The Hybrid pi model is over complicating things, but I should start with Ebers-Moll? I must be missing your point.
I intended to explain in the simplest way I knew why ic / vbe was unequal to the DC ratio Ic / 26mV in the sim I ran. How would you have done it without "over complicating" things?
The analysis also pointed out that we have one voltage controlling a current source, and another divided by a resistance producing a current. This led to the conclusion that we were not dealing with a simple transconductor.
I meant explaining the contributions of beta and VA in the performance of the full CFA circuit is unnecessary. The GBW behavior, for instance, will not be affected over a wide range.
You can try it SPICE does not care if VA and beta = 1e6. You will need a comp cap in the simplified circuit unless the transistor models have enough parasitic cap to comp the circuit.
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Considering the contentiousness of the discussion so far, my goal was to forestall refutation by examining a commercial CFA design in closed loop operation by using common transistors that came with LTspice. No inventing new CFAs, no playing around with transistor parameters, no arguments using ideal transistors. Can you imagine the lifesaver that an ideal transistor would be for those desperate to reject an argument's conclusions?
Small signal analysis revealed a contradiction of the "input transistor is just a transconductor" claim which needed further explanation. The Hybrid pi small signal model provided that explanation very simply and nicely.
With the "simple transconductor" claim debunked, we needed a replacement. Ebers-Moll says nothing about the relationship between ic and ie, but it certainly exists and to a good approximation, it's a very simple one: they're approximately equal. And so, vo = alpha T(s) ie.
I think that the argument you prefer has been made before and did not convince. That's why I wanted to look at an actual CFA under practical operating conditions. I don't see how the conclusion I drew can be refuted.
But then, I've been surprised before 🙂.
I partially disagree. By now many of us find only bits and pieces in a book that is something we didnt know or read else where. One area in this one is that series R compensation as well as series L et al.
However, some of the subject within could be applied to audio. Espec topologies. The explanation of current-mode operation is also very good. Not sure it is worth 106 USD, though. But I havent read all of it, yet.
Is it the first book to get on CMA? Probably not. I have recommended those before already. But those here with greater knowledge might find a few new bits and pieces of useful info in it.
THx-RNMarsh
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That claim is made by people who don't understand, it does not need debunking.
The second statement is wrong. The Ebers-Moll model gives Ie, Ic, and Ib under all operating conditions – active, saturation etc. SPICE does not
need to implement different equations for different regions of operation. It uses only four free parameters, the forward and reverse alpha parameters differ from 1 due to specific process issues and yield the non-idealities of beta and ro.
http://www3.imperial.ac.uk/pls/portallive/docs/1/7292573.PDF
BTW we agree (I think) but all this extra stuff is unnecessary for the basic argument. If the input devices were just transconductors it would make no difference in the basic properties of that particular connection of transistors.
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Well, if you hope to change their minds, it does!
I do not have the detailed Spice knowledge that you do, and I'm willing to be corrected regarding Ebers-Moll. It's unfortunate that a number of terms in the presentation you sent appear to have been defined in earlier versions of the lecture. It makes it slow to grok.
Even with Ebers-Moll, any argument that the transistor also had ve / ro as a significant counterpart to vbe gm in practical applications would have been rejected as theoretical only without a simulation, don't you think? Also, you can find transistor small signal models in their data sheets, but not Ebers-Moll parameters. So unless the Spice results are wholly inaccurate for the analysis I did, I believe that approach to be more likely to convince those who are not on board than an approach based on Ebers-Moll.
Those attached to the rigid academic definitions of the various feedbacks will never change their minds. The problem at hand is to take what we have available and put it together to solve a problem. Different amplifier topologies have different properties, use them for what they are.
You're giving them too much credit! Some of their reasoning is based on historical names; some is based on an inflexible or an inadequate understanding of how the circuit works. For my part, it was only in the past few days that I came to understand the importance of ro in a CFA input transistor.
And as in the past, I have benefited from tussles such as this one. I get a deeper understanding as I search for ways to defend my position. Also, as in this case, I get articles published on the topic!
No disagreement there.
They have very good reasons to do so
1. Thieving the name current feedback was and is still shocking.
2. Still using it today is absolutely contrary to the common sens, because it does not describe what is the global effect of the circuit but refers to its internal topology which usually one does not care.
3. Even the name for the internal topology is subject to controversy.
The few guys (there are also great names among them) who, according to you, show an inflexible and inadequate understanding of how the circuit works can return the compliment to the sender.
