shall we discuss about feedback in electronics?

Discussion in 'General Electronics Chat' started by xljin2014, Jun 12, 2015.

  1. xljin2014

    Thread Starter New Member

    Nov 11, 2014
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    hi friends, how do you think about feedback in electronics? i feel it not easy to understand, i am confused and crazy at signal flowing details, the time delays, though i can't say the point out clearly now, so i would like to discuss about it with you, and maybe i can find what the problem point of mine is.

    so maybe you start with your understanding and let me ask questions, ok?

    thank you for you help so much!
     
  2. WBahn

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    This would be like me posting on an automotive forum saying that I don't feel that modern fuel systems are easy to understand and asking them to post their understanding of them so that I can ask questions.

    Here's another approach that might get you more response: Pick an example of a simple circuit that had feedback that you don't understand and post it. Then try to describe where you are having problems.
     
  3. Hypatia's Protege

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    Your question is ambiguous in that it could refer to anything from servo-mechanisms to degenerative stabilization networks to oscillators -- please be more specific:)

    Best regards
    HP
     
    Last edited: Jun 12, 2015
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  4. xljin2014

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    Nov 11, 2014
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    ty WBahn and HP,

    so first of all, can i say this:

    there are mainly two types of electronics circuits:

    1. circuits without feedback, so inputs just decide outputs, signals go from inputs ahead to outputs as one direction;
    2. circuits that have feedback, so signals go from inputs to outputs and back to inputs, so there are two directions: go ahead and come back.


    so if we can analys how inputs decide outputs, and how feedback work, then we can understand all circuits, am i right?

    ty
     
  5. WBahn

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    I suppose you could look at it that way, provided you use such a broad brush as to be almost useless.
     
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  6. xljin2014

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    Nov 11, 2014
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    ty,

    now here is a circuit example for feedback, a simple opamp voltage follower.

    when Vdd and Vss are added, and then add Vin, from the time Vin is added, how is the Vout (or V_) change from the original value(what is it?) to Vin? and experimental fact shows that feedback progress lead to the final value of Vin, not an osillation, but we need to analyse how feedback works exactly.

    when Vin is added, how is Vout? from opamp, Vout is determined by Vin and V_, but V_ is determined by Vout, now from math, it is just an equation: f(Vin, Vout)=Vout , where f(Vin, Vout) is the function of opamp. but as for physical circuit, the problem is there is time delay , so for the Vout from beginning, then Vout transforms to V_ after a while (about the time of the distance from Vout to V_ divided by the speed of light), and V_ and Vin then decide a new Vout after a while (that opamp delay time due to paratistic capacitors etc ), and then Vout goes to V_ again, and the new V_ and Vin again determine Vout, and negtive feedback goes to the conclusion of Vout=Vin after a short time.

    ty, after careful analysis of feedback circuit , i seem feel better. is my analysis right?

    ty again.

    feedback.png
     
  7. Jony130

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  8. atferrari

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    Socratic method in action, it seems. Not for me; thanks.
     
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  9. Hypatia's Protege

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    Assuming I correctly understand your post, you seem to be confusing continuous vs discrete phenomena:

    As an aid to perspective please consider the fallacy in the following statement:

    "Motion is a practical impossibility because there are an infinite number of positions between any two points, ergo any motion requires an infinite number of acts and, hence, infinite time..."

    Get it?:)

    Best regards
    HP
     
  10. WBahn

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    It's a bit hard to follow your explanation (English as a second language, I'm assuming, which we just deal with as best we can), but your basic reasoning is on the right track. Note that the time delays you are talking about are there but are seldom of any consequence until you are talking about very high speed signals. Also, they are not fundamental to the operation of the circuit.

    The model for the opamp output as a function of the input, to first order, is simply

    <br />
V_{out} \; = \; A_{v}\(V_{+} \; - \; V_{-}\)<br />

    Where Av, the open loop gain, is typically a fairly large value of between 100,000 and 10,000,000.
     
