Current feedback - Voltage feedback, how do I see the difference? - Page 3 - diyAudio
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Old 21st June 2004, 06:48 PM   #21
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Quote:
Originally posted by mikeks


No....'current feedback' obtain their high slew by running the 'small signal' stage in class AB....

This has nothing to do with the class of operation of the output stage.....
I thought a current feedback stage applies feedback at a point where impedance is low (the emitter of the input transistor for example, vs. the base of the other input transisor in the differential pair).

and as far as I can tell, pretty much all input stages work in class A (within their input signal range, of course).

Quote:
Originally posted by mikeks

No.......it has a voltage feedback input stage.....

He merely chose a single common-emitter stage over the usual diff. stage....
the feedback in the jlh is applied to the emitter of the input transistor. why do you think it is a voltage feedback input stage?

maybe we can consolidate definitions before we can have a more productive discussion.
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Old 21st June 2004, 07:14 PM   #22
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Quote:
Originally posted by mikeks

[snip]No.......it has a voltage feedback input stage.....

He merely chose a single common-emitter stage over the usual diff. stage....
Exactly. The same case can be made for the so-called current-feedback opamps, which in reality are just a variation of a voltage feedback opamp, it is voltage-sensing at the output, voltage subtraction at the input. The variation is that the voltage subtraction is differently.
Another case where marketing succeeded in hi-jacking a perfectly sensible known topology to try to gain some extra market share.

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Old 21st June 2004, 07:32 PM   #23
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Quote:
Originally posted by mikeks
Nobody...but nobody can hear the difference between a current feedback amp. , a voltage feedback amp, etc....
Once again you have avoided answering the question.

Since you apparently actually believe this, why do you:

a) Bother to design audio circuits at all?

b) Waste everyone's time on these forums?

Are you just engaging in public mental masturbation, trying to show that you know more than others?

I am seriously asking this question, as I cannot fathom why you would waste your time trying to "improve" things which (in your judgement) cannot be improved.
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Old 21st June 2004, 07:40 PM   #24
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Quote:
Originally posted by Charles Hansen
[snip]b) Waste everyone's time on these forums?
[snip]

Sorry Charles, beg to differ. Mikek's not wasting MY time, quite the contrary. The contents of most of his posts are quite to the point as far as audio engineering is concerned. Although it is interesting from a historical point of view to read who did what to whom 30-odd years ago, it IS sometimes wasting my time.

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Old 21st June 2004, 07:45 PM   #25
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Quote:
Originally posted by millwood


I thought a current feedback stage applies feedback at a point where impedance is low (the emitter of the input transistor for example, vs. the base of the other input transisor in the differential pair).[snip]the feedback in the jlh is applied to the emitter of the input transistor. why do you think it is a voltage feedback input stage?[snip]

It is voltage feedback because the effective input voltage is the result of the subtraction of two voltages - the input voltage and the feedback voltage.

Why do you think it is current feedback, just because the feedback signal enters a low impedance? What's the limit then? 10 Ohms? 100 Ohms? 1k Ohms? A heavily biased input stage can have a lower impedance at the base than a starved stage at its emitter.

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Old 21st June 2004, 07:47 PM   #26
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Glad you're getting something out of Mikeks postings. I'm still not sure what he's getting out of it, except apparently your admiration. But maybe that's what he wants, I don't know.

Interesting new avatar. Is SY supposed to be able to identify the person standing on their head, or is it some sort of obscure message?
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Old 21st June 2004, 07:51 PM   #27
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Quote:
Originally posted by janneman
Why do you think it is current feedback, just because the feedback signal enters a low impedance?
I'm a bit reluctant to touch this one, as it will most likely lead to another dreary debate about semantics.

However it seems pretty obvious to me what the difference is, and it has nothing to do with impedance.

For example, if you put a 10 ohm resistor from the inverting input to ground of a conventional voltage-feedback amplifier and apply feedback, is it now a "current feedback" amplifier? Of course not.

