Current feedback - Voltage feedback, how do I see the difference? - Page 53 - diyAudio
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Old 24th March 2013, 08:58 PM   #521
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Quote:
Originally Posted by godfrey View Post
Not necessarily.
Reducing closed loop gain in any series (voltage applied) shunt (voltage derived) negative feedback amplifier, including so-called "CFAs", means either

1) increasing the value of the feedback resistor connected to ground

or

2) reducing the value of the resistor connected to the output.

The first of these, in a so-called "CFA", reduces first stage gain and therefore reduces bandwidth even as it reduces closed loop gain. This is because the effective impedance at the emitter of the input stage is increased.

The second option, in a so-called "CFA" slightly increases first stage gain and therefore increases bandwidth even as it reduces closed loop gain.

The second option is therefore preferable with a so-called "CFA" as it increases loop transmission if stability margins are not compromised by the increase in bandwidth.

However, users of so-called "CFAs" are often exhorted to alter closed loop gain by using option (1), presumably because option (2) may compromise stability.

Last edited by michaelkiwanuka; 24th March 2013 at 09:14 PM.
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Old 24th March 2013, 09:12 PM   #522
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Originally Posted by RNMarsh View Post
perhaps you havent been using the best model for current-mode feedack?
I haven't used any particular model for so-called "current-mode feedack".

To me it's abundantly obvious that so-called "current-mode feedack" amplifiers are nothing of the sort: they're self-evidently series (voltage applied) shunt (voltage derived) feedback amplifiers, i.e. voltage amplifiers.

Quote:
Originally Posted by RNMarsh View Post
1. lower noise potential because the devices are in parallel with each other (push-pull);
2. More consistent sound quality character because (?) the distortion's harmonic structure is the same at all frequencies -> at low-mid-high.
3. Very low distortion with only moderate levels of gnfb.
4. Very fast-high speed and wide bandwidth.

THx-RNMarsh
All these attributes are readily attainable in audio applications with VFAs that use a differential stage for their input stage.

I am not sure what is meant by "more consistent sound quality" though.

Last edited by michaelkiwanuka; 24th March 2013 at 09:15 PM.
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Old 24th March 2013, 09:42 PM   #523
dadod is offline dadod  Croatia
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Quote:
Originally Posted by michaelkiwanuka View Post
I am not sure what is meant by "more consistent sound quality" though.
Now its becoming interesting!
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Old 24th March 2013, 10:19 PM   #524
RNMarsh is online now RNMarsh  United States
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Well, I'm not going to defend the sound character of anything.... I will just put forth for all to consider -->

The typical THD curve with Voltage feedback toplogy is one of higher distortion in the high audio freqs than at the bass and mids. Rising thd as freq goes up.

This doesnt happen with CMode feedback circuits. The harmonics structure - relation of one harmonic to another -- their amplitutes and the freqs relationship of the harmonics stays the same across the BW when using current-mode or compimentary push-pull current-mode fb circuits. The sonic descriptions given to this class of amplifier topology is unattainable any tradional way. The diff/vfb circuits sound one way in the bass and other in the highs.

What is clear to you isnt so clear to the IC industry, it appears.

Noise can be lower, thd can be as low and etc. PSR and CMR can be boosted by cascoding and better Power supplies (which even vfb opamps benefit from) et al....

Evryone can study up on it and find out for themselves the pro-con of each and the means to overcome each cons. So, thanks for the input and thoughts and I'll leave the rest for others to explore for themselves in the published literature. The references are also interesting.

CMFB.jpg

Thx-RNMarsh

Last edited by RNMarsh; 24th March 2013 at 10:45 PM.
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Old 24th March 2013, 10:35 PM   #525
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Trolling its is. Combined with a solid measure of the old Gadfly ointment. Some of us will go on believing what we decide is fact, no matter what the evidence is to the contrary or despite perfectly valid alternative interpretations. And there we should leave it. For to soldier on in the face of such obstinance is pure folly.
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Old 24th March 2013, 11:09 PM   #526
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Quote:
Originally Posted by RNMarsh View Post
Since then the current-mode feedback as it has become named has been refined. They are super good, now. But they leave the vfb behind as you move higher and higher up in frequency. They are now a staple in all analog IC mfr'ers portfolio as their strengths overlap with vfb designs. Its above or below the over-lapping areas (grey) where one shines brighter than another.

Its important to know when to use one or the other and why.

Thx-RNMarsh
"This shows that the (CFB's) bandwidth becomes independent of the set gain G, because bandwidth is defined as the frequency at which the gain drops by 3 dB, no matter how large the gain is."

To that, add an open loop gain level as much as you can and you'll get the best amplifier as money can buy.
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Old 24th March 2013, 11:35 PM   #527
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Originally Posted by Bonsai View Post
Some of us will go on believing what we decide is fact, no matter what the evidence is....
Where is you evidence?

True. You and a few others have chosen to believe, without adducing a single shred of truth in rebuttal, that so-called "CFAs" are not VFAs.

Sad.
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Old 24th March 2013, 11:41 PM   #528
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Originally Posted by RNMarsh View Post
The sonic descriptions given to this class of amplifier topology is unattainable any tradional way. The diff/vfb circuits sound one way in the bass and other in the highs.
I really am afraid one shouldn't be able to tell the difference between two competently designed amplifiers in a controlled listening test, regardless of differences in their design philosophies.
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Old 24th March 2013, 11:52 PM   #529
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Quote:
Originally Posted by RNMarsh View Post
Well, I'm not going to defend the sound character of anything.... I will just put forth for all to consider -->

The typical THD curve with Voltage feedback toplogy is one of higher distortion in the high audio freqs than at the bass and mids. Rising thd as freq goes up.

This doesnt happen with CMode feedback circuits. The harmonics structure - relation of one harmonic to another -- their amplitutes and the freqs relationship of the harmonics stays the same across the BW when using current-mode or compimentary push-pull current-mode fb circuits. The sonic descriptions given to this class of amplifier topology is unattainable any tradional way. The diff/vfb circuits sound one way in the bass and other in the highs.

What is clear to you isnt so clear to the IC industry, it appears.

Noise can be lower, thd can be as low and etc. PSR and CMR can be boosted by cascoding and better Power supplies (which even vfb opamps benefit from) et al....

Evryone can study up on it and find out for themselves the pro-con of each and the means to overcome each cons. So, thanks for the input and thoughts and I'll leave the rest for others to explore for themselves in the published literature. The references are also interesting.

Attachment 338249

Thx-RNMarsh
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Old 25th March 2013, 12:08 AM   #530
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Originally Posted by RNMarsh View Post
The typical THD curve with Voltage feedback toplogy is one of higher distortion in the high audio freqs than at the bass and mids. Rising thd as freq goes up.
I am positive this need not be the case with VFAs with traditional differential pair as input stage.

Scott Wurcer's AD797 and the NE5534 spring immediately to mind.
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