Talk:Circuit Idea/How to Swap Circuit Inputs and Outputs

In order to provoke discussion about the unique feature of negative feedback to reverse the causality in circuits, I have extracted here a part of correspondence between me and Gordon Deboo (the inventor of Deboo integrator). Circuit-fantasist (talk) 19:54, 25 December 2007 (UTC)



From: Gordon Deboo To: "Cyril"  Sent: Tuesday, October 09, 2007 3:34 PM Subject: Re: Deboo Integrator

... Looking at a problem in a different way can sometimes lead to new ideas. Just to give a simple example (Fig. 1), consider the attenuator (Figure a) and the well-known non-inverting amplifier (Figures b and c) shown in the pdf attachment.

The equation (4-1) for the attenuator shows that the input voltage V1 is attenuated to V2 depending on the values of the two resistors. Equation 4-1 can be solved for V1 as in Equation 4-2, where V1 is great than V2, so that amplification has occurred. Of course this can’t happen with two resistors because it would involve the creation of energy. However, if the required energy is supplied from an external source, such as an operational amplifier (resulting in a  conventional non-inverting amplifier as in Figure b, more usually  drawn as in Figure c) the equation can be satisfied.

I often manipulated circuits in this way, drawing a “box” with the input and output I required and then figuring out what I needed to do to get the desired result. This may seem obvious, but the idea was think about supplying energy to an attenuator to make it an amplifier after rearranging the equation told me what was needed. This type of thinking can enable one to create...

Gordon Deboo California



From: Cyril Mechkov To: Gordon Deboo Sent: Saturday, October 13, 2007 8:32 PM Subject: How to reverse circuit ports by applying negative feedback (Re: Re: Deboo integrator)

Dear Gordon,

Reading your last email, I was deeply struk by your idea of how to transmute a voltage divider into a non-inverting amplifier. Will you believe me that for the first time I consider a valuable circuit idea with someone? Today's people do not like to think about fundamental ideas and principles. Every time when I tried to discuss some basic circuit idea I run upon the rocks...



Surprise. Imagine my amazement when I was reading your excellent explanations (I suppose that the scanned page is from your book) because the same great idea conceived in my mind in the early 1990s! Then I was writing down (regrettably, in Bulgarian) all the ideas arising in my mind on squared sheets of A5 paper. I have scanned especially for you the yellowed record that I made on May 14, 1992 at my home (I was writing the circumstances surrounding the recorded idea on the top of the sheet; now, I continue doing the same).

"Reversed" voltage divider. This is the title of the record, which is eloquent; the drawing too. As you can see, I have drawn (likewise you on Fig. b) the circuit with an unusual shape to show the basic idea in the best way; also, I have colored the added op-amp in a different blue color. The idea is the same: to make the passive voltage divider work conversely we supply it by an additional (properly supplied) op-amp that observes the difference between the divider's output voltage and the actual input voltage. As a result, the op-amp adjusts its output voltage Vout so that an R2/(R1 + R2) part of it to be equal to the input voltage Vin. Thus, the old output voltage VR2 is always equal to the new input voltage Vin; the old divider's input voltage serves as a new output voltage Vout = (R1 + R2)/R2.





Mechanical analogy. On another sheet, I have described a lever analogy of a non-inverting amplifier that exploits the same powerful idea. A man is looking continuously at point A (the output of the "lever divider") and is comparing its height Hin' with the input height Hin. If there is a difference between them, the man moves the end of the lever (the point B) to make Hin' = Hin. As a result, Hout/Hin = (l1 + l2)/l1. By the way, this second-class lever analogy is more truthful than the third-class lever analogy of non-inverting amplifier presented by Tony Kuphald in his Lessons in Electric Circuits.

