True Current Feedback N-channel Mosfet Amp

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lumanauw

lumanauw

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Well, it's all fine with me, since I think a CF amp is something entirely different anyway, but I noted that on this post many are of the opinion that there never is an input current, because the signal input is high impedance. They say that CF is input voltage and feedback current.
Click to expand...

This example should be the "missing link" between those "classical" and "modern" term in CFB definition.

If the modernist thinks that the feedback should be given to a lowZ point, then this examples fullfill it.

It even does more. In Ampman's schematic, the feedback still in form of voltage divider (still have R to ground), so the so called current fed-back to the input stage must be very little.

But in this example of inverting opamp, the feedback is 100% goes to the -input point. No R at all (voltage divider) to ground.
So the "modernist" must say that this is a perfect example of "Current Feedback". Current really goes 100% to the -input.

I'm more in the "classical" point of view, but trying to understand the "modernist" better. But this Inverting opamp example is really will be something.

About the second line that I wrote, it is another issue, not related to the example above it. I want to know what causes the statement "Inverting amp gives "better" sound, while the +input is grounded, compared to the non-inverting amp"

I really do not understand this, because the rest of the circuit is all the same. The difference is only in the input and feedback configuration.
 
Ouroboros

Ouroboros

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Joined 2003
lumanauw said:



About the second line that I wrote, it is another issue, not related to the example above it. I want to know what causes the statement "Inverting amp gives "better" sound, while the +input is grounded, compared to the non-inverting amp"
Click to expand...


This is often asserted and has some validity for op-amps (and similar designs).
A non-inverting op-amp input has the input signal appearing as a common-mode signal to the amplifier. An inverting configuration has no common-mode because the + input is at an ac ground.
Many op-amps have distortion that increases as the input common-mode gets larger.
 
Tube_Dude

Tube_Dude

Member
Joined 2002
lumanauw said:


But in this example of inverting opamp, the feedback is 100% goes to the -input point. No R at all (voltage divider) to ground.
So the "modernist" must say that this is a perfect example of "Current Feedback". Current really goes 100% to the -input.
Click to expand...


Yes in the inverting opamp the feedback goes to the - input...but is "decteted" by the op amp as a voltage because in a op amp the inputs draw no current...(except in ther case of a Current Feedback op amp).;)

And it seem that nobody do the home work and see the link that Jewilson share with us:
http://www.eecg.toronto.edu/~kphang/ece1371/cfa.pdf

The link is clear about the subject.:)
 
Tube_Dude

Tube_Dude

Member
Joined 2002
Ouroboros said:


Regarding the sound, I really can't say, but it is worth noting that because the way the error signal is derived, CFB amps have very poor control of the output DC offset and usually require a separate servo-controlled dc feedback loop.
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Current Feedback Amplifiers have sacrifeced the DC precision of Voltage Feedback Amplifiers in a trad- off for increased slew rate and bandwith...but distortion in the audio band (Our field ;)) is usually lower in a voltage fedback amplifier.
For me the sound seem with more definition in the Voltage Feedback Amplifiers (maybe for the increased precision of the VFA)

In the oposite the CFA seem more dark and veiled in comparison..

PS: this are a very personal and subjectif opinion...so it can be judged only as just one more opinion! :)
 
jan.didden

jan.didden

AX tech editor
Joined 2002
linearaudio.net
Paid Member
Tube_Dude said:
[snip]And it seem that nobody do the home work and see the link that Jewilson share with us:
http://www.eecg.toronto.edu/~kphang/ece1371/cfa.pdf

The link is clear about the subject.:)
Click to expand...

Tube_dude,

I thing most of us did our homework, the link does not add anything new to what has been discussed. This CFA architecture has been discussed and it has been said that this indeed is the "modern" way to describe a CFA.

I know I am repeating myself, but the confusion was about the "classical" use of CF which is a sample of the output current (often through a small resistor in series with the load) fed back to the input.

The author of the article in the link either has no clue that this "classical" definition existed (he may have been very young when he wrote it) many decades before his CFA, or he wisely decided to ignore it, to make a better (commercial) case for this new-fangled CFA stuff.

Whatever.

Jan Didden
 
C

Christer

Member
Joined 2002
Tube_dude,

I agree with Jan here. I did find that document earlier and it is
actually the one where I found the historical data on CFB op amps
that I posted. Howver, since it didn't really add anything
technically to the app notes I had linked to earlier, I never bothered
to remember the link or post it. This document is just another
one explaining CFB in the modern sense.

I really don't know what we are discussing anymore.:confused:
 
K

km

Member
Joined 2001
cadaudio.dk
>Hi FACTOR, STALLION, amp_man_1... are you back in town?
220.224.29.xxx.

/Hugo

hi.

no hugo you got it all wrong , they just look alike and have this ability to use the keyboard at the same time but they are 2 different persons (at least they got seperated when factor/kanwar was kicked out from here :)

karsten madsen - www.cadaudio.dk

ps. who has at least built a couple of amplifiers........
 
traderbam

traderbam

Member
Joined 2002
No. Just because the feedback node is low-Z is not enough. In the case above, even though the node is low-Z, it is still a VOLTAGE difference that is being used inside the feedback loop as the error signal. A CFB amp (modern usage!) derives the error (current) signal as the difference between an input current and the feedback current.
Click to expand...
I must disagree. Whether something senses voltage or current is merely a matter of it's impedance. V and I are related by Z, obviously. A so-called CFA audio amp is trying to minimize difference between output V and input V just as a VFA is. Surely the defining difference is the Z of the summing node(s).

By this definition an op-amp is not a CFA even wehn used in inverting mode, however, if the OL gain is very large it behaves as if it's inverting input has very low Z (at low f) and so the difference may not be important.
 
lineup

lineup

Member
Joined 2005
Re: Reply!

Workhorse said:
:smash: :smash: :smash: :smash: :smash:

ACTUAL PROTOTYPE COMPLETED!
Supply Voltage = +-50VDC
Slew rate = 250V/uS
Power = 200W TRUE RMS@4ohms@100Khz
Zero degree phase shift @50KHz
THD = 0.0012%
Frequency Response = DC TO 300KHz


ANY SUGGESTIONS ARE MOST WELCOMED!

:smash: :smash: :smash: :smash: :smash:
Click to expand...

Workhorse said:
I am very lucky guy to get a reply from "SIR ANTHONY HOLTON":D

EXTEREME THANX FOR UR INVALUABLE SUGGESTION SIR,

Regarding Dissipiation of IRF640 PLZ REFER to PHOTOS IT states 150W Pd.
Click to expand...

Keep up your good work, Workhorse
I see you have some good ideas.
Personally, I am a fan of N-Channel HEXFET designs.
Veru much so ... by logical reasons :cool:

Lineup July 2008 in Sweden - A small country in north of Europe
 
lineup

lineup

Member
Joined 2005
Current feedback or Voltage feedback differences.
We have had some topics discussing this.
There is in fact no difference.

all amplifiers are using current feedback
... because it always takes current to create a voltage drop
... that is: some electrons that are passed =electrical charge, Columb

so, I personally, never wants use those terms:
Voltage FB or Current FB
.. it really doesnt say nothing

instead I use:
Base feedback .. as in a LTP pair input
Emitter feedback .. as in a such a faster amplifier ( or Source FB for MOSFET/JFET )
.. because this describes where the feedback current hits the controlling transistor

Lineup ... is very much a fan of Source & Emitter FB amplifiers :cool:
 
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