Current Feedback Amplifiers, not only a semantic problem?

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Its not incoherent forr.

CFA is less intuitive than VFA, but that only makes it more interesting.

Just think about how the current to the TIS/TAS input node is controlled in the basic form of the two topologies:-

VFA - you steer the LTP tail current towards or away from the TIS/TAS input. The peak current is the LTP tail current. In its classic MC form, the tail current and compensation are not coupled in a VFA - you can set them independently.

CFA - the peak current to the TIS/TAS input is set directly by the CFA feedback resistor -so it can be very high. The value of the feedback resistor also sets the compensation of the amplifier, unlike a VFA.
 
Bonsai, let me repeat, please. It is a local feedback in the first stage, through a feedback resistance. It is more than one resistor to the ground; it is also a feedback resistor's value dvided by amplification factor, in parallel with it. With frequency this resistance (impedance, actually) changes, changing local (nested) feedback in the 1'St stage.
No miracle here, all is transparent, time to go to discuss boobs in The Lounge! I use similar nested feedbacks forever, plus other loops inside!
 
You have not changed the fundamental operation of the input stage compared to a VFA in my view - you have simply hidden it behind a buffer.

Now you see why I view the H bridge as a CFA and not a VFA.

CFA: lo z - input port, current on demand into TIS/TAS input, BW does not change with CLG

VFA: both input ports hi z, peak current into TIS/TAS = Tail current. Constant gain BW in its simplest guise assuming MC

There are more than two ways to make an amplifier input stage; I'm getting curious how you would categorize the input stage (first inverter) of a three-inverter transimpedance amplifier (three CMOS inverters in a row with a feedback resistor from the last to the first and some frequency compensation). Assuming a fixed supply voltage, the input stage operates in class AB, but its input is as high impedant as it gets.
 
Its not incoherent forr.
CFA is less intuitive than VFA, but that only makes it more interesting.
[....]
The incoherence lies in the name which does not reflect the fundamental process of feedback in the input stage, not relying on current and being the same in CFA and in VFA.

Sure, the CFA and VFA characteristics and the ways to exploit them differ but they are both rather easy to explain. For the first explanations of how a CFA works, it is beneficial for the understanding to replace the push-pull configuration by a single transistor with its emitter loaded by a CCS and the feedback network, Then, for the VFA, just add a buffering transistor and double the current of the CCS.
 
Whatever the topology, with increasing frequency, a primary effect of the usual compensation is to increase the potential difference between the non-inverting input and inverting input of the sensing device which controls the feedback loop.




Um, whatever the topology, the impedance of the feedback network does not effectively program the transconductance of the input stage unless the topology is what has been designated a "CFA". There appears to be a persistent conceptual block here, so we are bound to.......




Honestly, the "established nomenclature" of Current Feedback as a topology should be revised or at least, students and beginners in electronics should be seriously warned against its incoherence.





........repetition. Perhaps this will endure for another 300+ posts.
 
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There are more than two ways to make an amplifier input stage; I'm getting curious how you would categorize the input stage (first inverter) of a three-inverter transimpedance amplifier (three CMOS inverters in a row with a feedback resistor from the last to the first and some frequency compensation). Assuming a fixed supply voltage, the input stage operates in class AB, but its input is as high impedant as it gets.

Marcel,

all that will happen is we will go around in circles. I think you know exactly what I mean. I am more interested in helping people who have a 'conceptual block' about CFA, or people who believe CFA=VFA or that CFA is a bad name because it does not fit with their understanding. I will never convince people that hate CFA (plenty here on the forum).

I am going to step out of this discussion. Wavebourn is right.
Talking about Russian girls, drinking Vodka and in my case listening to Tchaikovsky is time better spent.
 
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thank you walt. I have said same thing from other sources.... excerpts from books in fact but no one here wants to read them. I have said similar to last paragraph re low internal impedances. Just argue about something they don't know much about. So, I'm with Bonsai on this. Cheers.

THx-RNMarsh
 
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I believe the the difference between VFA and CFA is down to passive or active V to I conversion CFA over a resistor pair VFA over bjt or jfet devices. Circuits like the old (relative) Hiraga Class A fits the CFA. Some may argue tha voltage out also implies voltage feeedback. But I believe we need to accept that CFA is a circuit concept rather than a current to current feedback
 
The wrongly named CFB is PVF ! :p
Mona
 

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