Re: Re: Re: Re: Re: Who's censoring the internet???
If you want to make a family tree of this, you must include the current conveyors. And the initial paper on them appeared 1968.
See
http://www.macs.ece.mcgill.ca/~roberts/ROBERTS/PUBLICATIONS/BOOK_CHAPTERS/Current_Conveyor_90.pdf
for a summary article.
Regards,
Peter Jacobi
Christer said:
If you want to make a family tree of this, you must include the current conveyors. And the initial paper on them appeared 1968.
See
http://www.macs.ece.mcgill.ca/~roberts/ROBERTS/PUBLICATIONS/BOOK_CHAPTERS/Current_Conveyor_90.pdf
for a summary article.
Regards,
Peter Jacobi
traderbam said:
Traderbam,
No, we are not at cross-purposes. I agree that most usefull (for me) is describing the INFORMATION. And your listing gives clear indication of what the amp is doing. It encloses full info on the amplifier's transfer function. A transconductance amp is a voltage dependent current source. Clear, no confusion. How do you make such an amp? You take a voltage amplifier and enclose it in a current feedback loop - the old-fashined definition: take a sample of the output current and compare it in the input stage to make sure the output current is really only depending of the input voltage. So, your definition of the impedance/etc transfer functions seamlessy connects to the "old" definitions of VF and CF.
Now go to the new definition of CF (to a low-impedance node): it really doesn't tell me diddly about the amp. OK, it tells me that probably I can set the BW with very little influence on the Gain, but what DOES this amp? V to I, V to V, I to V, whatever. No idea. With the old def, telling me it is a VF or CF amp I immediately know what the amp puts out.
Jan Didden
Re: Re: Re: Re: Re: Re: Who's censoring the internet???
Thanks for the link. I just had a brief look at it, but as I understand,
what we now call CFB amp is essentially the 2nd generation
current conveyor, CCII, first officially published in 1970. So, now
we have an alternative name which doesn't clash with old
terminology. I am afraid it would however cause new confusion if
we started to use that term instead, at least here on the forum.
It seems to be used in academia, though -- I stumbled upon some
papers mentioning CC and CCII and other things, but didin't
attempt to check out what it was.
pjacobi said:
Thanks for the link. I just had a brief look at it, but as I understand,
what we now call CFB amp is essentially the 2nd generation
current conveyor, CCII, first officially published in 1970. So, now
we have an alternative name which doesn't clash with old
terminology. I am afraid it would however cause new confusion if
we started to use that term instead, at least here on the forum.
It seems to be used in academia, though -- I stumbled upon some
papers mentioning CC and CCII and other things, but didin't
attempt to check out what it was.
Christer,
Such a thing was actually discussed here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=3231&highlight=PA630
The PA630 was an independent development (well, as far as independent development is still possible) by a clever engineer from Canada, Doug Wadsworth of Phototronics. In the mid 90-ies I contacted him and actually bough some PA630's and PA630A's (which have an output current buffer). These are CCII's, primarily for use in I/V converters for DACs. After several years of trying to interest the big guys, Doug sold the patent and remaining stock to Wadia. They used it in a zero-feedback I/V converter. I still have what must be the world's only remaining PA630 stocks!
I used them in a zero-fb preamp (the PA630 also has a voltage input) so this was a straight voltage-to-current amp. You then set the gain by selecting the load resistor.
I'll look up the data sheet with the internal construction - interesting.
Jan Didden
Such a thing was actually discussed here:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=3231&highlight=PA630
The PA630 was an independent development (well, as far as independent development is still possible) by a clever engineer from Canada, Doug Wadsworth of Phototronics. In the mid 90-ies I contacted him and actually bough some PA630's and PA630A's (which have an output current buffer). These are CCII's, primarily for use in I/V converters for DACs. After several years of trying to interest the big guys, Doug sold the patent and remaining stock to Wadia. They used it in a zero-feedback I/V converter. I still have what must be the world's only remaining PA630 stocks!
I used them in a zero-fb preamp (the PA630 also has a voltage input) so this was a straight voltage-to-current amp. You then set the gain by selecting the load resistor.
I'll look up the data sheet with the internal construction - interesting.
Jan Didden
How about this ?
Current feedback voltage amp
Current feedback transconductance amp
Current feedback transimpedance amp
Current feedback current amp
Voltage feedback voltage amp
Voltage feedback transconductance amp
Voltage feedback transimpedance amp
Voltage feedback current amp
traderbam said:
Current feedback voltage amp
Current feedback transconductance amp
Current feedback transimpedance amp
Current feedback current amp
Voltage feedback voltage amp
Voltage feedback transconductance amp
Voltage feedback transimpedance amp
Voltage feedback current amp
Bernhard,
Yes, I agree with you in a sense that we need eight cases, at least.
