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Feedback stuff

Discussion in 'Random Thoughts' started by ultrabike, Dec 12, 2016.

  1. ultrabike

    ultrabike Measurbator - Admin

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    I take that back. I think resistors and inductors in series are all zeros. Let's say, for all practical purposes... Or something like that.
     
    ultrabike, Dec 12, 2016
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  2. tomchr

    tomchr MOT - Neurochrome

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    Not sure if we're talking about the same zeros. A zero in the transfer function is a root in the numerator. A pole is a root in the denominator. You can make either with combinations of R with C and/or L or LC alone. That's not feedback, though. You need a summing junction and feed back some of the output to the input to have feedback.

    Tom
     
    tomchr, Dec 12, 2016
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  3. ultrabike

    ultrabike Measurbator - Admin

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    LOL! This may sound a bit autistic to some. What I mean is that if you have roots in the denominator I see that as feedback.

    It may be that I think of it more in the digital domain, where roots in the denominator correspond to an IIR filter which has feedback. I know the two domains are not exactly one to one (there is a bilinear transformation and even that one doesn't always work). But yes, poles in the denominator to me means there is some kind of feedback going on. On passives it may be unconditionally stable since it can't grow as the passive element does not contribute energy.
     
    ultrabike, Dec 12, 2016
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  4. tomchr

    tomchr MOT - Neurochrome

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    A lowpass RC filter has a real pole at 1/RC. That's just voltage division.

    Voltage division: Vout = Vin (R2/(R1+R2)) (R2 is the resistor from Vout to GND). Now replace R2 with a capacitor. So substitute 1/wC for R2 and work out the math. You should end up with a transfer function that has a pole at 1/RC. Unless you end up with X = X or 2 = 3, which are common results, but not the correct answer. :)

    Tom
     
    tomchr, Dec 12, 2016
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  5. ultrabike

    ultrabike Measurbator - Admin

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    Alright, moving this discussion here.

    @tomchr. Yup. I know voltage division. And I know Laplace.

    A resistive network will not have poles and zeros other than at zero in the complex plane. Add caps and inductors and all of the sudden, poles and zeros not at zero magically appear.

    Does it have feedback if you add a capacitor in series with a resistor? I believe so. The first order RC low pass can also be referred to as passive integrator, which has some sort of feedback and memory, otherwise it would not integrate. The feedback coefficient maybe less than unity, but non the less I don't think it's zero.

    I don't mean to be condescending in the very least. I just want to let you know where I'm coming from.
     
    ultrabike, Dec 12, 2016
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  6. Armaegis

    Armaegis Friend

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    Armaegis, Dec 12, 2016
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