Current feedback - Voltage feedback, how do I see the difference? - Page 61 - diyAudio
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Old 26th March 2013, 02:37 PM   #601
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Quote:
Originally Posted by kees52 View Post
I don,t get why we are so complicated here, just think in V or A it is just the impedance node of a active element
who decide current or voltage feedback
Kees, as I noted, there really isn't any current to speak of going into that emitter. The current going in is determined by the feedback factor more than the low emitter resistance.
How much curent you think is going into it?

jan
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Old 26th March 2013, 03:00 PM   #602
godfrey is offline godfrey  South Africa
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On a more philosophical note:

What do you call it when someone like Charles Hansen uses a "current feedback" opamp open loop? Is a CFA still a CFA when there's no feedback?
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Old 26th March 2013, 03:18 PM   #603
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Quote:
Originally Posted by godfrey View Post
On a more philosophical note:

What do you call it when someone like Charles Hansen uses a "current feedback" opamp open loop? Is a CFA still a CFA when there's no feedback?
Its then known as a current conveyer.
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Old 26th March 2013, 06:31 PM   #604
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by jan.didden View Post
Yet, Richard, that's not what I see. The voltage at the inverting input (the low impedance node) swings just as wide as the voltage on the non-inverting input. And the current into the inverting (low impedance) node is much lower than you'd expect.
I checked with a G=10 amp with an AD844 and the inverting input current was just a few 100 nA.

I do agree with you that the term seems to be cast in concrete now and no amount of arguing is going to change that. Not even Mike will pull that off

jan
Well that is one of my points -- where the name came from and that is what it is now called. How it works is easy to explain and measure. I developed the circuit and measured it and learned things from it and published an app for it..... then went off to do other new things. Seems like such a waste of time when you can just measure it to decide what's what.

However, in some of the newest CMFB opamps the reasoning has evolved...... but still depends on actual design circuitry as to what you get with a particular version being called Current-Mode. Now they are hybrids of VM and CM and neither exactly explains them... whole new stuff with same old names.

Thx-RNMarsh

Last edited by RNMarsh; 26th March 2013 at 06:35 PM.
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Old 26th March 2013, 06:37 PM   #605
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Originally Posted by RNMarsh View Post
Now they are hybrids of VM and CM and neither exactly explains them... whole new stuff with same old names.

Thx-RNMarsh
Sounds intriguing Richard, do you have any part numbers or something?

Edit: I know hybrid opamps as in having discrete dies on the chip but I guess that's not what you meant.

jan
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Old 26th March 2013, 07:10 PM   #606
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Quote:
Originally Posted by jan.didden View Post
Yet, Richard, that's not what I see. The voltage at the inverting input (the low impedance node) swings just as wide as the voltage on the non-inverting input. And the current into the inverting (low impedance) node is much lower than you'd expect.
I checked with a G=10 amp with an AD844 and the inverting input current was just a few 100 nA.

I do agree with you that the term seems to be cast in concrete now and no amount of arguing is going to change that. Not even Mike will pull that off

jan
Jan let me summarize one way of looking at it. A typical analysis starts with breaking the feedback loop into a forward path (A) and feedback path (f) to state the problem with the classic feedback equation A/(1+Af). With a VFA the break is easy and the forward path is essentiallv independent of the feedback. I learned using Y parameter two ports. The answer is easy by equating voltages. You cannot solve via input currents you simply get 0=0.

With a CFA you use H parameters and the forward path contains the feedback network. Point number one, the voltage across input to the amplifier is no longer the driving function of the forward path. You now can easily solve by setting the net input CURRENT to 0. That is by superposition the current in the input loop (out the -input) from the input source equals the current into the -input from the feedback network.

Surprise, surprise the answer is the same. Vin=Vo*Rg/(Rf+Rg). So the CFA case can be solved by only considering the -input CURRENT to be forced to 0.
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Old 26th March 2013, 07:35 PM   #607
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Thanks Scott, I get it.
I had come to the same conclusion empirically, in the limiting case the 'feedback current' approaches zero.

In hindsignt, since the inverting input current is determined by the delta V between Vin and Vfb across that low impedance, that necessarily leads to very low current since that delta V is basically Vout/OLgain.

Edit: Scott, I do understand and accept the difference in behaviour of a 'VFA' and a 'CFA', to be clear.
I believe I've said that already several times.
One of those differences are the reason I like those AD844's so much ;-)

jan
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Last edited by jan.didden; 26th March 2013 at 07:43 PM.
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Old 26th March 2013, 07:45 PM   #608
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Scott if I may pick your brain some more:

In your opinion, what is the defining property of those CFAs?
Is it the fact that you feed back into that particular topology, or is it just the low impedance?

jan
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Old 26th March 2013, 08:09 PM   #609
RNMarsh is offline RNMarsh  United States
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It can sometimes help to understand some circuits by knowing the meaning of 'virtual' as opposed to actual, physical. Check your understanding of virtual and a lot of things fall into place.

For example, an active node may be called a virtual ground node but doesnt actually measure that way with a volt/current/ohm instruments. Also, a port labelled low Z may not actually measure that way but behaves that way... as if.

Thx-RNMarsh

Last edited by RNMarsh; 26th March 2013 at 08:11 PM.
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Old 26th March 2013, 08:10 PM   #610
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Quote:
Originally Posted by jan.didden View Post
Scott if I may pick your brain some more:

In your opinion, what is the defining property of those CFAs?
Is it the fact that you feed back into that particular topology, or is it just the low impedance?

jan
I just used your post to put out that viewpoint. I think the constant BW for low gains and the much overlooked clean pulse response are most important. You might not have seen Nelson's comments about how he saw CFA's at military houses used for high speed signal processing and that got him going.

My main problem with Cherry is that he takes a narrow somewhat audio-centric view. He criticizes CFA's at high closed-loop gains (virtually no one uses them that way). He also shows basicly a 741 with variable compensation as an alternative. This is a poor choice, an undegenerated diff-pair is suboptimal for input linerarity and slew rate.

OTOH when all is said and done some of Mike's criticisms of CFA's for audio are perfectly good points, casting them as "bad" VFA's in disguise is not necessary.
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