Is the CFB topology superior, and why? - Page 16 - diyAudio
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Old 15th October 2012, 11:55 AM   #151
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Thanks for the explanation.

In my above comments; however, I was thinking about setting the amplifier closed loop gain to a proportion of the open loop gain--this is a Lower gain setting for VFB if you want transparency. The lower closed loop gain setting may be inconvenient. If so, use CFB.
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Old 15th October 2012, 01:00 PM   #152
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Quote:
Originally Posted by danielwritesbac View Post
Thanks for the explanation.
In my above comments; however, I was thinking about setting the amplifier closed loop gain to a proportion of the open loop gain--this is a Lower gain setting for VFB if you want transparency. The lower closed loop gain setting may be inconvenient. If so, use CFB.
You are right on this. It is easy to understand that TIM is in direct proportion of the open loop bandwidth. On fast transients, if the amp is not able to follow the signal's speed, it introduce ugly TIM distortion. That is the reason why Feedback has so bad (unjustified) reputation.
With current feedback, you can easily reach >3Mhz bandwidth, > 500V/µs of slewrate. and, so, 0° of phase error at 20 Khz. Here, more feedback you set, less distortion in the audio range and better damping factor.

The good practice, once you have tuned your amp (CFB or VFB) to maximize bandwith, is to add a low pass filter in the input, to ensure the amp will never have to deal with faster edges (transients) than he can deal with.

Comparing CFB vs VFB, the difference in sound impression is better transparency with CFB, better dynamic, in a strange way, more solid basses (that is because big transients are mainly in basses and Kick drums), more fluid treble and ease with dynamic.
Increased bandwidth lead to stronger and more separated basses and less aggressive treble impression.
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Old 15th October 2012, 01:48 PM   #153
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Christophe

If the designs are similarly optimized for speed (lets say essentially identical except for the input stage), then is it really the extra phase shift from the inverting side of a vfb that limits the bandwidth (and are there limits such as the driving source impedance)?
Is the error really different between the two, isn't it still just the non-linear voltage required to change the current into the current to voltage stage?
Does the class-B slew rate performance of the cfb ever come into play for audio signals?

Just trying to understand,
Thanks
-Antonio
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Old 15th October 2012, 02:35 PM   #154
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As you say, in the link i published, the two version are identical, except the feedback path.
You can simulate, (here simulation tools help a lot to understand) to see the signals differences in the feedback line.
About current vs voltages, it is just a matter of (bad) naming things. Because of the Ohm law, you can look at what happens both from voltage or current point of view. It is the same thing.
In fact, in the so called "current feedback" version, the feedback kind of modulate the gain of the first stage. And, because the bandwidth of the first stage is proportional to his gain factor, you can understand why bandwidth is increased. Not to forget than the low impedance of the feedback line help to minimize influence of parasitic capacitances at high frequencies.

One of the very interesting features of CFB is that the bandwidth remain nearly constant, whatever gain of the amp or feedback factor. For Current Feedback amps, the loop gain is set by the feedback impedance allowing an independent setting for the signal gain. The feedback impedance becomes the frequency response compensation.
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Old 15th October 2012, 03:19 PM   #155
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Thanks for the link, very helpful.
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Old 15th October 2012, 03:32 PM   #156
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Analog Design, Texas Instrumets, etc.. have published many papers to explain the theory of both VFB and CFB. Just Google for Voltage feedback vs Current feedback.
http://www.ti.com/lit/an/snoa376a/snoa376a.pdf
http://www.ti.com/lit/an/slva051/slva051.pdf
http://www.ieee.li/pdf/viewgraphs/cu...amplifiers.pdf
etc...
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Old 15th October 2012, 06:21 PM   #157
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Quote:
Originally Posted by Esperado View Post
It is easy to understand why, as current feedback remove one pole in the loop (the input inverting stage) , it increase the bandwidth.
It is not necessarily generally true: you can use the additional loop gain provided by the VFB to extend the closed loop bandwidth.
This may not always be possible or desirable, but it is a theoretical option.
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It is easy to understand why, because Current .Feedback is directly removed from the original signal, resultant signal to be amplified has lower level -> lower distortion.
The level has nothing to do in this, and anyway the level is not dependent on the input stage topology but on the voltage gain of the stages comprised between the input stage output and the speaker terminals
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.And why the inverting stage of a voltage feedback, adding his own distortion to the feedback signal; add total distortion.
Generally, the opposite will be true. If we look at the typical CFB example provided by Catalin:
Click the image to open in full size.

the main source of non-linearity (in the amplifier itself, leaving the IP buffer aside) will be the non-linearity of Q2's B-E.
When it is orphaned, as in this example, it uncompensated and outside the FB loop.
But when the amplifier has been converted to VFB:
Click the image to open in full size.

a similar non-linearity is inserted into the FB path and provides a first order compensation

Quote:
And, even on a good amp schematic, you can really and definitively hear the difference. Both in preamps and power amps.
I'd like to see the results of fully randomized fully blind A B tests...



