Current feedback - Voltage feedback, how do I see the difference? - Page 70 - diyAudio
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Old 30th March 2013, 06:12 PM   #691
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Quote:
Originally Posted by michaelkiwanuka View Post
The error signal is the difference between the input voltage and the feedback voltage;
Agreed. If the input voltage is changed, that causes a current difference into the collectors of the LTP, as the LTP is a voltagedriven current source.
The inv input is a common base stage from the point of view of the feedback network, so if the feedback voltage changes, it causes a current difference into the collectors etc bla bla.

I think you'd agree that looking into the noninv input you see a fundamentally different topology (common emitter) than looking into the inv input (common base).

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Old 30th March 2013, 06:48 PM   #692
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I think you'd agree that looking into the noninv input you see a fundamentally different topology (common emitter) than looking into the inv input (common base).

jan

I agree, but the fact that the inverting port of the "CFA" sources and sinks current to and from the feedback network does not imply that this current is the error signal driving the amplifier. That's my point.
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Old 30th March 2013, 07:36 PM   #693
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At this point, I don't understand what's the struggle about. I think I've proved that the "CFA" topology properties can be mathematically deduced by using the "VFA" h-parameters formalism (so exactly what Michael says, about sampling voltage at the output and providing the sample to the inverting input to create the error signal. The big difference is that in a CFA topology the feedback network loading at the input node can't be ignored, and it modifies the forward gain and forward UGF. I've also proved that for large closed loop gains (theoretical exercise, since it's kinda stupid to use a CFA for) the CFA topology degenerates to a typical VFA. So indeed a "CFA" category is theoretically not really required, although it can be somehow shoehorned using Mr. Didden et. al. arguments, I see no theoretical or practical damage in doing this.

I also don't believe the linearity of a CFA topology is anywhere worse than a VFA. That's a little harder to prove mathematically, but the number of DSL CFAs with distortions in the ppm range at MHz is witnessing.

CFAs are bad for audio? I haven't heard any serious arguments, based on measurements or controlled subjective tests. I personally have one big argument pro-CFA, the unconditional stability. It is very hard (if possible at all) to build a VFA with high loop gain at HF, which is also unity gain stable. I also like the high speed and high slew rate, but that's my purely subjective opinion.
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Old 30th March 2013, 07:37 PM   #694
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For a typical CFA the current flowing in/out of the inverting input is the difference between the +/- currents driving the high impedance gain node (subtle difference but important since each of these collector currents are more nonlinear than there difference).

This resulting current does impose an error on the feedback voltage as a measurement of the true output voltage.

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-Antonio
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Old 30th March 2013, 07:58 PM   #695
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CFAs are bad for audio?
Not necessarily, but one can do much better with traditional VFA design at audio frequencies.

Quote:
Originally Posted by Waly View Post
It is very hard (if possible at all) to build a VFA with high loop gain at HF, which is also unity gain stable.
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.

Clearly double pole compensation or even "TMC" cannot be used in the usual so-called "CFA" topology.

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I also like the high speed and high slew rate....
Don't these two phrases mean the same thing?

At any rate slew rate is not an issue with competently designed VFAs intended for the audio spectrum.

Last edited by michaelkiwanuka; 30th March 2013 at 08:15 PM.
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Old 30th March 2013, 08:32 PM   #696
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I found some interesting explanations of the CFA topology in several issues of Analog Dialogue. I wrote it up and linked to the pertinent pages from the 1987, 1988 and 1990 journals here (scroll to the bottom of the page).

jan
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Old 30th March 2013, 08:49 PM   #697
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One of those articles describes a source follower as a current feedback amplifier. This is utter nonsense and is on a par with describing a voltage follower as a current feedback amplifier.

A source follower is an example of 100% shunt derived series applied voltage feedback.

http://www.linearaudio.nl/linearaudi...AD_23-3-89.pdf
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Old 30th March 2013, 08:58 PM   #698
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Originally Posted by Waly View Post
I also like the high speed and high slew rate, but that's my purely subjective opinion.
Quote:
Originally Posted by michaelkiwanuka View Post
Don't these two phrases mean the same thing?
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. AS CFAs are essentially "current on demand", the "slew rate" can be very high (agreed, perhaps uselessly high for audio).

BTW, I suppose you are aware you can use a VFA in "CFA mode"? See for example the Alexander amp.

Last edited by Waly; 30th March 2013 at 09:03 PM.
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Old 30th March 2013, 09:11 PM   #699
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Originally Posted by michaelkiwanuka View Post
One of those articles describes a source follower as a current feedback amplifier. This is utter nonsense.
I wouldn't call it "utter nonsense", but a wrong example.

None of the four feedback types and canonical formalisms (hij, yij, zij, gij) are actually mandatory to analyze a feedback circuit. You may characterize a feedback circuit using any of these (or none at all , but only Mason, or Kirchoff, or whatever fancies you) and get exactly the same results (after all, real circuits are deterministic). It's a matter of convention and convenience to use the appropriate formalism, based on the assumed properties of the forward gain amp (which are not always a clean cut, as for CFA topologies).
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Old 30th March 2013, 09:25 PM   #700
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Quote:
Originally Posted by michaelkiwanuka View Post
One of those articles describes a source follower as a current feedback amplifier. This is utter nonsense and is on a par with describing a voltage follower as a current feedback amplifier.

A source follower is an example of 100% shunt derived series applied voltage feedback.

http://www.linearaudio.nl/linearaudi...AD_23-3-89.pdf
Yeah wasn't that a bad example? There's also a section where he says that current feedback is where you feed back a current that is subtracted from or added to an input current.
Funny how they wrestled with this - it's easy to see that everybody was releaved when they coined this 'CFA' term. As you know, acronyms can have 3 letters only

jan
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