Compact Power Amplifier from Elektor

[Tube_Dude]

Hi

Yes it is indeed a current fedback design!!

For another example of this tecnology see:

http://210.166.208.10/photo/e-530_e.pdf

The point here is that current feedback means that the sampled output voltage is injected in current form in the input stage that has very low impedance...
Some people here are confusing with feedback of current where a sample of output current is injected in the input stage...

In the former case the output impedance become small...in the second case the output impedance increase (it become a current generator)...

Regards

[jan.didden]

Well Jorge and others, nice story about the feedback signal injected as a current in the input stage, except that this doesn't make it CF. Voltage feedback means you feed back a sample of the output VOLTAGE (as in this Elektor amp). CF is when you feed back a sample of the output CURRENT.
Look at these circuits:

Fig a shows output voltage from output stage fed back by R and C - BY DEFINITION voltage feedback, even if it goes to the cathode of the first stage (functionally equivalent to the elector buffer output stage emitters), which is a low impedance, so one could indeed argue that the feedback signal is injected into the input stage as current. The point is that the feedback signal is proportional to the output voltage, thus VF...

Fig b : the output current develops a voltage across Rf, that signal is send back: the feedback signal is proportional to the output current, this is BY DEFINITION CF, even if it is fed back to a voltage-input (hi impedance, the grid).

Fig c: Clearly voltage feedback.

Fig d: Both VF from the output signal, and CF from the output current. The unbypassed Rk develops a voltage from tube's current (Ia = Ik) this current gives a voltage across Rk which is put in series with input voltage Es, but in opposite phase (after all, it IS negative feedback).

Fig e: This is the quiz. VF or CF?

Jan Didden

[JensRasmussen]

Originally posted by Tube_Dude

The point here is that current feedback means that the sampled output voltage is injected in current form in the input stage that has very low impedance...

Interresting, so you mean that all BJT (current controlled device)designs using a diff amp input is using current feedback, and FET (voltage controlled device) designs are voltage feedback?

\Jens

[DrG]

janneman: If I get you right then a sample of load current fed back constitutes CF, and the same with voltage and VF, irrespective of the impedance at the NI input.

If you are right then there are 2 problems for which I can only theorize a vague explanation. First problem is how to explain CF op-amps. Seems to me by your definition that they might not exist... and yet they exhibit certain blatant enhancements in performance consistent with CF.

The second problem I have stems from the first. What happens when you use a CF op-amp in inverting mode? The FB is then going to the high impedance input. Is this still CF?

I wonder if the definition of CF is not multifaceted... is the classical definition applicable to the Elektor circuit? Is it valid for CF op-amps? If it is simply that Vout~I(inv) for CF amps, then all can be explained easily. And then the design is indeed CF....

[jan.didden]

Originally posted by DrG
janneman: If I get you right then a sample of load current fed back constitutes CF, and the same with voltage and VF, irrespective of the impedance at the NI input.

If you are right then there are 2 problems for which I can only theorize a vague explanation. First problem is how to explain CF op-amps. Seems to me by your definition that they might not exist... and yet they exhibit certain blatant enhancements in performance consistent with CF.

The second problem I have stems from the first. What happens when you use a CF op-amp in inverting mode? The FB is then going to the high impedance input. Is this still CF?

I wonder if the definition of CF is not multifaceted... is the classical definition applicable to the Elektor circuit? Is it valid for CF op-amps? If it is simply that Vout~I(inv) for CF amps, then all can be explained easily. And then the design is indeed CF....

I know, CF opamps DO exist. But that is a marketing term, that has confused a long standing understanding and definition of CF and VF. The diagrams I posted are from Engineering Electronics, by John D Ryder, Dean of Engineering and Professor of Electrical Engineering at Michigan State University. Published in 1957. (I have later references, but they are in Dutch...)

The blatant improvements in CF you mention are not at all due to CF! They are due to the fact that the feedback node is low impedance. The effect is present in both CF and VF amps, PROVIDED the feedback node is low impedance. Again, if you were in marketing, and your application guru came rushing to tell you he had invented an opamp with a Low Input Impedance Inverting Input, allowing you to feedback current into this low impedance node and giving it very good specs, how would you describe it in the brochure? A LIIIIA or a CFA?

Your second problem actually isn't one. It is the EXTERNAL CIRCUIT that determines whether a circuit is CF or VF. An opamp is just that, and in and of itself cannot be VF or CF. Only when you put in an external FB network you make it into a CF or VF circuit. (Opamps have internal feedback, actually ALL opamps have internal CF because of the degenerated emitter circuits and current sources. But as a black box amplifier they become alive for us users only with an external feedback circuitry).



Jan Didden

[jan.didden]

Originally posted by Tube_Dude
Hi
[snip]The point here is that current feedback means that the sampled output voltage is injected in current form in the input stage that has very low impedance...[snip]Regards

The point here is that you take the liberty to re-define the definition for CF and VF. Like saying: "the point here is that 1 kilometer is 456,7 yards".

And, how would you then call a circuit where the output CURRENT sample is injected in a low impedance node?
Better yet, how about a sample of the output current being injected into a high impedance node?

Jan Didden

[OliverD]

Picture is taken from an Elektor article about their high power amplifier. (a) is conventional, (b) is what they and what I call CFB, as implemented in the schematic from the first post and all current Accuphase gear.

[ojg]

Originally posted by janneman

And, how would you then call a circuit where the output CURRENT sample is injected in a low impedance node?
Better yet, how about a sample of the output current being injected into a high impedance node?

There is indeed four types of feedback circuits. My professor had different names and different symbols (Z,G,H...?) for each. I only remember their general differences:

1. Voltage output sensing creates voltage error signal
2. Voltage output sensing creates current error signal
3. Current output sensing creates voltage error signal
4. Current output sensing creates current error signal

For audio we usually only talk about types 1 and 2. For type-1 we use a VFB op-amp and for type-2 a CFB op-amp. That is why the op-amp manufacturers offers two types of op-amps.

Type 3 and 4 are essentially current-sources with high output resistance and not often used in audio. You can make type 1 and 3 with a VFB op-amp and you can make type 2 and 4 with a CFB op-amp.

Jan is different. If I understand him right, he is calling types 1 and 2 for VFB and types 3 and 4 for CFB independent of which op-amp is used, so his semantics differs from the rest of the world's... It doesn't really matter who is "right", but the rest of the world has come up with a de-facto standard, or marketing term as Jan calls it, and I recomend we stick to it to avoid further confusion.

[DrG]

ojg:

Jan is different... so his semantics differs from the rest of the world's... but the rest of the world has come up with a de-facto standard, or marketing term as Jan calls it, and I recomend we stick to it to avoid further confusion.

DrG:

I wonder if the definition of CF is not multifaceted...

Seems my suspicion was on the money, albeit without the technical explanation. Thanks for clearing that up, ojg.

[DrG]

janneman:

It is the EXTERNAL CIRCUIT that determines whether a circuit is CF or VF.

This is not correct. The external FB circuit only feeds back a portion of the output signal:-

i) If the node into which this is fed is high-Z, then we are dealing with an error voltage - ie type 1 FB circuit listed by ojg.
ii) If the node is low-Z, then an error current is generated - ie type 2.

At no time is the CFB/VFB "mode" of operation determined by the external circuitry comprising the FB loop.

I know of one exception though, the Alexander amp. This uses a conventional VFB op-amp (SSM2131) as a CFB device by taking output from the power pins via current-mirrors, and feeding back amp signal into the op-amp output, which is a low-Z node...