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Fair enough, I'll buy that.No, Q1 is common base, seen from the feedback network.
Perspective is everything
More importantly though, I think this whole argument is silly. "CFA" may not be a particularly appropriate name but it is the name that's been given. I reckon it will be simpler all round to just stick with the name that's already widely known and used.
Unless we want to pull a Doug Self and insist on calling it "FTALIN" or something (short for "feedback to a low impedance node"). Trouble is, if everybody starts inventing their own acronyms, it won't be long before nobody understands each other anymore.
P.S Actually FTALIN's starting to grow on me - it has a nice ring to it. Maybe we should make an exception to the "no renaming" rule just this once.....
Is that not precisely that some people started to do with acronym CFA, which was formerly attributed to amplifiers where the feedback controls the output current ?
No, Q1 is common base, seen from the feedback network.
Perspective is everything
jan
As Q1 uses the property that the emitter voltage follows the base voltage, can't it be also seen as an emitter follower despite not being a common collector ? Called "splitted loads" I think.
Yes, it's an emitter follower (common collector) as seen from the Vin. It's a common base as seen from the feedback.
jan
Indeed. And I agree with Godfrey, nobody is going to change this at this stage in the game. But it might (and did on several occasions in this forum) lead to confusion. CFA really only refers to that particular class of opamps - someone commented that in the tube world they have used feedback to the cathode of a prior stage, and nobody ever called that CFB.
I guess this is just another sign how alive language is.
jan
How do you figure that out? The output is from the collector, so surely it's a common emitter stage as seen from the Vin.
My view is that a common collector stage has some specific attributes for the input signal, like increase of input Z, which is the result from the Re and is not influenced by whatever you do at the collector.
In this case where we take Vout from the colloector, I can see your point, maybe we should call this a common emitter with local feedback/ local degeneration? Don't know if that makes any sense though.
jan
Hi Claude,
This is incorrect. The input stage of fig. 6 is a common emitter stage with its output taken from its collector and used to drive the second common emitter stage.
It is from the collector of the second stage that major loop, shunt (voltage) derived series(voltage) applied, feedback is introduced to the emitter of the first stage.
Further, you've taken issue with the fact that in this case feedback is taken from the collector of the second stage whereas with a so-called "CFA" the feedback network is buffered from the second stage collector.
This is irrelevant, and neither alters the fundermental operation or analysis of the circuit, nor alters the fact that in this case and with a so-called "CFA" we are dealing with voltage feedback amplifiers.
No; Q1 is a common emitter stage from the perspective of the input signal.
Yes, that's the way I see it.
IMHO an emitter follower has input to the base and output from the emitter, a common emitter has input to the base and output from the collector, and a common base has input to the emitter and output from the collector. Local feedback/ local degeneration is often used with common emitter gain stages.
True. The input stage of a so-called "CFA" is simply a common emitter stage with degeneration provided by the Thevenin impedance of the major feedback network (as seen from input stage's emitter).
Not true. VFA refers to the fact that the transfer function of the feedback network is itself a voltage.
Moreover, the error signal that drives the forward path in both so-called "CFAs" and traditional VFAs is a voltage:
Verror=Vin-Vfeedback
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Nonsense Michael. Absolute nonsense.
1. CFA - the feedback quantity is current.
2. VFA - the feedback quantity is voltage.
3. The terrm CFA and has been used in technical literature since a least the mid 1980's and commercially available CFA opamps since 1987
Period.
A neat aspect of the CFA is that this Verror will still be small and stable if the forward loop is broken and the output simply grounded (the emitter follower aspect) whereas the collector current will substantially increase to follow the emitter / load. So although there is of course an i-v relationship between this b-e voltage and collector current it is more straight forward to use this current (part of the forward loop) as the error signal.
edit: Actually since this input stage can supply current to the feedback divider then this divider does not represent the output node where as the amount of current put into this divider does represent the actual error.
I think if we call them A and B we can better find the most appropriate usage and subtleties for each.
Thanks
-Antonio
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