Is the CFB topology superior, and why? - Page 7 - diyAudio
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Old 7th September 2012, 08:49 AM   #61
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Quote:
Originally Posted by Elvee View Post
Hi,

I would like to investigate about the actual/perceived/imagined superiority of current feedback, and possibly to pinpoint what exactly makes it superior (if indeed it actually is).

As a study subject, I propose to use a very simple, quasi-canonical form of a CFB amplifier, the CFP with gain.

The two first pics show the linearity and frequency performances of the circuit.

The interesting thing with the sim is that it allows the transformation of this ideal CFB amp into an identical ideal VFB stage.
That can be done with an arbitrary voltage source (the next two pics).

As could be expected, the linearity has improved thanks to to the increased loop gain, but somewhat more surprisingly the bandwidth also increases.
I recall at a circuit in that kind for a simple preamp (line) stage. By listening tests the value of R1 exerted a great influence to the sonic performance.
The best sonic results were achieved by such values, that the idle current for Q1 goes up to 10-15mA (instead only 1-2mA in your proposal).
By simulation no significant differences were observed.
What reasons could be responsible therefore ?
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Old 7th September 2012, 10:55 AM   #62
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Quote:
Originally Posted by wahab View Post
Apart from slew rate a CFB is no better than a VFB and usualy
not as good in matter of linearity contrary to your sayings
wich was the point that i didnt agree with.

Indeed , the best opamps in this matter are VFBs.

As for high slew rate, it is pointeless if linearity is worse ,
wich is often the case with CFBs.
Please name the VFB opamps that can beat the THD performances of CFB. CFB is in actual fact as linear and even more so than VFB. Show me a VFB that can do - 90 db THD at 1Mhz. There are many of CFB that can do this and some do even better. I think you need to do some research regarding CFB. When you have slewrate and linearity which is better than VFB it makes for excellent topology.

If what Im saying is wrong please provide proof, name the parts or show us the circuits. VFB is better in some respects but its nothing to do with linearity or slewrate.
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Old 7th September 2012, 12:31 PM   #63
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by homemodder View Post
Please name the VFB opamps that can beat the THD performances of CFB.
For example http://www.ti.com/lit/ds/symlink/ths4271.pdf -92dB @30MHz

Which doesn't mean that VFB is anywhere better than CFB. CFB is just a different approach, with pros and cons (cons examples: offset, noise).
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Old 7th September 2012, 04:36 PM   #64
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by tiefbassuebertr View Post
By simulation no significant differences were observed.
What reasons could be responsible therefore ?
That is quite surprising: R1 does have a huge impact on many aspects of the circuit, the linearity, loop gain, slew rate etc.

For example if R1 is doubled in the example, from 560 ohm to 1K, the THD is reduced by a factor of ~3.

There are many tradeoffs of impedance, gain, bias currents etc in this circuit, and the value of R1 does have a pivotal role in them
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File Type: png CFB9.png (115.1 KB, 349 views)
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Old 7th September 2012, 05:07 PM   #65
wahab is offline wahab  Algeria
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Quote:
Originally Posted by homemodder View Post
Please name the VFB opamps that can beat the THD performances of CFB. CFB is in actual fact as linear and even more so than VFB. Show me a VFB that can do - 90 db THD at 1Mhz. .
Low THD at 1Mhz doesnt mean lower THD at audio frequencies ,
wich is the case that interest us.
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Old 8th September 2012, 09:24 AM   #66
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Quote:
Originally Posted by wahab View Post
Low THD at 1Mhz doesnt mean lower THD at audio frequencies ,
wich is the case that interest us.
Yes it does in 99,99 % of cases and what is even more important is the fact that typically you dont see a sharp increase in THD at frequencies higher than 1k as one usually does with VFB. The THD curve is virtually flat, from DC till around 20Khz. What is the use of only having low THD at 100 to 500 hz.
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Old 12th September 2012, 08:48 PM   #67
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Default cfb to vfb

Elvee

Saw this one and thought you might enjoy it:
http://www.ti.com/lit/ds/symlink/lm7171.pdf

See the simplified diagram near the bottom.

-Antonio
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Old 12th September 2012, 09:48 PM   #68
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A thing that I have looked at for the LM7171 would look something like this.

This is intended to be a high q bandpass filter/buffer.

The first two devices are connected as a transconductance amplifier to provide high impedance drive to the filter, and the output device is connected as a q multiplier.
rcw
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File Type: gif qmultnotabuff.GIF (4.0 KB, 255 views)
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Old 13th September 2012, 04:47 PM   #69
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by magnoman View Post
Elvee

Saw this one and thought you might enjoy it:
http://www.ti.com/lit/ds/symlink/lm7171.pdf

See the simplified diagram near the bottom.

-Antonio
Yeah, nice one: it shows that in reality too the differentiation between the two categories is dwindling.
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Old 26th September 2012, 01:02 PM   #70
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Quote:
Originally Posted by suntechnik View Post
the reason is feedback has no clue if it is timbre of instrument or a power supply noise that NFB is currently suppressing
It is probably unfair to compare an amp run from clean power with an amp run from dirty power. Whether or not your LTP with global negative feedback can hide a lot of power noise along with a little bit of your signal, highlights the very real problem of using power noise rejection as an excuse for cheap abbreviated power supplies. The resulting problem is harder to measure by design with many generations of amplifiers repeatedly refined for the purpose of hiding errors from scopes (too much like selecting beer flavor by forensic analysis). However, the consequences of dirty power inside the feedback loop remains obvious to the ear as a distorted or shrunken perceived soundstage size. The LTP can be hobbled and clean power can be used. Note that there's less distortion of soundstage when using global negative feedback with NTP, SSA and Singleton input amplifiers. Therefore, the concept of global negative feedback wasn't at fault, and an LTP might not be blameless after all.
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