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Insights The What and Why of Circuit Analysis Assumptions - Comments

  1. Oct 30, 2017 #1

    anorlunda

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  3. Oct 30, 2017 #2
    A good topic to discuss! There are certainly many cases in real circuits where non-ideal behavior can have a huge affect. Just to offer a few suggestions:

    I think the main point you are making here is that there are non-ideal behaviors in real circuits which do not fit the assumptions of CA. You gave some of these assumptions, so maybe you could show some explicit cases where the assumptions are violated and what the correct solution would be.

    I think the references to QED and FT somewhat detract from your main point. Perhaps the focus should be on how EM fields from one part of a circuit can alter the operation of another part, such as with the multiple inductors in the schematic shown.
     
  4. Oct 30, 2017 #3

    anorlunda

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    Thanks for the kind words, but that was not my focus. Non ideal behavior would make an interesting Insights article, but others like @berkeman who designs circuits, or @jim hardy who troubleshoots misbehaving ones are better qualified than I to write it. My expertise in on the tools that analyze circuits that do follow CA assumptions.
    You may have heard of Spice. That is the kind of tool I mean. In power system analysis PSS/E is the tool. I wrote the first version of that software in 1971.

    Analysts have tricks to use CA even when circuit behavior is non ideal. Consider the following picture showing two three phase circuits sharing the same transmission towers. The mutual coupling between the two circuits violates the CA assumptions big time. But analysts simply adjust the values of series reactance for both lines to compensate, then put them into the analysis as if they weren't mutually coupled. In other words, they fudge the data to avoid the need to use Maxwell's equations to model the entire power grid (which would be impossibly difficult.) The practice is akin to the transformer equivalent circuit that I showed in the article. Non-ideal behavior is approximated by ideal components and standard CA methods are used to solve the circuit.

    1476699574.jpg
     
  5. Oct 31, 2017 #4

    donpacino

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    Great article. A classic CA example are batteries.

    It can be simplified to an ideal voltage source. But as you need more and more accuracy you add capacitance, inductance, and non linear elements. Before you know it you have a fourth order non-linear filter. That is before add series and parallel cells.

    It might be needed, depending on your application, but if you are making a pwm based low power led driver run off a voltage regulator, is all that really necessary. Making assumptions simplifies design and analysis so much, as long as one is aware of the conditions of the assumption.
     
  6. Oct 31, 2017 #5

    donpacino

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    Another common CA that greatly simplifies things, but can cause disastrous results if mismanaged...
    Forgetting that PCB traces and wire harnesses and wires can and will act as transmission lines, not ideal 0 impedance connections.
     
  7. Nov 1, 2017 #6
    Great Insight and great comments!
     
  8. Nov 1, 2017 #7
    From the thread on helical antennas: How about the fact that between its 1/4WL self resonant frequency and 1/2WL self resonant frequency, an inductor exhibits capacitive reactance? So much for inductance being proportional to the square of the turns. :eek:
     
  9. Nov 1, 2017 #8

    Dale

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    Great insight article! That was also my first circuit analysis textbook, and I remember that introductory chapter with its explicit list of assumptions to this day! Personally, I wish that every textbook would start off with such an explicit list of the simplifying assumptions used in the theory.

    As far as when to teach these assumptions, I think that it doesn't make sense to teach QED first, you would lose a vast number of students many of which can make good use of CA throughout their life but will never use QED. So CA needs to come first in a program of study. But even if a student never learns the more advanced courses, they should still know these assumptions. That way the students can tell if a given physical scenario is one that they can model with these tools or not. They may not know how to treat assumption-violating scenarios, but they will at least know that their familiar tools may not be up to the task.
     
  10. Nov 1, 2017 #9

    bhobba

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    CA as used by electronic engineers has some rather strange implications when applied to active devices like transistors.

    There is a device called a 'diamond' amp - its a little IC that acts like an ideal transistor - current can flow both ways through the base-emitter. The current is then amplified and appears at its collector.

