Is the CFB topology superior, and why? - Page 13 - diyAudio
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Old 30th September 2012, 02:41 PM   #121
catalin is offline catalin  Romania
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Well I think that you can't understand that Emitter common introduce a pole at low f and common base introduce a pole at ultra high f .But wathever ,if you are happy with vfb nothing else matters .
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Old 30th September 2012, 02:56 PM   #122
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Quote:
Originally Posted by Lazy Cat View Post
I'm glad that the topic attracted you and you're beginning to understand, nevertheles we could still have eternal theoretical discussion about VFB vs. CFB.

My suggestion would be more of a practical nature: try to build simple-basic 6 transistors VFB and simple-basic 6 transistors CFB amp and compare them sonically.
Lazy Cat,
Sorry I'm in it for the eternal theoretical discussion (I can barely tell the difference between speakers).

The supposed theme, what are the true differences and what advantages or disadvantages do they bring.
I can accept the cfb current on demand aspect resulting in much higher slew rates, as well as eliminating the current source pole which is what I think Catalin is referring to.
As for the remaining parts of a typical amp structure I still fail to see why they can't be made similar. A vfb can have a common base stage as well as current mirrors driving the voltage stage.
Typically it does seem that cfb op amps inherently use their input structure combined with the natural current mirrowing to minimize additional voltage gain phase shifts at the expense of lower frequency gain.

So does it comes down to typical implementations of cfb versus vfb and the comparison of higher open loop bw at the expense of lower frequency loop gain?

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-Antonio
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Old 30th September 2012, 04:24 PM   #123
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by catalin View Post
Elvee, you want to bring a deviation to the true. You forget that the vfb is uncompensated.Therefore the group delay is the same.But an uncompensated vfb will not work ever.
So please compensate the vfb and I will bring also a cfb with good phase reserve.Let see than the real group delay.

Let 's do the things correctly and not loose time.

Thank you
In this case, there is no need to bring compensation, because the source CFB circuit is supposed to be stable (it is not the case, but that is nor my fault nor my problem) and the conversion into VFB changed practically nothing to the loop stability parameters.
I could always overcompensate the circuit, but then the comparison wouldn't be fair
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Old 30th September 2012, 05:20 PM   #124
catalin is offline catalin  Romania
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Quote:
Originally Posted by Elvee View Post
In this case, there is no need to bring compensation, because the source CFB circuit is supposed to be stable (it is not the case, but that is nor my fault nor my problem) and the conversion into VFB changed practically nothing to the loop stability parameters.
I could always overcompensate the circuit, but then the comparison wouldn't be fair
Well exactly this is the main difference between the VFB and CFB .It will not work without compensation .And when the miller cap appear at vfb (discrete or parasitic) then the pole at low f appear .
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Old 30th September 2012, 07:08 PM   #125
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by catalin View Post
Well exactly this is the main difference between the VFB and CFB .It will not work without compensation .And when the miller cap appear at vfb (discrete or parasitic) then the pole at low f appear .
The CFB you gave as an example will not work without compensation (it might work practically, but not in a stable and reliable manner).
The derived VFB version is almost identical in this respect: in theory, it should need a slightly heavier compensation, but in practice, the difference is smaller than normal tolerance on components.
This simply shows that the conversion from CFB to VFB is almost costless: it is true that a very high frequency pole is added by the follower, but it is minuscule compared to the rest of a multitransistor amplifier.

The difference could be felt for a single stage common emitter amplifier: then , the added phase shift wouldn't be negligible anymore, but in a multistage amplifier, the supplement in phase is very small, almost negligible.
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Last edited by Elvee; 30th September 2012 at 07:11 PM.
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Old 30th September 2012, 08:04 PM   #126
catalin is offline catalin  Romania
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Elvee I didn't said that the cfb from picture is working. The cfb circuit was only to see and understand how cfb works .

If you want a real working cfb amp there are a lot also in this forum also in the most expensive amps like accuphase ,goldmund,etc .The accuphase e 213 for example has a plot for the function transfer like no one else .Google it
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Old 30th September 2012, 08:37 PM   #127
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I looked at your reference Catalin and only saw a bunch of marketing hype for an over complex circuit about which they are making vague and unsubstantiated claims.
rcw
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Old 30th September 2012, 09:27 PM   #128
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Catalin, Elvee

I finally figured out (I think, yes it takes me a while) what you guys are talking about, anyway I'd have to agree with both of you.
I do agree the cfb will have slightly less phase shift at the higher frequencies and that in general this will hold for a cfb.

Thanks
-Antonio
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Old 30th September 2012, 10:16 PM   #129
wahab is offline wahab  Algeria
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Quote:
Originally Posted by catalin View Post
But an uncompensated vfb will not work ever.
That s not rigorously true.

I used an uncompensated symmetrical differential amp for many
years and it was perfectly stable , used for almost 30 years
and still working perfectly to this day.

I published the schematic somewhere at DIY Audio as a comparison
to check the effect of increasing high frequency open loop gain in respect
of resulting low order intermodulation products.
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Old 30th September 2012, 10:45 PM   #130
Bigun is offline Bigun  Canada
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Sometimes the compensation is intrinsic. The original JLH 10W Class A has no external compensation but relies on the parts choices - using modern high Ft devices can be a problem.
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