Quote:
Originally Posted by Esperado View Post
One of the very interesting features of CFB is that the bandwidth remain nearly constant, whatever gain of the amp or feedback factor. For Current Feedback amps, the loop gain is set by the feedback impedance allowing an independent setting for the signal gain. The feedback impedance becomes the frequency response compensation.
For a general purpose building block like an IC, this is a selling point.
For a discrete amplifier you design yourself, the usefulness of such a feature is more debatable, unless you want to locate the volume control in the feedback network
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Old 15th October 2012, 09:04 PM   #158
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Quote:
Originally Posted by Elvee View Post
I'd like to see the results of fully randomized fully blind A B tests...
Please, in that matter, believe the old sound engineer i was.
And i know the material i'm listening too, because it can be my own mixages.
I do not believe so much in blind tests neither. For numerous reasons too long to detail here. :-)
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Originally Posted by Elvee View Post
For a general purpose building block like an IC, this is a selling point.
For a discrete amplifier you design yourself, the usefulness of such a feature is more debatable, unless you want to locate the volume control in the feedback network
I partially agree, with some remarks.
One is you can set bandwidth to his maximum with CFB and keep-it there.
Second you can play with local gain of each stage VS loop global gain to optimize distortion.
Third, you can tune damping factor to your needs without affecting bandwidth..

One example where VFB is a no no is in the stereo bus of a mixing desk. More you plug tracks in it, more you increase the gain of the mixing stage, more you reduce its bandwidth, and get a dull sound. One example where CFB change your life.

I have no time, for now, to fight again your other controversial argues, it would be too long and boring. But some of your you asserts are interesting, like "similar non-linearity" compensation. I use that kind of perversity to increase bandwidth, using current mirror in the first stage because efficiency decrease after FT, and so more current for the same level of HF, more level at the output: local auto compensation. One of my answers would be to talk slew rate instead of closed loop bandwidth.

The main one is, if you prefer VFB for some obscure reason, chose your poison. My own religion is made since more than 30 years, correlated by numerous experiences, objective or subjectives, and many measurements .

Those who are interested with CFB can read this very interesting thread: Simple Symetrical Amplifier and read how DIYers talk about the sound of their CFB amps in a very similar manner.

PS: in your schematic, Feedback resistance do not have a proper value for CFB. Too high impedance. And the feedback is not applied to the first stage; witch limit the bandwitch by itself. And here, you apply CFB to a x1 voltage gain stage...
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Last edited by Esperado; 15th October 2012 at 09:28 PM.
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Old 16th October 2012, 12:43 AM   #159
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I find it incomprehensible that you are an engineer, and yet do not "believe" in double blind tests.

If a test you do is not double blind then it is subject to systematic error such that it has an in built bias that biases the outcome to the one you wanted in the first place.

All that an increased sample size does is to reinforce this bias.

Statisticians know this well and marketers use single blind tests for precisely this reason, to give "scientific", respectability to what is marketing hype.

You gloss over your objections to double blind testing, I am curious as to why.
rcw
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Old 16th October 2012, 08:33 AM   #160
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Quote:
Originally Posted by Esperado View Post
Second you can play with local gain of each stage VS loop global gain to optimize distortion.
Third, you can tune damping factor to your needs without affecting bandwidth..
THD and DF need no no "tuning", simply minimization for one and maximization for the other. DF can easily be tuned (degraded) by adding a series resistor.
Quote:
But some of your you asserts are interesting, like "similar non-linearity" compensation. I use that kind of perversity
No perversity in that, simple application of sound engineering practices
Quote:
The main one is, if you prefer VFB for some obscure reason, chose your poison. My own religion is made since more than 30 years, correlated by numerous experiences, objective or subjectives, and many measurements .
I believe in the right tool for the right job.
I am under the impression that CFB has a (mostly unjustified) aura, whereas VFB is accused of all evils of the earth.
When you look at the subject from a purely objective point of view, the picture is far from that kind of manicheism.
Quote:
PS: in your schematic, Feedback resistance do not have a proper value for CFB. Too high impedance. And the feedback is not applied to the first stage; witch limit the bandwitch by itself. And here, you apply CFB to a x1 voltage gain stage.
.
The source schematic isn't mine, it is an example provided by a keen CFB supporter
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