    It's sometimes used as the buffer for a current output DAC chip that likes to see a virtual earth.. What you do is you clamp the base to ground and the emitter to the output of the DAC chip and have a load such as a resistor on the collector. Because the current flows both ways and it has gain it will always try to keep emitter also at earth - if not current will flow through the collector and whatever is connected to the emitter until it is. This is since the base is clamped to the earth it can't go there - no potential difference so it must flow through the collector and its load - yet still sees a virtual earth. I have shown the circuit to a number electronic engineers I know - one even incorporated it in a DAC he built - but it takes a while to see it, its just a simple application of current laws - current must go somewhere.

    That of course is the 'naive' analysis - what's really going on I have tried and failed to understand - it is rather tricky.

    Just as an aside I always thought the best output for a DAC chip was just a simple transformer. I got one of my electronic friends to try it - it failed miserably with the PCM1704. But he told me when he switched to an AKM chip it worked great. Interesting.

    Thanks
    Bill
     
    Last edited: Nov 1, 2017
  11. Nov 1, 2017 #10

    anorlunda

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    That was interesting. Thanks. Are you able to make an equivalent circuit for a diamond amp?
     
  12. Nov 1, 2017 #11

    bhobba

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    I am friendly with a number of electronic engineers in the audio field and believe me they know circuit analysis backwards. I have tried and failed miserably to explain QM to them - they just don't get it. QED - well that's way off the landscape for them. Yet of course transistors rely on holes that are really quasi particles. They understand well how transistors and diodes work using the hole idea - but quasi particles - that they have great difficulty with.

    Thanks
    Bill
     
  13. Nov 1, 2017 #12

    bhobba

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    For the gory detail of what is actually inside a diamond transistor see:
    https://www.edn.com/electronics-blogs/analog-bytes/4438881/Quest-for-the-Ideal-Transistor-

    In a circuit diagram it has the same symbol as a normal transistor but with two arrows in each direction - one from base to emitter and the other emitter to base indicating current can flow both ways - see fig 3 in the above link.

    It's also sold as a little IC eg the OPA 860:
    http://www.ti.com/lit/ds/symlink/opa860.pdf

    For some reason its not actually used much by the electronic guys I know, and when I was into electronics many many moons ago none of the circuits I mucked around with or saw in magazines used it, so may be out of production these days - but has some advantages - so I don't know why.

    Understanding whats going on at the naive level is a bit tricky as I explained - but it's simple when looked at the right way - its just current must go somewhere - if the base is clamped to earth it cant go there so must go through the collector and its load. Of course when you look at the actual circuit of one of these things as in the above link its not that at all - that's the tricky bit I have never really been able to figure out. Abstraction hides all sorts of difficulties.

    Thanks
    Bill
     
    Last edited: Nov 1, 2017
  14. Nov 1, 2017 #13

    Averagesupernova

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    I would assume the diamond transistor is not very popular because what you describe it being able to do can be done easily with op amps. Unless I misinterpret what you are describing.
     
  15. Nov 1, 2017 #14

    donpacino

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    I thought a diamond amp was a transconductance amplifier.
    To boil it down to one line, it would be an op-amp with a current output instead of a voltage output
     
  16. Nov 2, 2017 #15

    bhobba

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    That's another name for it.

    The advantage is you do not have to bias it on - current travels both ways.

    Don't know why for sure, but if you use it instead of an a normal op amp for current to voltage conversion it tends to sound better according to those that have tried it - possibly because its better slew rate -
    'Another dynamic advantage of CFAs is their relative immunity from slew-rate limitations because Cc is driven directly by the input buffer, which can supply virtually any current to rapidly charge/discharge Cc.'

    Thanks
    Bill
     
    Last edited: Nov 2, 2017
  17. Nov 3, 2017 #16

    LvW

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    Regarding the "Diamond Transistor": I think, the device we call "transconductance ampolifier , OTA" has two high-resistive inputs.
    In contrast, the device called "Diamond Transistor" has one high-resistive as well as one low-resistive input.
    That`s why it is a kind of current conveyor (CCII).
     
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