to be quite low in modern op amps. Certainly thermal drift is much higher in discrete circuits but thermal coupling as seen in op amps is absent due to the much larger distance between input and output transistors. An output buffer eliminates thermal coupling in op amps as Walt Jung describes.

Walt's later articles on this and other op amp limitations are from 1998 and are available here . Still well worth reading.

Indeed, and thanks for the links. But then again, Walt mentions that modern opamps have 1uV thermal feedback effects from output to input IIRC. I don't think I will worry too much about that. Also, these effects would show up in THD measurements below 1kHz anyway. I agree these are important issues, but would not be a reason to badmouth IC's and switch to discretes for these reasons.

Jan Didden
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Old 17th May 2007, 08:35 PM   #30
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Quote:
Originally posted by FastEddy
" ... I still think that it is intuitive that if you have less feedback you have less problems with stability...."

... and your favorite op-amp cookbook??

My first one was Walts opamp cookbook from the late 70's IIRC. But I have moved beyond cookbooks after many years of studying on and working with opamps. I have a few books on feedback systems and theory I use to look-up subjects. I am still not too strong in complex calculations with s and j, but I am at a point that I have to master it to go further. One does what one can

Jan Didden
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