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Feedback needed for an oscillator or multivibrator

  1. Jan 11, 2017 #1
    i want to ask why the feed back at multivibrator must be positive feedback ?
     
  2. jcsd
  3. Jan 11, 2017 #2

    berkeman

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    Staff: Mentor

    Can you post links to the reading you've been doing? Why fundamentally do you need positive feedback to form an oscillator?
     
  4. Jan 11, 2017 #3
    i read chapter 18 (switching circuits ) at (principles of electronics by v.k mehta) ..
    section 18:10
    he mention that the multivibrator is a two stage amplifier that work with a positive feed back .. i can take a screenshot if you need ..
    the use of positive feed back at oscillators i already understand ..
     
  5. Jan 11, 2017 #4

    berkeman

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    That would help, thank you. :smile:
     
  6. Jan 11, 2017 #5
    here is the photo of the beginning of the section
     

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  7. Jan 11, 2017 #6
    here the second photo for the rest of the section (the 1st one at previous reply)
     

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  8. Jan 12, 2017 #7

    LvW

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    To answer this question it is necessary for you to know the meaning of the term "stability" and to be familiar with stability criteria (Nyquist, Barkhausen,).
    Intuitively, each harmonic oscillator and each multivibrator needs a kind of self-excitement (autonomous build-up signal amplitudes). For this purpose, a part of the output voltage must be coupled back to the input - without any phase shift: That`s positive feedback.
     
  9. Jan 12, 2017 #8
    it seems helpful .. but at multivibrator say- astable- how the capacitor and resistor make positive feed back
     
  10. Jan 12, 2017 #9

    LvW

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    Which circuit are you referring to? With opamp or with transistors?
    Please, show the circuit. Otherwise, explanations are problematic.
     
  11. Jan 12, 2017 #10

    Svein

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    Here is the simplest possible circuit for an astable multivibrator:
    schmitt-trigger-osc.png
     
  12. Jan 12, 2017 #11

    jim hardy

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    Hmmmm

    since we're dealing with an introductory textbook
    maybe a boiled down answer is best for right now ?

    The multivibrator is described as an amplifier
    but it has no input

    meaning

    its gain, output/input, must be infinite.

    Since no amplifier can have truly infinite gain , in order to have an output it must get an input from someplace.
    That someplace is from its own output.
    It must get its input from a fraction of its output and that input must be in a direction to reinforce output not oppose it. And, that is the definition of positive feedback.
    Were feedback in a direction to oppose further output it'd soon drive output to zero which is what negative feedback does.

    When you study feedback they will explain that
    An amplifier with gain A
    and feedback B
    has gain A/(AB-1)

    note that when product AB = +1 the denominator becomes zero so gain becomes infinite and it'll make an output with no input and that's an oscillator..
    but if product AB is negative , which is negative feedback, gain is always less than A.

    For simplicity set A = 1 and plot gain for B = -1 , -0.5, 0, +0.5 and +1
     
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