It is a current feedback amplifier when the feedback is applied to the emitter (cathode, source), because then the current of the feedback signal is added to (or subtracted from) the current of the input stage.
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Old 21st June 2004, 08:06 PM   #28
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Quote:
Originally posted by millwood


I thought a current feedback stage applies feedback at a point where impedance is low (the emitter of the input transistor for example, vs. the base of the other input transisor in the differential pair).

and as far as I can tell, pretty much all input stages work in class A (within their input signal range, of course).

the feedback in the jlh is applied to the emitter of the input transistor. why do you think it is a voltage feedback input stage?

maybe we can consolidate definitions before we can have a more productive discussion.
millwood, if you are truely interested in CFB amps, there are lot's of materials on the net. When I see your answers/questions I notice that you have absolutely no idea what's all about.

Good reading. If you read those, then you can discuss more and you will learn also all definitions.

http://www.linear.com/prod/datasheet.html?datasheet=315
http://mirand.dk/
My own CFB amp
http://www.national.com/an/AN/AN-597.pdf
http://focus.ti.com/lit/an/sboa071/sboa071.pdf
http://focus.ti.com/lit/an/slod006b/slod006b.pdf
http://www.national.com/an/OA/OA-31.pdf
http://www.national.com/an/OA/OA-30.pdf
http://www.national.com/an/OA/OA-25.pdf
http://www.national.com/an/OA/OA-20.pdf
http://www.national.com/an/OA/OA-13.pdf
http://www.national.com/an/OA/OA-07.pdf

Those links were only a selection. If you search for "current feedback" you will find tons of documents.
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Old 21st June 2004, 08:48 PM   #29
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Default inverting vs. non-inverting feedback

Hi Mikeks,

I just saw your post #260, could you please explain your conclusion about no difference between inverting and non-inverting amplifier configurations.

For me, the inverting feedback approach makes use of the inverted signal so that subtraction can be done using just highly linear resistors, any subsequent diff. amp. is just more gain in the forward gain path, just like the VAS and output stage, so is within the feedback loop's ability to minimize its errors.

For non-inverting feedback, the diff. amp. is doing the subtraction. If one of the diff. amp. transistors distorts the signal in a different way than the other, then it is the same as corrupting the input signal reference. (Try putting a diode in series with one of the input diff. amp. transistor emitters and see if you don't get an output offset, or right to the rails if in backwards. Or similarly, use a low resistance tail instead of a current source tail, and watch the distortion go up) And the feedback will not correct this error. The only reason non-inverting feedback passes muster in audio amplifiers is by keeping the difference levels within the 50 mv (or more with emitter degeneration resistors) range of relative linearity before the input diff. amp. begins to display increasing odd order distortion which the feedback blissfully ignores (ie. doesn't correct). Further, if the two diff. amp. transistors are not well matched, one will get even order distortion as well, that will not be corrected by the feedback. (One of the reasons for output offsets, the feedback doesn't see any error because its reference is effectively corrupted.) Also, any distortion processes in these transistors that are emitter current sensitive or collector to base voltage sensitive (unless collectors are cascoded) are not tracking due to the complementary currents, so don't cancel out. On top of all this, there are common mode errors made due to the whole input swinging around. So how can you claim the input diff. amp. stage is within the feedback loop's correction process for the non-inverting gain configuration? Output offsets would be history if this were so, no one would bother to use matched transistors either.

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Old 21st June 2004, 08:56 PM   #30
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Quote:
Originally posted by Charles Hansen
Glad you're getting something out of Mikeks postings. I'm still not sure what he's getting out of it,

janneman: Why is that so important to you, Charles?

[snip]
Interesting new avatar. Is SY supposed to be able to identify the person standing on their head, or is it some sort of obscure message?
It is some sort of message, yes. Note that the person is not so much standing on his head, as much as having his head in a certain position/substance.

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