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Old 1st September 2003, 11:41 PM   #1
kyrgeo is offline kyrgeo  Greece
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Question Active Feedback Amplifier

In this article http://www.chipcenter.com/analog/c014.htm (Are Op Amps really linear?) the great Barrie Gilbert mentions an AFA - active feedback amplifier:

"...Later, another fundamental building block, which I call the Active Feedback Amplifier, or AFA, will be discussed. In my opinion, this structure, which, unlike the OPA (vfb opamp), has high open-loop linearity and excellent closed-loop linearity due to its distortion-canceling topology, and a very high degree of versatility arising from its dual fully-differential inputs, has the potential to eclipse the OPA in all applications involving the manipulation of purely voltage-mode signals. Being a super-set, it can do anything that a conventional op amp can do, plus a whole lot more."

Well, if that doesn't sound promising...

However, I can't seem to find this later column he is referring to. He mentions that he will discuss the AFA in a later column, in two more of his articles (All of his articles are great read - recommended). But a google search on this AFA thing did not help at all. Has anybody any info on this? Or seen anything published?
Nothing's new under the sun (in amlifier electronics at least), so there must be some implementation of this, obviously under a different name... (I guess Barrie Gilbert loves to invent names!). Any info would be greatly appreciated!

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Kyriakos Georgiadis
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Old 2nd September 2003, 07:26 AM   #2
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The AD830 is an active fb amp. It's data sheets explains the principle. There is a newer version, not sure of the type but it also ends on ...830 IFAIK.

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Old 26th February 2013, 02:36 AM   #3
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Zombie Thread!!1!!!

OMG! Zombies!!!!11! zombie3.gif

I just found this doc ... and was gonna post ... but this thread already exists ... from way, way back!

There is an interesting trend here: operational amplifiers started out (back in analog computing days) with a totally different set of objectives to those surrounding amplifiers for ac amplification with low distortion. In the 'op amp paradigm,' the idea was to make the open-loop gain so very high that the function is determined solely by the external components. However, as we've seen, that is far from the case for a typical OPA operating at moderate gains and frequencies. In the second design style, typified by audio power amplifiers, the objective is to achieve almost perfect linearity without feedback, and then use a small amount of feedback to squeeze out the last gram of linearity. Designers of modern high-performance op amps now understand the critical importance of pursuing the latter paradigm in all high-frequency applications. This equates almost completely to achieving extremely high slew rates; value of over 5,000 V/ms are nowadays available in monolithic OPAs.


In another approach to improving HF linearity, the gm input stage is replaced by a current-conveyor, which is another type of Class-AB cell. (In fact, high-slew OPAs often used what is effectively two such cells in a differential arrangement.) A current conveyor is the starting point of a current-feedback amplifier, also called a transimpedance amplifier (TZA). This type of amplifier can, in principle, be designed to be totally free of slew-rate limitations, one of its main attractions. Another is that, unlike an OPA, in which the so-called 'virtual ground' or 'summing node' exhibits a low impedance only by the action of global feedback, becoming very high at frequencies close to w1, the TZA provides a low summing-node impedance (ohms) even with the feedback completely removed, from dc to many hundreds of megahertz.


This useful and interesting special-purpose amplifier topology will be the subject of my next column. Later, another fundamental building block, which I call the Active Feedback Amplifier, or AFA, will be discussed. In my opinion, this structure, which, unlike the OPA, has high open-loop linearity and excellent closed-loop linearity due to its distortion-canceling topology, and a very high degree of versatility arising from its dual fully-differential inputs, has the potential to eclipse the OPA in all applications involving the manipulation of purely voltagemode signals. Being a super-set, it can do anything that a conventional op amp
can do, plus a whole lot more.


Surely he's talking about more than just current FB opamps?
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File Type: pdf Are%20Op%20Amps%20Really%20Linear.pdf (56.6 KB, 91 views)
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Old 26th February 2013, 03:58 AM   #4
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Yeah he's not talking about CFB amps, rather a completely different front-end topology for an opamp. I'm in love with these AFAs at present, they do sound so much better than traditional LTP-fronted opamps They do measure somewhat worse than the top-of-the-range opamps though - so much for THD measurements eh?
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Old 26th February 2013, 04:16 AM   #5
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I guess it depends on how you like your THD ......
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Old 26th February 2013, 04:26 AM   #6
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Yes indeed - some work hard to put two or more zeros after the decimal down at 1kHz and don't much worry how many are still there at 20kHz. I'm of the other school, I tend to think one zero is enough at 1kHz but still want the same well beyond the audio band to mitigate RF IMD.
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Old 2nd March 2013, 02:17 PM   #7
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Hey abraxalito!

I've been searching and found all your good info and blogs etc...

More people should be trying these out!

Problem is, there's many threads each with a bit of info and they may not be easy to search for.

I really think we need one thread that references them all and brings it all together in a coherent, easy-to-find thread, something like:

"Gilbert's chips, AFA (active feedback amplifier): AD830, LT6552, TDA856X"

I'm good with info: all the spec sheets, part schematics from the specs, graphs, technical summary, suggested circuits. This would really get people going!

But I don't want to steal any of your thunder, you're doing a fantastic job!
What do you think?

PS Are there anymore AFA chips we could add to the list in the thread title (hard to edit later)

Last edited by AudioLapDance; 2nd March 2013 at 02:28 PM.
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Old 2nd March 2013, 02:23 PM   #8
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I reckon there's more than enough thunder to go around, so why not start the thread yourself? Then I'll chip in if I find some new stuff out or if you have specific questions. I agree these chips should be more widely known, but then if too many people find out they might be harder to find on the second-hand market (which is where I get a lot of my supplies). Many of the really interesting NXP/Philips parts have been obsolete a while, but who knows if they undergo a renaissance perhaps NXP will bring them out of retirement? That would be a win all round

Anyway, glad that you're a 'convert' now
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Old 2nd March 2013, 02:32 PM   #9
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I'm on it! Sweeeeet

Any other part numbers (or 'key' words) I should reference in the thread title?

hmmm, maybe a section for power amp chips and then one for line level chips. Any ideas?

For me it's not so much whether a part is listed as obsolete. If there's a few dozen actually around still, then us DIYers are good-to-go!

True, it would mean less chips for us but spreadin' good sound and a new (hopefully more successful) way of looking at circuits (high freq issues in our cellular/wifi world) is just toooooo important ...

Last edited by AudioLapDance; 2nd March 2013 at 02:39 PM.
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Old 2nd March 2013, 02:37 PM   #10
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I'd not reference them in the title, rather in the first post - that's one that can be edited by the thread starter, so you can add more if we find them. The title can just be 'Audio applications of Barrie Gilbert's AFA' or something like that.

Hard to make a hard-and-fast distinction between line level and power amp - there are a few headphone amps for example, and also at least one power amp which has a line driver mode selectable by a pin. I'd lump them all in together and let the readers sort out which they want.
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