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Old 2nd July 2016, 08:12 AM   #1
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Default Triodes and feedback theory

Triodes and feedback theory

A few days ago jan.didden posted an article written by Jerald Graeme of Burr-Brown fame. The article is aimed at feedback in op-amps. Jerald Graeme and his crew have been prolific in the number of white papers and books they have written. Almost every document references the original electronics feedback paper written in 1934 by H S Black. You might want to take a look. https://ia802703.us.archive.org/24/i...bstj13-1-1.pdf Feedback theory is now applied to medical research and many other disciplines beyond electronics. Triodes Too?

I went looking at Amazon to see if Graeme had written a book on this topic, in fact he has: Optimizing Op Amp Performance 1997. I purchased a “like new” copy for about $40. The single most interesting thing to me that Graeme had to say was this: “intuitive observations, guide the model derivation, and then comparison of the model’s transfer response with that of the circuit confirms the model’s validity”. How about that, sharing the process, and not just the results as if it was correct the first time. I think of it as intuitive deduction, starting with you have worked hard to learn, filling in the blanks with what you think best fits and then do reality testing in the lab, wash rinse and repeat. See page 30 heading 2.1.

In chapter 1 Graeme applies Black’s feedback model to the non-inverting input of the op-amp. Graeme develops the concepts including feedback factor represented by β, β+for positive feedback and β- negative.

In Chapter 2 Graeme applies and adapts Black’s feedback model to the inverting input of the op-amp. Chapter 2 is where the feedforward factor represented by α is introduced. α accounts for the inverted op-amp input, and attenuates the Vinput.

Too many words, more about triode non-inverting and inverting outputs later.

DT
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Old 2nd July 2016, 08:16 AM   #2
forr is offline forr  France
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Originally Posted by DualTriode View Post
Feedback theory is now applied to medical research
Our body is a giant feedback/feedforward/error-correction system.
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Old 2nd July 2016, 10:29 AM   #3
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Feedback also applies to economics, population biology and lots of other areas.

The triode has lots of feedback, which is why when used properly such an intrinsically nonlinear device can provide such linear amplification.
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Old 2nd July 2016, 11:01 AM   #4
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Originally Posted by DualTriode View Post
Triodes and feedback theory

A few days ago jan.didden posted an article written by Jerald Graeme of Burr-Brown fame. The article is aimed at feedback in op-amps. Jerald Graeme and his crew have been prolific in the number of white papers and books they have written. Almost every document references the original electronics feedback paper written in 1934 by H S Black. You might want to take a look. https://ia802703.us.archive.org/24/i...bstj13-1-1.pdf Feedback theory is now applied to medical research and many other disciplines beyond electronics. Triodes Too?

I went looking at Amazon to see if Graeme had written a book on this topic, in fact he has: Optimizing Op Amp Performance 1997. I purchased a “like new” copy for about $40. The single most interesting thing to me that Graeme had to say was this: “intuitive observations, guide the model derivation, and then comparison of the model’s transfer response with that of the circuit confirms the model’s validity”. How about that, sharing the process, and not just the results as if it was correct the first time. I think of it as intuitive deduction, starting with you have worked hard to learn, filling in the blanks with what you think best fits and then do reality testing in the lab, wash rinse and repeat. See page 30 heading 2.1.

In chapter 1 Graeme applies Black’s feedback model to the non-inverting input of the op-amp. Graeme develops the concepts including feedback factor represented by β, β+for positive feedback and β- negative.

In Chapter 2 Graeme applies and adapts Black’s feedback model to the inverting input of the op-amp. Chapter 2 is where the feedforward factor represented by α is introduced. α accounts for the inverted op-amp input, and attenuates the Vinput.

Too many words, more about triode non-inverting and inverting outputs later.

DT
Yes very good book.

As to studying feedback/feedforward on a triode, the best way to start may be to model the triode as any other (opamp like graphic) block with two inputs and an output. All the known feedback equations can then be applied, and it is the open loop gain expression that locks it in.

Just like with opamps there are more inputs like the power supply connections / anode. Possibly these can me modeled as part of the open loop gain.

If this works, it would open up a huge knowledge base to the triode without a lot of reinventing the wheel.

Another book tip: David Mindell 'Between human and machine - Feedback, Control and Computing before Cybernetics'.

The title doesn't give it away but a large part of the book is devoted to the struggle to apply what Harry Black developed to everyday problems in feedback control. For instance, at the start of the war BuOrd (the US Navy ordnance development organisation) saw the lesson from the Blitzkrieg and the need for accurate anti aircraft fire control. A classical feedback problem. Yet the Navy was very much focused on manual control for big guns. After all, an enemy battle ship doesn't move that much while your shells are on their way towards it. You look were they land and adjust your elevation and azimuth (or train as the Navy called it) and fire again. Human in-the-loop feedback.

