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Old 14th August 2013, 02:49 PM   #101
banat is offline banat  Serbia
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Originally Posted by forr View Post
I think that, in an LTP, (emitters of same polarity coupled) each transistor can be seen as working in the three configurations all together : EC, BC, CC. A picture of considerable help is to consider that a single transistor works as a differential device and its behaviour mainly depends on the value of the potential difference between its base and its emitter : it is voltage driven by Vbe.

The parasitic capacitances should be defined by their value and their effects described. "High" alone is not informative and comparisons are to be made with a push-pull input.
Hi forr !

My personal view is that in CFB Amp with single input BJT device ( singleton) input device work in same time in common emitter mode for input signal but in common base mode for FB signal , where sum of this two signals appear on collector .
Same operation is valid for BJT complementary input stage pair .

But maybe I`m wrong !

Best Regards !
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Old 14th August 2013, 03:26 PM   #102
dadod is offline dadod  Croatia
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Originally Posted by Bonsai View Post
Here are some of the things about CFA I would like to see discussed and resolved on this thread:-

4. Are comp techniques like TPC, TMC applicable to CFA (I suspect the answer to this is not straightforward)

The focus for all these questions is discrete CFA audio power amplifiers.
Here is VSSA simulation I did a way back in the VSSA thread. It is with MC and TMC compensation.
THD20k with MC only was 0.009388% at 30W/8ohm
THD20k with TMC was 0.002370% at 30W/8ohm
Nobody commented that at that time. Not so close to what is easily achieved with VFA but we can try to improve those figures.
Attached are LoopGain for MC and TMC, and schematic used in the simulation.
BR Damir
Attached Images
File Type: jpg VSSA-LG-MC.jpg (240.7 KB, 490 views)
File Type: jpg VSSA-LG-TMC.jpg (243.5 KB, 456 views)
File Type: jpg VSSA-TMC-sch.jpg (136.1 KB, 468 views)
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Old 14th August 2013, 04:54 PM   #103
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Here is VSSA simulation...
Very nice one and interesting, Dadod.
It seems a little early in this thread to talk about different TPC, TMC, pure Cherry compensation schemes. I hope we will have too the kgrlee participation to this passionating subject as well.
I asked Andrej and he gave the authorization to use his VSSA schematic for simulations.
I'm sure he will be very interested by those compensation studies too.

What do -you think, Dadod, because you yet have done 80% of the work, found the models etc,. to design a VFB version of this amp adding a LTP to the input stage ?
So, we can compare each aspect of the two topologies ?
Then, compensation networks studies will make this thread fantastic.

To finish by an attempt to understand better what are the characteristics related to listening experience.

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Originally Posted by dadod View Post
Nobody commented that at that time.
Welcome in my anti acronym lobby ?
http://www.acronymfinder.com/TMC.html
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Last edited by Esperado; 14th August 2013 at 05:11 PM.
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Old 14th August 2013, 05:52 PM   #104
forr is offline forr  France
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Originally Posted by Esperado View Post
Please, refer on your data-sheets.
They do not tell the story once the components are mounted.
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Old 14th August 2013, 06:39 PM   #105
forr is offline forr  France
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Originally Posted by banat View Post
Hi forr !

My personal view is that in CFB Amp with single input BJT device ( singleton) input device work in same time in common emitter mode for input signal but in common base mode for FB signal , where sum of this two signals appear on collector .
Same operation is valid for BJT complementary input stage pair .

But maybe I`m wrong !

Best Regards !
Hi Banat,

I think your view is right concerning a singleton input amplifier.

Refering voltages to AC ground :

The emitter rougly replicates the voltage of the base.
The input BJT can then be seen as an emitter follower (CC).

If there is no input signal at the base, the loudspeaker acts as a microphone and delivers a signal which is submitted to the feedback network, and after attenuation, to the low impedance at the emitter. The emitter is then an input.
The input BJT can be seen as being in common base (CB).

