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Current Feedback Amplifiers, not only a semantic problem?
Current Feedback Amplifiers, not only a semantic problem?
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Old 11th August 2017, 05:01 PM   #1
forr is offline forr  France
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I would like to discuss about the first article below and its comments :

Michael Kiwanuka, AudioXpress, June 2017
"Current Feedback and Voltage Feedback Fallacies"
https://www.audioxpress.com/files/attachment/2621

Currently, invalid links :
http://www.waltjung.org/waltsblog/catego...mplifiers/
Some Useful Background Reading on Current Feedback Amplifiers | Walt's Blog 2014


Walt Jung, Sergio Franco, Michael Kiwanuka, AudioXpress, July 2017
"Current Feedback and Voltage Feedback Dispute"
http://www.audioxpress.com/article/curre...ck-dispute

EDIT december 2018. I added links having correlations with the topic :

Christian Henn, Burr-Brown, May 1993
"New Ultra High-Speed Circuit Techniques with Analog ICs"
http://www.ti.com/lit/an/sboa047/sboa047.pdf

E.M.Cherry, IEE Proc/-Circuits Devices Syst; Vol 147, No 6, December 2000
"Feedback amplifier configurations"
https://drive.google.com/file/d/1n03...zjYJWwLNU/view

Walt Jung and technical staff of Analog Devices, 2005
"Op-amp applications handbook / Current feedback basics"
https://studfiles.net/preview/429636/page:5/

Cyril Mechkov, Technical University of Sofia, Lutz von Wangenheim, discussion started in 2013
What is the truth about the exotic current feedback amplifier? Is it something new or just well known old? Is it really a current feedback device?

Sergio Franco EDN August 23, 2017
"In defense of the current-feedback amplifier"
https://www.edn.com/design/analog/44...back-amplifier

Christopher Paul, AudioXpress, July 2018
"Current Feedback: Fake News or the Real Deal ?"
https://www.audioxpress.com/files/attachment/2660

Last edited by forr; 21st December 2018 at 02:50 PM. Reason: Updating links
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Old 11th August 2017, 06:21 PM   #2
jcx is offline jcx  United States
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Default just accept "CFA" as an informal, "trivial name"

I think some of the objection to "cfa" is that the name doesn't fit the existing formal classification schemes - as a Voltage output device it is the Voltage Output that is sampled by the feedback

a more abstract, formal op amp classification: http://web.archive.org/web/200701282...ns/2000cas.pdf

but I don't think there is any real dissagreement on how the circuit works

but a lot about whether it is advantageous in audio poewer amplifiers

Last edited by jcx; 11th August 2017 at 06:28 PM.
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Old 11th August 2017, 09:37 PM   #3
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by jcx View Post
I think some of the objection to "cfa" is that the name doesn't fit the existing formal classification schemes - as a Voltage output device it is the Voltage Output that is sampled by the feedback
True, and I myself was also upset when I found out, and I wrote many angry posts here.
But if you take some sensible distance you realise that the term 'CFA' is firmly established for a particular opamp circuit configuration. It is not unusual for a term to have different meanings in different areas or contexts, and if the context is clear like in this case, the smart thing is to accept it.

Jan
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Old 12th August 2017, 04:53 PM   #4
r_merola is offline r_merola  Brazil
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I agree totally with you.

Regards
Ronaldo
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Old 12th August 2017, 06:27 PM   #5
Nelson Pass is offline Nelson Pass  United States
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Current Feedback Amplifiers, not only a semantic problem?
What shall we call an amplifier that actually has current feedback then?
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Old 12th August 2017, 07:51 PM   #6
Michael Rothacher is offline Michael Rothacher  United States
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Current Feedback Amplifiers, not only a semantic problem?
I'm dealing with the semantic issues of feedback naming in the article I'm working on now.
My preference is for the wordy but explicit convention which names how it is derived and how it is mixed back. In my research, I've identified a whole bunch of confusing names, some which sound identical but mean completely different things. It's not hard to understand how confusion creeps in. And don't even get me started on so-called Schade fedback
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Old 12th August 2017, 08:52 PM   #7
Mark Johnson is offline Mark Johnson  United States
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Current Feedback Amplifiers, not only a semantic problem?
Why not name it after its most famous implementation?

We call the circuit in Figure 1 "the LH0002 buffer" after the NSC product.

Instead of "current feedback amplifier" why not call cicuits like Figure 2 "EL2020 type amps" after the Elantec product? I looked at a bunch of Comlinear datasheets but they don't have internal schematics.