With aircraft it's much different. Those f*cking pilots refuse to fly level with constant speed. Then those Japanese Zero pilots added insult to injury by changing their altitude as well, all the time! By the time the shell arrived where you thought the plane was, it was already a mile away. Good firing directors can predict a lot from pilot behaviour, especially those who were former pilots, but basically it was a guessing game.

Interestingly, the process to smooth track history and extrapolate to a future position is EXACTLY the same as electronic filtering problems, too much smoothing and you miss the next manoeuvre, not enough smoothing and your aim point jerks all over the place.

Anyway, what I was getting at was that while at Bell Labs they were routinely building feedback amps, traditional Navy developers like at Sperry Gyroscope still say a servo amp as an open loop amp with the input-output error returned to the input. They did not realise it is a closed loop, and couldn't get their head around the stability problems they got! Only after Bell Labs and MIT got involved in the firing director problem did the two realise they were talking about the same thing! Then Bode and Nyquist chipped in and that was the start of our current body of knowledge on feedback and feedback stability. Fascinating!

Buy the book.

Jan
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Last edited by jan.didden; 2nd July 2016 at 11:04 AM.
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Old 2nd July 2016, 10:01 PM   #5
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Hello,

In the Chapters 1 and 2 of the book Optimizing Op Amp Performance the op-amp is treated as the basic building block. Black’s feedback model was adapted to the 3 legged creatures with one output, one non-inverting input and one inverting input. On paper the op-amp conceptually looks pretty basic. Zoom in on the schematic of an op-amp made of discrete parts and it does not look nearly so simple after all. For example take a look at this op-amp made up of FET’s.

http://www.forsselltech.com/media/at...JFET_Opamp.PDF

I want to start more basic, perhaps a JFET or a vacuum triode. I like the idea of a triode better. I want to approach the feedback model in a fashion similar to triode noise modeling. Each source of noise is computed as if it is an Ein at a single input, the grid. With a model that assumes a single input we need to address the two potential triode outputs and program the feedback math (loops) so that feedback from these outputs to the single input matches actual circuit performance.

Assuming the grid is the single input what are the two potential outputs? We only have the anode and cathode to look at. Say we apply a DC power supply to the triode, positive voltage to the anode and negative to the cathode. Not so much voltage as to melt anything. Next we apply a AC voltage to the grid making sure that the grid is always negative in relation to the cathode, we do not want the grid to be a second anode. If we probe around with our oscilloscope probe we will only see AC voltage at the grid not at either the anode or the cathode. We will see DC voltage across the tube and there will be current flow through the tube. Now add a resistor between the B+ and the anode, also add a resistor between the cathode and negative terminal of the DC power supply. Try probing around again, we will find non-inverted AC voltage between the cathode and the cathode resistor and we will find inverted voltage between the anode and anode resistor. These are the 2 basic possible triode outputs, non-inverted and inverted. The feedback model will need to loop these voltages back or forward to the input and predict the behavior of a real circuit on the bench.

Jan, thanks for the book idea. I placed an order at Amazon. The fire control modeling you are speaking of is leading to next generation AI fighter planes with no pilot.

DT
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Old 2nd July 2016, 11:10 PM   #6
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Quote:
Originally Posted by DualTriode View Post
The single most interesting thing to me that Graeme had to say was this: “intuitive observations,
guide the model derivation, and then comparison of the model’s transfer response with that of
the circuit confirms the model’s validity”.
This is the scientific method. See Karl Popper's book, "Conjectures and Refutations:
the Growth of Scientific Knowledge".
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Old 3rd July 2016, 05:28 PM   #7
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Quote:
Originally Posted by DualTriode
Assuming the grid is the single input what are the two potential outputs?
If you make a false assumption then you may draw false conclusions.

Assuming conventional biasing, the normal valve input is grid-cathode (not grid) and the output is anode-cathode (not anode and cathode separately - the same signal current flows though both). Put the valve in a circuit, and then you might have circuit input from grid-negative supply and circuit output from cathode-negative supply (CF) but the valve sees this as exactly the same as the grounded cathode stage with circuit output from the anode-negative supply.
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Old 3rd July 2016, 06:55 PM   #8
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Good points. There never is 'a' input - input signal is ALWAYS between two nodes, just as output is always between two nodes. That is why a multimeter has two probes, you cannot measure a voltage on a point, only with respect to another point!

The reason why it is seldom looked at it like that is because it is assumed that the implicit reference to 'the' input is ground. In your triode, the grid is only 'the' input if you assume the cathode is grounded.

It is not by accident that 'voltage' is really 'potential diference' and difference needs two nodes.

Jan
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Old 3rd July 2016, 07:10 PM   #9
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Didn't someone do a drawing recently illustrating that point explicitly?
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Old 3rd July 2016, 07:48 PM   #10
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Didn't someone do a drawing recently illustrating that point explicitly?
Yes but I can't access it right now!
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