The usual four cases (those that Jan listed) assumes that either
both inputs are high impedance or both inputs low impedance,
while the CFB amp has one high and one low impedance input.
All,
Further, one must never forget that all such classification are
theoretical constructions that make assumptions. The ideal
op amps can fit the different cases. Real opamps are what they
are claimed to be only under certain assumptions. High (low) input
impedance, for instance, is not really a property of the op amp
itself, since this depends on the source impedance too. A bipolar
VFB op amp, for instance, might turn into a low-impedance input
amp (relative to the source) if driven from an extremely high
impedance source. Hardly a common case, but I am sure such cases
exist. Then it is suddently not a voltage amp any more but a
transimpedance amp, or even a current amp if we also attach it
to an extremely low-impedance load.
Yes, I agree with you in a sense that we need eight cases, at least.
The usual four cases (those that Jan listed) assumes that either
both inputs are high impedance or both inputs low impedance,
while the CFB amp has one high and one low impedance input.
All,
Further, one must never forget that all such classification are
theoretical constructions that make assumptions. The ideal
op amps can fit the different cases. Real opamps are what they
are claimed to be only under certain assumptions. High (low) input
impedance, for instance, is not really a property of the op amp
itself, since this depends on the source impedance too. A bipolar
VFB op amp, for instance, might turn into a low-impedance input
amp (relative to the source) if driven from an extremely high
impedance source. Hardly a common case, but I am sure such cases
exist. Then it is suddently not a voltage amp any more but a
transimpedance amp, or even a current amp if we also attach it
to an extremely low-impedance load.
Interesting current feedback amp here:
http://www.stephenwmoore.com
under the Current-Feedback Amp link
http://www.stephenwmoore.com
under the Current-Feedback Amp link
Ouroboros said:
No it is correct. It is current feedback, but according to the "old"
way of using the term. Haven't you guys got it yet, there is no
single truth here. Several different interpretations of the same
terminology exist alongside each other. There is not much we can
do about it, however confusing it is.
Sure, it's an amplifier that is defining an output current, but it is doing so by feeding back into the amp, a VOLTAGE that is proportional to the output current, sensed across a low value resistor.
It may be overall a transconductance amp, but it is still using voltage feedback.
(you can of course design a transconductance amp using current feedback, but this isn't one, and I wonder what the stability of such an amp might be).
I take your point about the confusion of nomenclature, but I don't think an amp that senses the output current by feeding back a proportional voltage should be called 'current feedback'.
It may be overall a transconductance amp, but it is still using voltage feedback.
(you can of course design a transconductance amp using current feedback, but this isn't one, and I wonder what the stability of such an amp might be).
I take your point about the confusion of nomenclature, but I don't think an amp that senses the output current by feeding back a proportional voltage should be called 'current feedback'.
Ouroboros said:
I am not supporting the old terminology, just persist in pointing out
that both terminologies exist in parallel, so it is wrong to claim that
it is not a current feedback amplifier. It is, however, wrong to say
that it is a current feedback amplifier in the modern sense of the
term. I also question whether the old uses of the terms voltage
and current feedback are appropriate. I do not find them as logical
as Jan does. Is the modern use of the term more appropriate? I
am not so sure. It is not for us to standardize the terminology,
however, so we have to live with it. Like it or not.
It intersting to note, though, that the author has taken his
actual diagram from the APEX app. note which does not use
the term "current feedback" at all, but refers to this as a "VCCS"
which seem correct. More surprisingly, he also refers to the Intersil
app note 9420 which is about CFB op amps in the modern sense,
so I suppose this guy is confused or unaware of the different
terminologies.
Tube_Dude said:
Do you mind taking a trip to a -10 deg. C. Sweden? I can serve you
an espresso to warm you up from the cold while you have a
look in my old textbook to see that I am indeed not lying to you.
You can have a cognac with the coffee too if you wish.
Yes, the classification used to distinguish the four types of
amplifiers you list, but the terminology was different. In my
book a transconductance amplifier is referred to as a "current
feedback voltage error amplifier". I am not so sure it is even
possible to fit the modern meaning of CFB into this classification,
since it has different-impedance inputs.
The term 'current feedback' in its modern use is perfectly reasonable because the feedback summation is done by subtracting a feedback current (flowing through the feedback network), from an input current (from the input transconductance buffer). The result of this subtraction is an output current which is then the internal error signal feeding into the current-mirror stage.
I'm biased I guess, because I never have used the term to refer to an amp that performs an overall VCCS function!
I'm biased I guess, because I never have used the term to refer to an amp that performs an overall VCCS function!
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