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Old 16th March 2013, 12:04 AM   #21
Waly is offline Waly  United Kingdom
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Quote:
Originally Posted by jan.didden View Post

Your interpretation is the newer interpretation that came into swing after the 'CFA' came onto the market.
I'm for sure way to young to know about the former definition you mentioned. But my point is that there's nothing marketing related in the current definition. It's just a simple interpretation of a circuit theory formalism.
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Old 16th March 2013, 12:06 AM   #22
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Actually there is nothing new about feeding back a divided voltage to a low open-loop impedance node. Most 1950's valve amplifiers do exactly that by feeding back to the cathode of the first valve. No-one called that current feedback back then, because current feedback only had the 'traditional' meaning that Jan explains in posts 15 and 18.

To make everything even more confusing, Nordholt reserves the term current feedback for amplifiers that take a sample of the output current and subtract that from the current of the input signal source. That is yet another way of using the term.
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Old 16th March 2013, 12:12 AM   #23
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Quote:
Originally Posted by Waly View Post
I'm for sure way to young to know about the former definition you mentioned. But my point is that there's nothing marketing related in the current definition. It's just a simple interpretation of a circuit theory formalism.
At some moment in history, someone started using the then well-defined term current feedback to mean something it had never meant before, thereby creating a lot of confusion. I guess Jan's point is that someone who is stupid enough to do that must have been a marketeer (-;
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Old 16th March 2013, 01:16 AM   #24
Bonsai is offline Bonsai  Taiwan
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Well, up until a few weeks ago there was no confusion in the minds of most people as to what the two types of topology were, and certainly in the semi industry there is no confusion. CFA as used in hundreds of opamp data sheets over the last 30 odd years should attest to that. We seem to have gotten into a semantic argument here on DIY.

Firstly, rather than look at it from the input to output voltage view, let's look at what's happening with the feedback network. Here is how I understand it from an intuitive POV.

VFA
You provide a sample of the output voltage to the inverting input by dividing it down, and then feeding it into a diff pair, which in turns steers the LTP current into the TIS input through the LTP collector load resistors, or a mirror. If you keep the ratio of the feedback resistors constant, (within reason) you do not affect the rise fall time or the bandwidth of the amplifier. These are controlled by the LTP current and Cdom. Note, that if you change the feedback network ratio, you can affect the stability, but that is a different issue. Importantly, the maximum practical current into the TIS input stage is limited by the LTP current source.

CFA
In the CFA case, you also sample the output voltage, but the primary feedback quantity to the feedback node is current. Why? Because this feedback current, set of course by the value of the feedback resistor and its associated lower leg to ground, directly moderates the TIS drive via the emitters of the input buffer summing junction. There is no interposing LTP stage. If you change the resistance of the feedback network, but keep the ratio constant, you can affect the rise fall times and the loop bandwidth. Hence, on CFA's the feedback resistor is the recommended way to compensate for loop stability. And that is why is called a CFA. Note, that because of the use of low feedback resistor values, the drive into the TIS input stage can be much, much higher than a VFA - hence the great speed of CFA's. You could do this on a VFA, but the LTP current would have to be very high (100 mA) - a totally impractical proposition. VFA designers can get around this by going for MIC, so I am just pointing this out to try to highlight the differences.

The two compared:-
VFA- higher DC gain, lower offsets, lower noise; generally, slightly more complex (generally!). Bandwidth related to closed loop gain.

CFA - lower gain, wider bandwidth, higher offsets, faster rise fall times. Bandwidth, within reason, not related to closed loop gain.