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File Type: png LH0002.png (53.1 KB, 2561 views)
File Type: png EL2020.png (86.6 KB, 2559 views)
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Old 12th August 2017, 10:32 PM   #8
forr is offline forr  France
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Quote:
Originally Posted by jan.didden View Post
True, and I myself was also upset when I found out, and I wrote many angry posts here.But if you take some sensible distance you realise that the term 'CFA' is firmly established for a particular opamp circuit configuration. It is not unusual for a term to have different meanings in different areas or contexts, and if the context is clear like in this case, the smart thing is to accept it. Jan
Semantically

According to the comments [1], the CFA term is firmly "established since 3-4 decades" (Jung).

However, there is a lack of fair acknoledgement that :
- it was already established much earlier to mean the control of the output current of an amp by negative feedback.
- it is probably still now the main usage of the term.

So the term current feedback can be considered as having been appropriated by marketing services (Steffes : "there is certainly a bit of marketing slang involved there") to describe a topology which was a novelty because its uses of a push-pull for an inverting input but was not a novelty at all in the way how the difference between the input signal and the feedback signal is obtained.

I extract a few words written by Peter Baxandall in Michael Talbot-Smith's book
Audio Engineers Reference Book Focal Press

Quote:
2.8.8 The 'current feedback amplifier' notion
The 'current feedback amplifier' term has appeared in op-amp manufacturers (Ogden, 1992) since 1985, and also in audio power amplifer context (Alexander, 1990). It implies that a new basic principle is involved, but really it is an old one in disguise.
[...]
It is felt that the term 'current feedback' in the above connection is a rather unfortunate one, since this is a long established term employed when negative feedback is derived from the voltage across a small value resistor in series with a load for the purpose of obtaining a high output impedance or, in combination with normal voltage feedback, an output impedance of some specific value
[1]http://www.waltjung.org/waltsblog/current-feedback-amplifiers/useful-background-reading-on-current-feedback-amplifiers/#comments

***

Quote:
Originally Posted by Nelson Pass View Post
What shall we call an amplifier that actually has current feedback then?
In what context : function intended of the feedback or topology of an input stage ?

***

Technically

Now technically, the term CFA for topology is highly questionnable.
I prefer to describe the input stage than how it is supposed to work :
single device, diamond or differential.

Note that if a diamond input stage is said to belong to the topology of current feedback amplifiers, a single device input stage should also be said to belong to the same topology. But nobody does it. This is a first clue of the weakness of the expression.

Feedback relies on the difference between the signal at the non-inverting input and the signal at inverting input. The input signal at the non-inverting input is a voltage. So the feedback signal can't be a current, it's a voltage.

The circuit which operates the difference is a single transistor or an arrangement of transistors to provide better performances but keeping the main fundamental property of a single transistor which is to be a transconductance device, voltage controlled (by its Vbe) current source (Ic).

These are two more clues about the weakness of calling CFA an amplifier using a diamond circuit input.

Now what is the exact role of the current in the topology called CFA ?
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Old 13th August 2017, 07:49 AM   #9
jan.didden is offline jan.didden  Europe
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Forr, I fully agree with you regarding the semantic part. It was unfortunate that this term was hijacked. I wonder whether it was a delibered move, or maybe by someone who was not aware of the original meaning, could also have been the case.

For the technical part, an easy way out is to note that in a 'VFA' the feedback is returned to a common emitter stage which generally is assumed to be voltage driven.
In a 'CFA' the feedback is returned to a common base stage which is generally assumed to be current driven.

I say 'generally' because you can always come up with specific cases that are different. And as we all know, you can't have current without voltage (unless Z = 0).
But as a naming convention to make a broad distinction between two quite different circuit topologies, I believe CFA and VFA are usefull and non-ambiguous identifiers. Has been for 30+ years, and I am not aware of a single case of confusion on this in the context of opamp structures.

Jan
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Last edited by jan.didden; 13th August 2017 at 07:52 AM.
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Old 13th August 2017, 08:04 AM   #10
MarcelvdG is offline MarcelvdG  Netherlands
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You could do what philosophers do in such cases: first define what you mean by a certain term, then use it.

By the way, the term current feedback has at least three meanings. First it meant series feedback at the output, then it changed into series feedback at the output combined with shunt feedback at the input, then it changed into plain old voltage feedback using an amplifier with low open loop impedance at its negative input.

Last edited by MarcelvdG; 13th August 2017 at 08:08 AM.
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