The AC voltage difference between the base and the emitter controls the collector current which is then exploited to drive the Voltage Amplifier Stage.
The input BJT can be seen as being in common emitter (CE).

The working configuration of the input BJT is not semantically well defined except if we say it works as a combined CC-CB-CE.
This is my personal view of things, not sure everybody shares it.

Considering a BJT as a Voltage Controlled (by Vbe) Current Source (collector current) device (VCCS), albeit simplified, makes many circuits easy to understand.
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Old 14th August 2013, 08:18 PM   #106
banat is offline banat  Serbia
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forr

Excuse me if I made some gramatic mistakes since my English is pretty poor !

But must say that generally I share your views !

Actually CFB `singleton` input BJT device principle of operation remind me pretty much on principle of operation of additive RF mixing by input triode tube in RF receivers ,
where signal from antena is feed on the relative high impedance triode grid ( BJT base ),
but in the Same Time from separate oscillator unit signal is feed in to the relative low impedance triode cathode ( BJT emitter ) , sum of this two signals appear on triode anode ( BJT collector ) , later that signals sum output being further amplified , detected , ....etc .
But of course this additive signal mixing principle by Audio CFB Amps works different , since here we mix just two audio signals , one from Amp input and one from Amp output , and not just only that two audio AC signals but two DC signals to .
I think that this same principle of operation it is implemented by complementary input BJT pair by symmetric CFB Amps , for example by VSSA .

Last edited by banat; 14th August 2013 at 08:25 PM.
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Old 14th August 2013, 09:11 PM   #107
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Originally Posted by banat View Post
by symmetric CFB Amps , for example by VSSA .
With a single transistor in CFA, you cannot achieve a DC coupled input and feedback in the same time, because the voltage drop in the p-n junction between the base and the emitter.
Dividing the serial resistance in two paths in a symmetrical implementation, like in the Nelson Pass F5 or the VSSA makes those DC +0.6V and -0.6V to cancel themselves across the serial feedback resistances, allowing direct coupling both for input and feedback. A simple and clever idea.
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Old 14th August 2013, 09:15 PM   #108
dadod is offline dadod  Croatia
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Quote:
Originally Posted by banat View Post
forr

Excuse me if I made some gramatic mistakes since my English is pretty poor !

But must say that generally I share your views !

Actually CFB `singleton` input BJT device principle of operation remind me pretty much on principle of operation of additive RF mixing by input triode tube in RF receivers ,
where signal from antena is feed on the relative high impedance triode grid ( BJT base ),
but in the Same Time from separate oscillator unit signal is feed in to the relative low impedance triode cathode ( BJT emitter ) , sum of this two signals appear on triode anode ( BJT collector ) , later that signals sum output being further amplified , detected , ....etc .
But of course this additive signal mixing principle by Audio CFB Amps works different , since here we mix just two audio signals , one from Amp input and one from Amp output , and not just only that two audio AC signals but two DC signals to .
I think that this same principle of operation it is implemented by complementary input BJT pair by symmetric CFB Amps , for example by VSSA .
This is very bad analogy, mixing in heterodyn receivers used unlinear behavior of triode (or hexode) to produce sum and difference frequencies and for negative feedback nonlinearity is not good thing. With NFB two very similar signals are subtracted and result is error signal.
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Old 14th August 2013, 09:49 PM   #109
banat is offline banat  Serbia
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Hi Esperado !

By term -DC coupled input - did you mean that audio AC signal goes straight to the BJT`s input complementary pair bases without of using of any input coupling capacitor ?

Best Regards !
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Old 14th August 2013, 09:55 PM   #110
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Hi Esperado !

By term -DC coupled input - did you mean that audio AC signal goes straight to the BJT`s input complementary pair bases without of using of any input coupling capacitor ?
Yes you can.
The bases are at 0V, both emitters at 0.6 and -0.6V, means the common point of the two serial feedback resistance at 0V (-0.6+0.6) that you can connect to the output of the amplifier.
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Last edited by Esperado; 14th August 2013 at 10:00 PM.
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