That's how I understand it. I heard somewhere, IIRC, that Scott Wurcer rewrote the loop equations for CFA to express them more fully in current terms, but I may have got my wires crossed on that one.
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Last edited by Bonsai; 16th March 2013 at 01:19 AM.
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Old 16th March 2013, 01:21 AM   #25
Bonsai is offline Bonsai  Taiwan
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At some moment in history, someone started using the then well-defined term current feedback to mean something it had never meant before, thereby creating a lot of confusion. I guess Jan's point is that someone who is stupid enough to do that must have been a marketeer (-;

Nonsense. Stop hammering marketers.
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Old 16th March 2013, 01:27 AM   #26
CBS240 is offline CBS240  United States
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Not to be windy about it, but I have found that using a common mode amplifier to provide a common mode error correction can greatly enhance the linearity in operation of the CFA input stage.



Ohhhh!! but at the expence of more complexity.....
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Old 16th March 2013, 05:51 AM   #27
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Quote:
Originally Posted by Ken Newton View Post
To me, perhaps, the most relevant operational distinction is that in VFB, the feedback input electrode typically drives the base/gate of a grounded emitter/source stage. In CFB, the feedback input electrode typically drives the emitter/source of a grounded base/gate stage.
To me the fundamental difference, the 'difference that makes a difference' is the asymmetry of the input stage. VFB has symmetry between + and - inputs and CFB does not. There might though be exceptions to the asymmetry come to think of it - the LM3900 springs to mind I suppose a CFB amp with both + and - inputs as current (low impedance) still counts as CFB.
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Old 16th March 2013, 06:55 AM   #28
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Originally Posted by abraxalito View Post
To me the fundamental difference, the 'difference that makes a difference' is the asymmetry of the input stage. VFB has symmetry between + and - inputs and CFB does not. There might though be exceptions to the asymmetry come to think of it - the LM3900 springs to mind I suppose a CFB amp with both + and - inputs as current (low impedance) still counts as CFB.
This is one reason I started playing with a CM amplifier stage, to mitigate asymmetrys of the input transistors, straighten out the transfer per se.
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Old 16th March 2013, 06:55 AM   #29
Bonsai is offline Bonsai  Taiwan
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Jan Didden wrote
"There might be a semantic issue here. Traditionally, the name 'current feedback' has been given to the kind of feedback that returns a sample of the output current. This could be for instance the voltage across a small resistor in series with the load. Current feedback thus makes the output impedance very high.

With the new so-called 'CFA' opamps that appears to have taken on a new meaning (blame it on the marketeers) namely: feedback to a low impedance node. Personally I think this is an ambiguous way to name it because, what's a low impedance node? The emitter impedance of a starved BJT can be pretty high, while 'voltage feedback' to an invering input fed from a low-impedance source may be pretty low impedance."

I would call that current sense mode feedback amplifier and indeed the output quantity controlled is the current. I do recall pointing this out somewhere else on the forum a few years ago.

Going back to VFA and CFA point a little earlier in this thread, note that they are both still voltage amplifiers - voltage in, voltage out. Might also be that's where the confusion is.


Note also that CFA opamps are not new. The first integrated circuit devices were produced by Elantec and Comlinear in the late 1980's and the CFA term was already used then to describe them. I recall reading that the Japanese were already using a similar topology in discrete designs earlier than this - but cannot be sure. When I was still doing linear design in the 1980's, the was a flurry of papers and articles about the subject - Sergio Franco wrote a very nice article in EDN that I read.

Wiki is your friend : http://en.wikipedia.org/wiki/Current...onal_amplifier
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Last edited by Bonsai; 16th March 2013 at 07:05 AM.
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Old 16th March 2013, 08:39 AM   #30
Bonsai is offline Bonsai  Taiwan
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One other point that should be emphasized is that because the VFA front end tail current is quite low, you will get a pole formed the the LTP current an the TIS input capacitance. Since the VFA open loop gain is much higher, and you generally have to consider the output stage pole, you would end up with excess phase and instability. The cure of course is to lower the TIS pole frequency with Cdom so you get 20 dB/ decade slope and a ULGF intercept below the output stage pole.

The CFA does not have this issue, because the current available to charge and discharge Cdom is usually very high, you you get a much wider bandwidth and slew rates.

In power amps, you normally have to apply
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