The discussion is very interesting. It did not change my mind but permitted to refine some details.
“Thieving the name current feedback was and is still shocking. “
I can think of many things that are shocking but calling a circuit a CFA is not one of them. Why the anti-CFA position I will never understand.
Scott is right, nothing more to do here.
😕
I can think of many things that are shocking but calling a circuit a CFA is not one of them. Why the anti-CFA position I will never understand.
Scott is right, nothing more to do here.
😕
Aw shucks, there's so much to challenge in what you wrote. I just can't let it go. (Sorry, Scott.) Simply another one of my character flaws...
I have little respect for a name so poorly chosen that it contradicts what is actually happening in the circuit. Historical "current feedback" circuits do no such thing. They employ voltage dividers with a load on top and a small sense resistor on the bottom which provide voltage feedback to the circuit that drives them. The name deserves to be re-purposed. Still, I acknowledge that it is unfortunate that this is causing confusion.
Thank you for explaining your thought process. You don't care how the circuit works, or whether its name reflects its inner workings. Perhaps that explains why the name makes no sense to you. I guess then that you would argue that there's no reason to call amplifiers Hollow and Solid State, nor voltage regulators Linear and Switching. After all, they have the same "global effect."
So is the proposition that the Earth is an oblate sphere.
Again, arguments not in evidence, and from Authority. As for flexibility, let's contrast the flexibilities of people who don't even bother to respond to multiple posts which challenge their claims, and those who do.
I have little respect for a name so poorly chosen that it contradicts what is actually happening in the circuit. Historical "current feedback" circuits do no such thing. They employ voltage dividers with a load on top and a small sense resistor on the bottom which provide voltage feedback to the circuit that drives them. The name deserves to be re-purposed. Still, I acknowledge that it is unfortunate that this is causing confusion.
Thank you for explaining your thought process. You don't care how the circuit works, or whether its name reflects its inner workings. Perhaps that explains why the name makes no sense to you. I guess then that you would argue that there's no reason to call amplifiers Hollow and Solid State, nor voltage regulators Linear and Switching. After all, they have the same "global effect."
So is the proposition that the Earth is an oblate sphere.
Again, arguments not in evidence, and from Authority. As for flexibility, let's contrast the flexibilities of people who don't even bother to respond to multiple posts which challenge their claims, and those who do.
Blame it on me. i said CMA/CFB amps sound better almost always.
Now those who must know why and dont, wont accept it.
Oh well. Their loss.
🙂
THx-RNMarsh
There won't be such reactions if my previous posts had not touched sensitive points.
I point out that my analysis considering the effect of the load of the inverting input has not been dismissed.
Overall, I remark :
- that despite all the efforts on both sides, nobody of one side had arguments such as to convince somebody of the other side.
- that I (and, as far I remember, the other people of the VFA side) respectuously never wrote that people of the CFA side understand nothing. Not the case of the reverse.
I point out that my analysis considering the effect of the load of the inverting input has not been dismissed.
Overall, I remark :
- that despite all the efforts on both sides, nobody of one side had arguments such as to convince somebody of the other side.
- that I (and, as far I remember, the other people of the VFA side) respectuously never wrote that people of the CFA side understand nothing. Not the case of the reverse.
When amplifiers with differential input (VFA now) appeared, were'nt they said to better sound than those with a single device input stage (of CFA status by now, not at the time)? One must admit that, at the time, the subjectivism was a baby.
Now, how do subjectively compare CFAs between them : those with single device input, those with two devices in push pull input, and those with a diamond buffer input (four devices).
I bet for the last ones as the best.
But how do compare amplifiers with Double Diamond Buffer inputs which are indisputably voltage driven ?
If Double Diamond Buffer inputs sound better, does not that means that the explanation is not in the current input but the current which the input stage can deliver to the next stage with "ease"?
If you'd like, please indicate the post that was not responded to. It's not that I wish to continue to argue, but that I have tried to make a point of not ignoring what others are saying.
And in return, if you wish a response, will you finally reply to the many posts that you have so far not answered?
No one wrote that one camp understands nothing. But surely you must agree that both camps believe that the other has an "inadequate understanding" of how the circuit works. Otherwise, what is the basis for disagreement?
As for inflexibility, one hallmark of this trait is not responding to arguments which contest accepted beliefs. We've seen too much of that here.