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Old 26th September 2012, 01:31 PM   #71
DF96 is offline DF96  England
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Quote:
Originally Posted by Elvee
That is quite surprising: R1 does have a huge impact on many aspects of the circuit, the linearity, loop gain, slew rate etc.
R1 sets the quiescent current through Q1. Alternatively, it sets the point where Q2 begins to turn on. Boosting the current in Q1 boosts the transconductance of the pair, which is roughly given by (Q1 gm) x R1 x (Q2 gm) at low currents and (Q1 gm) x (Q2 beta) at higher currents. The transition between these two regions depends on where the input resistance of Q2 starts to load R1. [Roughly, R1 ~ (Q2 beta) / (Q2 gm)]

In each case, gm varies with current in the usual BJT way.

I wrote an article about this for Wireless World about 10 years ago, but unfortunately it is not on the web and I think they still have copyright in it.
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Old 27th September 2012, 04:56 AM   #72
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...I wrote an article about this for Wireless World about 10 years ago, but unfortunately it is not on the web and I think they still have copyright in it.
That would be interesting for many here to read, DF. It seems absurd that an inaccessible archive forbids further distribution of its content for 25 years. Then there is your own perpetual copyright over the material. D. Self has the same issue with posting even ancient tube material from E&WW magazine on his website.

Cyril Bateman's material is everywhere about the web now so perhaps Svetlana would entertain the idea of posting the material on your site?
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Old 27th September 2012, 05:42 AM   #73
RNMarsh is offline RNMarsh  United States
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let me throw in a couple things to think about --- Most VFB amps have a rising thd as freq goes up. Cfb amps have a flat thd over a wide freq range. In power amps especially, the high freqs are not as clean sounding as the lower freqs. But cfb circuits have the same sound character across the bandwidth. Cfb circuits dont depend so much on a Cdom. -RNM
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Old 27th September 2012, 07:13 AM   #74
catalin is offline catalin  Romania
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Wink Phase deviation at low F and THD at high F

Fact 1:
Nobody talks about transfer functions; about that we have a first pole at low freq in VFB and the first pole in CFB is more more higher .This will shift the phase for VFB below 20khz with -90 degrees ,but in CFB ,because the pole is at a freq very high ,we have no phase shifting in all audio band !
So even the gain is high at VFB at low freq ,the phase is shifting from 0 to -90 in audio band ;It starts with 180 in DC to very low freq ,then at about 10-200Hz we have 45 degrees(the pole) ,and after we have 90degres at 1-3 khz.
All these in open loop .

So if the designers consider the open loop gain for thd why they don't consider also the open loop phase ? Why accuphase amps with CFB sounds better ?

Also phase for negative feedback input in VFB is turned by 2 poles but in CFB is turned only once and this is at high freq.If in VFB we have 2 common emitter stages for the negative feedback ,in CFB we have a common emitter and a common base .

So logical is to choose the circuit with less transformation of the negative feedback signals .Because the non-linear distortion is important .

So the CFB at this hour has high speed ,high bandwith ,excelent phase response .
The VFB has speed but not as much as CFB ,low bandwith ,poor phase response .
May be we don't need much speed,may be 50V/us is correct for 100W SACD information .But why not double it ?

Fact 2 :
What about the THD ?
Well , we can say that the THD is almost the same with VFB and CFB ,but at low freq .
Is more important the THD at high freq than at low freq ?
I am thinking only at the fact that the low frequencies in music is at least 3-5 times higher in amplitude than the high freq and that the SNR is also higher at low F and low at high F.
So to have a good separation between the instruments we need more low thd at high freq compared to the thd at low freq .The small amplitude signals in music are the high freq signals majority !

When we listen music we are searching to have good separation of instruments ,we are searching also to hear the complex pieces clear ,clear means that the high amplitude signals don't distort the small signals .The brain recognize the high amplitude easy but the low amplitudes (which is the high frequencies ) is a little more hard to understand .Also the small amplitude signals(HF) have a smaller SNR .
So then we need more accuracy at high frequency .

And here again the CFB has a lower thd at high freq compared with the VFB .

Fact 3.

Negative feedback circuit should have a high speed ?
What speed will have the negative feedback signal to compensate the output signal in CFB compared to VFB ?


So what topology should we choose ?
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Last edited by catalin; 27th September 2012 at 07:40 AM.
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Old 27th September 2012, 08:03 AM   #75
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by catalin View Post
Fact 1:
..../....
The fact of labelling preconceptions, personal beliefs and prejudices as facts doesn't change their nature or make them true.
That is a fact
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Old 27th September 2012, 08:57 AM   #76
catalin is offline catalin  Romania
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Originally Posted by Elvee View Post
The fact of labelling preconceptions, personal beliefs and prejudices as facts doesn't change their nature or make them true.
That is a fact
Elvee ,I think that all 3 "preconceptions" are theoretical correct .
Please say where I am wrong in these theories .
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Old 27th September 2012, 10:17 AM   #77
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Originally Posted by catalin View Post
Elvee ,I think that all 3 "preconceptions" are theoretical correct .
Please say where I am wrong in these theories .

Catalin next someone else will post a VFB opamp with better specs than CFB ones in another lame attempt to convince or mislead, what they do not say is that these VFB opamps consist of three opamps in one chip like the one Waly posted to manage what one CFB opamp can do.
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Old 27th September 2012, 10:52 AM   #78
dadod is offline dadod  Croatia
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Originally Posted by homemodder View Post
Catalin next someone else will post a VFB opamp with better specs than CFB ones in another lame attempt to convince or mislead, what they do not say is that these VFB opamps consist of three opamps in one chip like the one Waly posted to manage what one CFB opamp can do.
There were many VFB amps simulation with THD20k in ppm or sub ppm range(Edmond's is one of them)) presented here, but I never saw CFB simulation here in that distortion level, so maybe you could present such simulation. What you mean by "better specification", it could be many things?
dado
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Old 27th September 2012, 11:02 AM   #79
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And now for some more interesting news on CFB topologies, hope elvee doesnt mind.

For years me and the world have been under the impression that comlinear was the developer of the famous CFB topology that uses the diamond buffer as input. They also were the first to patent it in 1985 if Im correct and hold more than 20 other patents regarding CFBs.

Yesterday I got proof of what I have been was suspecting for a while now, a Japanese company had a amp on sale in 1980 with that topology and quite surprisingly very advanced form of that topology. Take about two years for development and production and Id guess the topology dates from about 1977. This company is Pioneer. One has to wonder where comlinear got the idea whether themselves or ...... In those days circuit topologies were a sacred secret in japan and its very difficult to find schematics of their amps which were sold only in their homeland.

Here is a preview although I hope to get the rest of the schematic into electronic form.
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Old 27th September 2012, 11:09 AM   #80
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Originally Posted by dadod View Post
There were many VFB amps simulation with THD20k in ppm or sub ppm range(Edmond's is one of them)) presented here, but I never saw CFB simulation here in that distortion level, so maybe you could present such simulation. What you mean by "better specification", it could be many things?
dado
Just look at the CFB opamps by National
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