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Old 29th August 2012, 11:59 PM   #11
Bigun is offline Bigun  Canada
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for me they (CFB and VFB) as described are both voltage feedback, but this is one of those annoying semantics issues. what I mean is that the feedback is sensing the voltage at the output and feeding a fixed portion of it back to the inverting input. what people call a current feedback loop is one in which the input impedance of the inverting input is relatively low and what people call a voltage feedback loop is one in which the input impedance of the inverting input is high.

I don't understand all the differences between them, but I've consistently read that the CFB type feedback has higher bandwidth / higher stability / less compensation. Or put it another way, it has less phase shift. For a feedback loop, this is highly desirable. I have only explored these differences by using either an LTP or a JLH style singleton and they have other factors that differ between how they operate to obscure an understanding of the differences between CFB and VFB in my mind. so what you are doing Elvee looks interesting.

for me, true current feedback is where the output current is sensed and converted to a voltage which is then applied to the inverting input. again semantics. an example is the Aleph current source, in which the output current is sensed by a resistor, the same resistor conveniently converting it to a voltage signal for feedback.

sorry for the ramble !
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Last edited by Bigun; 30th August 2012 at 12:02 AM.
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Old 30th August 2012, 02:30 AM   #12
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The essential difference is that the current feedback amplifier can be seen as a transimpedance amplifier in which the output voltage is controlled by a fed back current.
The transimpedance can be put as

Zt = Go/(1 + joGoCc)

Go= the open loop d.c. gain and Ct is the Miller compensation capacitor.

This type of amplifier has the advantage of a constant bandwidth despite closed loop gain because it does not have a constant gain bandwidth like a voltage feedback op amp.
rcw

Last edited by rcw666; 30th August 2012 at 02:31 AM. Reason: ambiguity
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Old 30th August 2012, 01:39 PM   #13
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Jay View Post
But I'm waiting... for the conclusion
I have no conclusions ready. I am freely reflecting on the subject, and I am sharing my thoughts.

I think there are a number of misconceptions about all this: firstly, in actual circuits the difference between the two is not as clear cut as many would like to believe, and secondly the virtues or flaws attributed to one or the other seem to disappear as soon as you normalize the conditions of the comparisons.

Bigun's opinion looks convergent with mine.


What could emerge of all this is a better understanding of both topologies (if they are actually distinct), and better circuits, making optimum use of the phenomenons involved, to create ideal and pure VFB or CFB circuits.
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Old 30th August 2012, 02:08 PM   #14
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As Bigun commented semantics have a lot to do with it.

For instance the standard type of op amp can be looked at as a current feedback amp when connected in the inverting mode, because the feedback is converted to a current by the feedback resistor and summed at the virtual earth point.

There is also the direct coupled three transistor amplifier, (much used in audio at one time), that uses d.c. feedback to the emitter of the input transistor. Since this arrangement varies the open loop gain with the feedback ratio it also satisfies another property of current feedback op amps.

In the expression I posted note that it has a compensation capacitor in it and the assumption is made that the amplifier has a fixed integrated capacitor for this, the current feedback topology allows a constant bandwidth over a range of gains unlike in a similar arrangement in a voltage feedback amp, these having a constant gain bandwidth product and not a constant bandwidth.

In the end I can see no particular advantage for audio anyway and they are most useful at frequencies well above the audio band.
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Old 30th August 2012, 04:38 PM   #15
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by rcw666 View Post

There is also the direct coupled three transistor amplifier, (much used in audio at one time), that uses d.c. feedback to the emitter of the input transistor. Since this arrangement varies the open loop gain with the feedback ratio
Could you elaborate on that, I don't quite visualize it?
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Old 30th August 2012, 04:51 PM   #16
Elvee is online now Elvee  Belgium
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One more paradox: in the test circuit, if an inverting configuration is adopted, not only is the HF performance not improved, it is even slightly degraded.
Here are both versions, for comparison:
Attached Images
File Type: png CFB8.png (100.1 KB, 471 views)
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Old 30th August 2012, 10:54 PM   #17
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This is the three transistor configuration as it appears in a Burr Brown application note.

The second configuration is the addition of an emitter follower that isolates the feedback loop and gives constant open loop gain unaffected by the feedback components this is then the classical voltage feedback op amp topology.

The third diagram is the basic current feedback topology. You can see that it is a symmetrical version of the three transistor amp, with a complimentary emitter follower on its output and emitter followers buffering its input.
rcw


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Attached Images
File Type: gif threetransdcamp.GIF (5.3 KB, 467 views)
File Type: gif voltage amp1.GIF (5.7 KB, 461 views)
File Type: gif cfbbasic1.GIF (6.4 KB, 465 views)
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Old 31st August 2012, 12:04 PM   #18
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by rcw666 View Post
This is the three transistor configuration as it appears in a Burr Brown application note..
OK, this circuit is identical to my first one, except for the emitter follower output.
This addition means that you can lower almost arbitrarily the gain setting resistors, and get a matching increase in OL gain (and departing from the CFB "philosophy")



Quote:
The second configuration is the addition of an emitter follower that isolates the feedback loop and gives constant open loop gain unaffected by the feedback components this is then the classical voltage feedback op amp topology.
That one is the "real life" version of my second circuit, with the spice buffer replaced with an emitter follower
Quote:
The third diagram is the basic current feedback topology. You can see that it is a symmetrical version of the three transistor amp, with a complimentary emitter follower on its output and emitter followers buffering its input.
Yeah, for high enough transimpedance gains, this structure will have a behavior indistinguishable from a regular VFB in all practical circuits (except comparators).
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Last edited by Elvee; 31st August 2012 at 12:27 PM.
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Old 31st August 2012, 12:39 PM   #19
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Quote:
Originally Posted by Elvee View Post
...
Yeah, for high enough transimpedance gains, this structure will have a behavior indistinguishable from a regular VFB in all practical circuits (except comparators).
Elvee,
Not sure I'm following your train of thought here, are you saying that as long as the VFB topology does not "run out of available input stage current change" it behaves similarly to CFB?

Thanks
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Old 31st August 2012, 04:57 PM   #20
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The amplifier bandwidth is limited by the current available to slew the compensation capacitor.

In an amplifier with no access to the compensation capacitor that is stabilised for unity gain the bandwidth is limited by the value of this capacitor, and in the voltage feedback scheme the current available to slew this capacitor is fixed.

In the CFB topology however the feedback resistor sets the current available to slew this capacitor, thus over a given range the bandwidth is only dependent upon this resistor, this is the Go term in the expression that I posted, Zt is constant for a given Cc provided that Rf is also constant, this is true over the range specified in the data sheets.

Your modeled circuits do not have the pole splitting capacitor, so I am assuming that the slew is limited by the transistor characteristics alone, putting the capacitor in the simulation might be informative.
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