rc phase shift oscillator

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Discussion in 'General Electronics Chat' started by maxpayne, Nov 23, 2008.

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  1. Nov 23, 2008 #1
    maxpayne

    Thread Starter New Member

    Nov 23, 2008
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    why a fet rc phase shift oscillator?
    fet doesnt bring any 180 degrees phase shift then what use of those three rc networks?
    i'm simply a starter!
     
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  2. Nov 23, 2008 #2
    KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    401
    ANY class A amplifier with a common emitter, source or cathode, will have 180 degree phase shift (or close to it)

    ERic
     
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  3. Nov 24, 2008 #3
    DickCappels

    Moderator

    Aug 21, 2008
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    To oscillate, you need to have 360 degrees phase shift.

    With the RC phase shift network, the FET provides the first 180 degrees. Each RC stage can supply up to almost 90 degrees (so just two stages won't get you 90 degrees), so using three stages gets you a total of 180 derees for the phase shift network, with each stage contributing about 60 degrees.

    FET 180 degrees
    RC stage 60 degrees x 3 = 180 degreest

    TOTAL = 360 degrees
     
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  4. Nov 24, 2008 #4
    Wendy

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    Mar 24, 2008
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    Gain tends to be critical in these circuits too.
     
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  5. Nov 24, 2008 #5
    maxpayne

    Thread Starter New Member

    Nov 23, 2008
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    sorry i was wrong
    fet really brings that 180 degrees phase shift and it is used for the rc phase shift oscillator
    but how is it proved?(that fet brings a 180 degrees phase shift?
     
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  6. Nov 24, 2008 #6
    KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    This is etremely simple. Model the FET and the load resistance as a simple voltage divider. The greater the FET current, the lower the voltage at the drain.
     
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  7. Nov 24, 2008 #7
    maxpayne

    Thread Starter New Member

    Nov 23, 2008
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    it sparked to glow
    radio extincted
     
    #7 Reply
  8. Nov 25, 2008 #8
    Wendy

    Moderator

    Mar 24, 2008
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    When the techs glow,
    then you know
    You're Radioactive.
    Time to Throw
    the Breaker.
     
    #8 Reply
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