The difference between current feedback and voltage feedback for audio amplifiers is a non issue. Voltage feedback amplifiers for audio applications are superior to their current feedback peers.
Given a set C of current feedback amplifiers and a set V of voltage feedback amplifiers, there exists an amplifier V(x) that has higher bandwidth than an amplifier C(z)
How much of a phase difference? One or two be junctions or gs junctions. Yes these can make a difference to the stability of an amplifier.
There is an advantage of having a symmetrical design with an LTP and that is LTP error cancellation, especially if the LTP is matched and essentially in the same package. Better still if the LTP package is paralleled.
Current feedback is a term coined from the nature of the low impedance input, nothing more other than the phase and nothing less. Add an active device and it becomes a high impedance input and now we call it voltage feedback
Given a set C of current feedback amplifiers and a set V of voltage feedback amplifiers, there exists an amplifier V(x) that has higher bandwidth than an amplifier C(z)
How much of a phase difference? One or two be junctions or gs junctions. Yes these can make a difference to the stability of an amplifier.
There is an advantage of having a symmetrical design with an LTP and that is LTP error cancellation, especially if the LTP is matched and essentially in the same package. Better still if the LTP package is paralleled.
Current feedback is a term coined from the nature of the low impedance input, nothing more other than the phase and nothing less. Add an active device and it becomes a high impedance input and now we call it voltage feedback
I think the terminology should be current source and voltage source amplifiers. The so called 'current feedback amplifiers' are also voltage source amplifiers.
When the output current is fed back the output impedance is increased with the opposite happening with voltage feedback.
Looks like the OP is talking about the regular so called 'current feedback amplifier' where the constant gain bandwidth restriction does not apply.
When the output current is fed back the output impedance is increased with the opposite happening with voltage feedback.
Looks like the OP is talking about the regular so called 'current feedback amplifier' where the constant gain bandwidth restriction does not apply.
Last edited:
I would think this does not match the common definition of a CFA as per their OpAmp manufacturers.
The point is that for a CFA the open-loop gain Aol of the amp is adjustable via the network impedance at the inverting input, whereas for a VFA the network impedance does not affect Aol. It has nothing to do with actual input impedance as such, though of course it will be generally low with an CFA.
When you buffer the inverting input of a CFA (probably using a series degenerating resistor) then you are restricted to the Aol (and min gain) dialed in to stabilize the amp at whatever the target gain. But the gain does not depend on network impedance.
For a very specific target gain like for an audio power amp I don't see any significant inherent advantages for either topology. What counts is the implementation of the actual circuit.
I think the terms are coined from the perspective of how the feedback is used
In common terminology, voltage feedback means a feedback signal that is proportional to the output voltage, while in current feedback the feedback signal is proportional to the output current. Hence, the amp's output impedance is dreased by negative VFB, while it is increased by negative CFB, but can be decreased even beyond zero to negative values by positive CFB. Both topologies basically don't have anything to do with the input impedance, unless the FB signal is applied in common with the input signal to the amp's inverting input.
Best regards!
Best regards!
Well, whose terminology? What you are talking about is often labelled VCVS, VCCS, CCVS and CCCS (Voltage Controlled Voltage Source, etc, plus their sub-variants) in EE textbooks. This is about amplifier black box behavior.
https://en.wikibooks.org/wiki/Circuit_Theory/Dependent_sources
On the other hand, CFB and VFB are clearly defined as working principles of the feedback mechanism inside the black boxes, by those who design and manufacture this stuff.
https://en.wikipedia.org/wiki/Current-feedback_operational_amplifier
VCVS's etc can be realized with VFB or CFB amps, these are completely independent things.
Ah OK, I see what you mean. I would think the Tietze/Schenk is still the standard textbook like it was already decades ago?
For the sake of precision, the terms "Current Feedback Amplifier" and "Voltage Feedback Amplifier" lack the explicit notion what exactly they are referring to.
Is it about a mechanism (impedance at the -IN is shaping the Aol)? That's the interpretation that I had in mind and this what I think is the common meaning in English EE.
Or is it about the quantity that is fed back, voltage or current? Then it is the meaning of the German terms (translated).
Those damned semantics, haha ;-)
Yeah, Tietze/Schenk 
!
Anyway, feedback concepts were developed long before the semiconductor aera. The German FUNKSCHAU magazine for example published tube amplifier designs in the mid 1950ies with combined negative voltage feedback and positive current feedback to minimize output impedance.
Best regards!
!Anyway, feedback concepts were developed long before the semiconductor aera. The German FUNKSCHAU magazine for example published tube amplifier designs in the mid 1950ies with combined negative voltage feedback and positive current feedback to minimize output impedance.
Best regards!
Yes and no, degeneration of the CFB emitters amount to degeneration of a differential emitters in some way, the result
is the same since in both cases this will limit the OLG and max frequency.
The difference is that with a VFB you can have no degeneration at all and this will provide a higher OLG contrary to the CFB
where a minimal degeneration is mandatory.
