mikeks said:
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).
mikeks said:
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.
mikeks said:
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.
Jan Didden
mikeks said:
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.
Charles Hansen said:
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.
Jan Didden
millwood said:
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.
Jan Didden
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?
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?
janneman said:
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.
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.millwood said:
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.
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.
Don
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.
Don
Charles Hansen said:
Funny that you say that. Let me switch sides, to (hopefully) demonstrate the futility of trying to arbitrarily classify some circuits, that depend on voltage subtraction, as *current feedback*.
In what most people call "conventional voltage feedback", the diffence input signal is created from the substraction of the two input currents in the input transistors (assuming BJT's for this moment). So, are really *all* opamps current feedback?
Jan Didden
Charles H, it is good to be reminded that the more things change the more they stay the same. One way that cliché applies is in that people make discoveries and apply them to "create" things when they are really forming some type of assemblage, either abstractly or concretely.
I consider your definition of current feedback to be both elegant and concise and appreciate such input. I thank all the voices of experience which contribute to the forum.
I consider your definition of current feedback to be both elegant and concise and appreciate such input. I thank all the voices of experience which contribute to the forum.
janneman said:
I don't know if "important" really applies to any of the goings-on in this forum. Nonetheless I find it Mikeks entire position rather odd.
It would be like going to a beer-making forum and finding some poster vociferously debating the finer points of zymurgy, making claims, arguing and chastising, only to find out that the poster actually believes that all beers taste the same.
I don't have a problem with someone that believes all beers taste the same. But I don't understand why they would feel compelled to discuss beer making with people who believe there are differences between beers. Seem "off" to me...
janneman said: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.
It's so tiny I can barely make it out. I loaded it into a bitmap editor and blew it up. If I squint, it looks to me like the guy in the foreground has his head buried in the earth. If that's the case, I wonder how he breathes...
janneman said:
If you want to be deliberately obtuse, yes.
And in that same way, all circuits have feedback.
And there is no such thing as "black" and "white", so everything is really just grey.
And there is no such thing as "hard" or "soft", so diamonds and feathers are both "somewhat malleable".
And on and on and on...
Charles Hansen said:
I guess one could just as well ask why you debate feedback
vs. non-feedback amps since you seem to have your mind
made up about this. Most, but not all, people tend to have
a more or less strong belief in a particular standpoint, yet
they debate it. Why do rightwing and leftwing politicians
bother to debate with each other. They know they are not
going to change each others opinion, yet they findt it
worthwhile to grind their opinions against each others.
Even if you don't convince the other part that you are right,
you may sometimes make them understand a little better
why you have your standpoint. Would there be any debates
at all or any progress at all if people didin't argue for their
point of view? Personally, I find it interesting to see what
arguments people are able to come up with to support their
own point of view.
Regarding VFB vs. CFB, I think it is important to note what
Eva said recently in another thread. There are no true VFB
or CFB amps in reality. However, I think it still makes sense
to discuss whether a particular amp is designed with the
intention of being VFB rather than CFB or vice versa. Always
viewing an amplifier as a mix of them both or something in
between complicates the analysis, and if it is VFBish enough
or CFBish enough, one can usually approximate most analyses
with the corresponding theoretical mode. As for CFB, at least
all papers I have read model them as having a unity gain
input buffer with a infinite-impedance input which is the positive
input and a zero-impedance output which is the negative
input (an output being used as input probably causes some
confusion), then the supply current to the buffer is used to
create the output signal. I think this model is pretty clear
although one could probably misunderstand it and misuse
it if playing the devils advocate.
Christer said:
There is a very big difference here, in my opinion. With regard to no feedback I am debating what I believe to be true, based on my direct experience. (There is also another lesser point, which is that very few people have had the opportunity to decide for themselves which is better, as zero-feedback amps are not too common yet. Also I am trying to spread the good news to the DIY crowd. In general it costs no more to build a zero-feedback amp compared to a feedback amp.) Similarly, Pavel believes that high-feedback amps sound better, based on his direct experience. And you are right, we may never change each other's minds, especially if we don't have the opportunity to sit in the same room, listening to the same system, and share our experiences with each other.
But Mikeks seems to be debating something in which he believes not at all. It would be as if I were to passionately debate Pavel on the merits of feedback (or lack thereof) when I felt that there was no audible difference either way. Why would I want to do that?
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