Human analogies. Thinking about the essence of this phenomenon, I began discerning similar manifestations in many everyday situations (some of them -quite funny:) Secretary > boss. Imagine a male chief that has a beautiful female secretary (or v.v.:); so, he indulges the pretty woman satisfying all her wishes. As a result, the boss thinks that he manages the secretary but, actually, the secretary leads the chief by the nose:) Wife > husband. Similarly, husbands that indulge their wives think that they manage their wives; actually, wives manage their husbands:) Generally speaking, women lead men by the nose in this world:)



Cat > owner. I am also a victim of this phenomenon. I think that I manage my (female) cat Futie; but giving a portion of food to "her" every time when "she" says "miaou" actually, the cat leads me by the nose:(

Generalization. Finally, I made a general conclusion that systems with negative feedback have the unique property to reverse the cause and effect relations between the input and output quantities of the objects. In this way, they can "reverse" the objects. Imagine a block diagram where a converter implementing a function y = f(x) is connected in the feedback loop; as a result, the whole nfb system will implement the reverse function Y = F(X), where Y = x, X = y and F = 1/f.

Applications. For our purposes in electronics, we can use this technique to swap the converter's ports (especially of the one-way devices) by connecting them into the feedback loop. From this viewpoint, an active voltage-to-current converter is actually a reversed passive current-to-voltage converter, an op-amp non-inverting amplifier is a reversed voltage divider (your idea), an op-amp integrator is a reversed differentiator and v.v., an op-amp logarithmic converter is a reversed antilogarithmic converter and v.v., an analog-to-digital converter is a reversed digital-to-analog converter, etc. Another very interesting example is a transdiode that is actually a transistor connected in the op-amp feedback loop; in this odd and never explained circuit configuration, the collector current is the input quantity (?!?) while the base-emitter voltage is the output quantity (?!?) Maybe, the same trick is used in the input part of a current mirror - the active diode, which is actually a transistor with parallel negative feedback (again, although it seems strange, the collector current serves as an input quantity while the base-emitter voltage serves as an output quantity)...

Popularization. Gordon, I am sure you will say "Oh" when you see the picture below (Fig. 5):) Here is the story about it.



In the end of 1990s, I prepared a series of 22 short 1-page articles under the title How to invent circuits. I designed them for Popular electronics (later Poptronics) magazine. Only, Poptronics ceased and I refused all the print undertaking and moved to the web. A few years later I put all the ready stories into a pdf file. As you can see, the story on the page 14 (with a title Compensating by injecting a current) is dedicated to your great integrator. The last story (Let's reverse causality!), which is not completed, is dedicated to the present idea. See the picture on the right - it is exactly as your picture! Is it not amazing that the same idea and the same picture can conceive in different minds? Below the left figure is written in red "a normal divider"; above the right figure is written "a reversed divider".

In the beginning of 2000s I began creating circuit-fantasia.com. Then I included this basic idea in a hierarchical circuit idea classification (time was when I was thinking that this collection was my supreme achievement but others were not thinking as me; so, I did not obtain the recognition that I had expected). Later, I used this technique (likewise you) as a possible way to "invent" the non-inverting amplifier. By the way, another possible way of "inventing" is to "disturb" an op-amp voltage follower by putting an attenuation R2/(R1 + R2) into the feedback loop. As a result, it "reacts" to this intervention by (R1 + R2)/R2 amplification (see Class Exercise 8: Op-amp Amplifiers with Negative Feedback). I have named this great principle "attenuation causes amplification". Actually, it represents the main property of negative feedback systems to compensate disturbances).

Circuit idea. I have reserved a special place for this "reversing causality" phenomenon in the contents of Circuit idea (see the section about negative feedback). I imagine that this part has to contain an introductory "philosophical" story and a series of specific stories dedicated to various circuits based on this idea. I would like to include in some way your materials in these stories. There are a lot of possibilities to do that.

For example, you can develop a story about "inventing" a non-inverting amplifier based on your idea exposed on the scanned text. It may consist of a text based on the image or a few paragraphs and the very scanned image included. If you do not want to do that, I might cite your text and include the original image into the story. Or, I can share with readers in the discussion page how I thought that... but ...

Gordon, believe me, your way of thinking in this material is even more valuable than the same idea. Thinking in this way, young people can invent new circuits. But yet, have you reveal this great idea? Have you written this text (I mean the scan)? Is this published and where? Can I place a link pointing to the source where it is published? It is extremely useful for Circuit idea, if you send me more clever circuit ideas...

Circuit-fantasist (talk) 18:39, 25 December 2007 (UTC)