  11. xljin2014

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    Nov 11, 2014
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    ty all, jony130, atferrari, hp, and wbahn.

    1) to jony130: you gave some explanations of how feedback work, and here is something about the first several moments of feedback that you didn't focus on.

    look at this: it gives the inside circuit of opamp, but it is still a voltage follower (with Vout feedback to V_) , or unity-gain buffer.

    now look at when Vin changes from low voltage to high voltage, how Vout changes. after some time, Vout is equal to Vin(such as point C ), yes. but at the begining, such as time A and B, Vout is also equal to Vin, but does it just stay there? no! it may still go up (point A) or down(point B). actually it oscillates somehow and then gets to the stable situation.

    of course feedback occurs through all time, and if some studies been done , feedback surely should be of concern.

    you may say there is output capacitor (and capacitor inside opamp) there that may explain the Vout curve, yes, and that is what i suspect, i think the detail how the capacitors shape the Vout curve (with feedback) should be studied . P50614-113451.jpg



    2) to wbahn,

    now let me tell you what i am puzzled about feedback:

    what puzzled me is that i cann't find the initial reason of how a feedback circuit behaves in the end.

    let me explain : look at this pic of a feedback system, the output of the circuit comes backward to the input through a feedback circuit, and added or subtractted by the input, gets a net input of the forward circuit, throught it, gets the output.

    sys28.gif

    now problem comes: on the analysis above , we got an output at first , but where does it come from? it should be come from the forward circuit by the net input, and the net input is circuit input ( θi ) plus ( or sub ) the feedback input ( θf ) , and the feedback input ( θf ) comes from output through feedback circuit ... ... so and so , thus we get an circle of signal flowing, output determine input , and input determine output , so my question is : which one ( θf or θo )is the initial factor that start the signal flowing circle? from the discuss above, can't see which one is superior .

    regards,
    ty again,

    jin
     
  12. Hypatia's Protege

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    Again, you are attempting to superimpose a discrete paradigm upon a continuous system -- 'Step wise' analysis is simply not applicable here --- Given a chance, even reality can be beautiful:D

    Best regards
    HP
     
  13. Alec_t

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    Both are present at the same time. Neither is superior.
     
  14. xljin2014

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    Nov 11, 2014
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    in every book it is said that feedback is output back to input, and thus determine new output.

    is it a discrete paradigm or a continuous paradigm?
     
  15. xljin2014

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    then how can we make sure that they abey rules that output determine the input by feedback circuit and the input determine output by forward circuit?
     
  16. Hypatia's Protege

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    'Feedback' (as applied to analog systems) describes a continuous process (i.e. a single, indivisible, ongoing, 'act') application of discrete analysis to which will render useless, often ludicrous, conclusions (à la the example in post #9)

    If it's any consolation, the very fact that you've taken note of this (seeming) paradox speaks very favorably of your intellect and industry!:) --- Moreover, I encourage you to continue your pursuit of this matter until such time as you reach a comfortable perspective...:cool:

    Sincerely
    HP
     
    Last edited: Jun 14, 2015
  17. Alec_t

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    "We" don't have to enforce the rules; the opamp circuit does that. Clever, ain't it? :D. Those two conditions apply simultaneously. It isn't a chicken-and-egg situation.
     
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  18. kubeek

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    You are missing the point that the circuit is continous both in time and amplitude. So, the output doesn´t have to wait for the feedback input, there simply allways is some voltage present at the output, and the actual value doesn´t matter. The feedback then makes the opamp steer the output in the direction that gets the output to the voltage that is set by input voltage and feedback gain.
     
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  19. WBahn

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    In reality, yes, there will be a delay going through each of the blocks, but the model for G(s) and for H(s) can include that if it is important (and sometimes it is and sometimes it isn't).
     
  20. xljin2014

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    Nov 11, 2014
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    i think that is the right answer to my question of how the initial reason comes into being of a feedback circuit .

    it comes from nothing at the very begining i think , and later circuit starts action , including feedback .

    ty all,

    regards,

    jin
     
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