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Need help to understand the amplifiers circuit

Discussion in 'General Electronics Discussion' started by joey Kou, Jun 2, 2016.

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  1. joey Kou

    joey Kou

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    Jun 2, 2016
    Recently I am reading a paper including the circuit below. Could anyone help me analysis this circuit? The paper said the first stage is to get the voltage difference and the second stage is to compare with the power ground. I do not know why the the first stage get the voltage difference.
     

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  2. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    The first stage is "looking" at three-phase power, two phases at a time, A-B, A-C, and B-C. The second stage compares each of these to ground, outputting square waves.

    Is this a homework problem?
     
  3. joey Kou

    joey Kou

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    Jun 2, 2016
    Thanks for your reply. No, this is my personal project. Could you explain a little about the first stage? The paper said the output of first stage is voltage difference. But my calculation is different. Thanks
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Show us your calculation.
     
  5. joey Kou

    joey Kou

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    Jun 2, 2016
    For the top one, (Vc-Va)/R=(Va-Vac)/R, then Vac=2Va-Vc.
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    I get the same result. Looks like the non-inverting input needs a resistor of value R connected to common.
     
  7. joey Kou

    joey Kou

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    Jun 2, 2016
    Thanks.Maybe the paper has mistakes.
     
  8. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    No "maybe" about it. The description in the paper is wrong, unless you add another resistor of value R between the non-inverting input and circuit common. That could just be an oversight in drafting the schematic, but someone should have proof-read it and corrected it before publication. If I find more than one "mistake" like that in a text I throw it away and find a better text. Life is too short to have to go around and correct other people's mistakes. Hell, I make enough of my own mistakes! Thanks for taking the time to question what you read. There is no substitute for critical thinking. And good luck in your personal project. Come back here if you run into problems. Oh, and welcome to Electronics Point. :D
     
    chopnhack likes this.
  9. AnalogKid

    AnalogKid

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    OR, the intent of the original design is not a balanced subtraction...

    ak
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    Who knows what the "intent of the original design" was? Why all the resistors in series with the non-inverting inputs of six op-amps? Assuming "ideal" op-amps, these resistors serve no purpose since no current flows through them.
     
  11. joey Kou

    joey Kou

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    Jun 2, 2016
    Hi Hevans, this is the terminal voltage sensing circuit for BLDC motor control. And the paper is from http://ieeexplore.ieee.org/xpl/logi...6109934/6119266/06119564.pdf?arnumber=6119564
     
  12. joey Kou

    joey Kou

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    Jun 2, 2016
    Maybe. Now I am trying to check whether I have wrong understanding about the paper
     
  13. hevans1944

    hevans1944 Hop - AC8NS

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    I think it was probably an oversight in proof-reading the artwork before the paper was submitted and subsequently published after the conference you cited. This happens more often than you might think with this type of submission. Even peer-reviewed papers can have mistakes of omission or commission.

    After reading the abstract (the entire document is hidden behind an IEEE paywall), it is clear the author intended a differential sampling of the three phases to derive zero-crossing pulses, claimed to be representative of the commutation points of a brush-less DC (BLDC) motor, said commutation points normally being derived from Hall Effect sensors.

    It is common in papers of this type to use "simplified" diagrams to get the point across, perhaps later including actual circuit diagrams if a working system is built. I did this in the (one and only) paper I submitted and presented in person to a conference on lasers many years ago. My paper described how a commercial plane-mirror distance-measuring laser interferometer was interfaced to a precision x-y scanning stage and a personal computer. It was not appropriate to show all the circuit details for this type of presentation, so I didn't.

    I took questions from the audience after my presentation, but judging from their glassy-eyed stares, most of what I had said went right over their heads. This was not a peer-reviewed conference presentation. At the insistence of my employer, I submitted an abstract prior to the conference and was subsequently invited to submit a full paper. To give you some idea of how this works (or does not), my presentation was followed by someone who claimed to have solved the mysteries of the Universe with their Grand Unified Theory of Everything... based on numerology! He wasn't exactly booed out of the room, but after about ten minutes of his babble most of the audience got up and left. I endured as much as I could, but didn't stick around long during the Q&A period. Most of the remaining audience really raked him over the coals during that. It was truly embarrassing

    I should note that this was an industry-sponsored conference, not an IEEE sponsored event. The quality of most of the presentations was quite good and revealed current research, but not in enough detail to actually provide instruction on how to reproduce it. I attended many similar conferences during my career but never walked away thinking, "Hey! What a neat idea! I think I'll go back home to the lab and do that too."

    When I was working a few years ago to develop a high-voltage (kilovolts), high-current (kiloamps), high-speed (nanoseconds), gallium arsenide Photo-Conductive Semiconductor Switch (PCSS), based on research performed at Sandia National Laboratories by a team of government scientists in the 1990s, I read a lot of papers this team had submitted at various high-power pulsed-energy conferences. The quality of their papers was excellent, but their implementation was unsuccessful in the practical sense. Their switches were expensive to construct and self-destructed after (typically) only a few hundred "shots". It took us more than two years to solve that problem, and in the end it seems no one was interested. There is other technology (silicon carbide) commercially available that performs in a similar manner.

    If there is a point here, it is this: Conference papers provide a "heads up" view of current technology developments, but seldom provide sufficient detail for an outsider to continue the work. If you want to build a BLDC motor controller based on the paper you cited, use that paper as a guide post. But realize that the actual design will have to be your own, perhaps based on the principles revealed in the paper. Good luck with that.

    Hop
     
  14. joey Kou

    joey Kou

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    Jun 2, 2016
    Hi Hop, thanks for your long reply. I learn a lot and I am now working on realizing it in my lab.
    Thanks again
     
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