Current feedback - Voltage feedback, how do I see the difference?
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Waly
diyAudio Member

Join Date: Jan 2011
Location: In the lab
Quote:
 Originally Posted by michaelkiwanuka We need to be clear what "HF" means: from my perspective it's not hard to build a VFA with relatively high loop gain at the top end of the audio band, siginicantly higher than that provided by a so-called "CFA" for comparable stability margins. Just use double pole compensation..
Take your own example: 60dB loop gain at 60KHz. This makes (for a single pole) a ULGF of 60MHz. That wouldn't be possible anywhere close for a power amp.

In a VFA, given a single pole non-compensated ULGF the only way to increase the loop gain at (say) 60KHz is to insert N poles and then N-1 zeroes to bring the phase back to the Bode stability condition. The larger N, the steeper the rolloff of the loop gain, but anything with N>3 is a practical nonsense, at least because of implementation and thermal stability issues. The only realistic example with N=3 I have ever seen is the Cherry NDFL topology.

Now show me a N=2 topology (TPC, TMC, whatever you fancy) with 60dB of loop gain at 60KHz and an ULGF of (an optimistic) 3-4MHz and I'll be fully in your VFA boat . Or with N>=3 which is also unconditional stable.

diyAudio Member

Join Date: Apr 2006
Location: Zagreb
Quote:
 Originally Posted by michaelkiwanuka Clearly double pole compensation or even "TMC" cannot be used in the usual so-called "CFA" topology.
Why not, look here. http://www.diyaudio.com/forums/solid...ml#post3434395

Waly
diyAudio Member

Join Date: Jan 2011
Location: In the lab
Quote:
 Originally Posted by dadod Why not, look here. http://www.diyaudio.com/forums/solid...ml#post3434395
Because of the x20 feedback resistor (R5/R6) over the emitter resistors (R36/R40), this is barely a real "CFA", but more like "a CFA topology in VFA mode" as described above. 1kohm barely loads the input node and therefore modifies the forward gain (through the transconductance of the input stage).

michaelkiwanuka
Banned

Join Date: Oct 2010
Location: Chigwell, Essex
Quote:
 Originally Posted by dadod Why not....
Because minor loop compensation of the shunt derived shunt applied variety can only be implemented around an inverting gain block (transimpedance stage). Nothing of the kind is to be found in the usual so-called "CFA".

michaelkiwanuka
Banned

Join Date: Oct 2010
Location: Chigwell, Essex
Quote:
 Originally Posted by Waly Take your own example: 60dB loop gain at 60KHz.
I think you mean 60dB at 20KHz....But that was just an approximation.

michaelkiwanuka
Banned

Join Date: Oct 2010
Location: Chigwell, Essex
Quote:
 Originally Posted by Waly No. IMMV, but to me the difference is like the small signal slew rate (aka "speed") vs. the large signal slew rate (aka "slew rate"). The former is a measure of the bandwidth, the latter is a measure of a non-linear input stage limiting effect.
There is no such thing as "small signal slew". There is only one definition for slew rate and power bandwidth, and it can be found in any good analog electronics textbook.

I suspect, however, that what you're alluding to is the distortion arising from the increased loading of the minor loop compensation network with increasing frequency on the transadmittance stage (the LTP) in a VFA. Some have called this "slewing induced distortion", but this is not a very descriptive term in my view-it will have to do, nevertheless.

michaelkiwanuka
Banned

Join Date: Oct 2010
Location: Chigwell, Essex
Quote:
 Originally Posted by Waly BTW, I suppose you are aware you can use a VFA in "CFA mode"? See for example the Alexander amp.
I know the Alexander amp, which is really a voltage feedback amp. in all respects.

But, perhaps, I do not understand what you mean by "...you can use a VFA in "CFA mode"?

Waly
diyAudio Member

Join Date: Jan 2011
Location: In the lab
Quote:
 Originally Posted by michaelkiwanuka I think you mean 60dB at 20KHz....But that was just an approximation.
Ok, say 60dB @ 20KHz or 50dB @ 60KHz, is still a 20MHz ULGF, unattainable for a power VFA... 6dB extra loop gain for a two pole compensation, still to high for a power VFA.

Waly
diyAudio Member

Join Date: Jan 2011
Location: In the lab
Quote:
 Originally Posted by michaelkiwanuka I know the Alexander amp, which is really a voltage feedback amp. in all respects.
Here we go again . Not gonna bite the bait this time, good night .

 30th March 2013, 10:32 PM #710 michaelkiwanuka   Banned   Join Date: Oct 2010 Location: Chigwell, Essex Good night me old mate!

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