FET help

Discussion in 'General Electronics Chat' started by asoljazworld, May 4, 2013.

  1. asoljazworld

    Thread Starter New Member

    May 4, 2013
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    Dear members
    I am having trouble figuring out the common ground FET that is in this circuit. I know what the tank circuit is doing, but I get stuck at the FET with the capacitor across the source and drain. Could someone please help me to understand what is going at that point in the circuit. Thanks
     
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  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    That's not a Fet its a Bipolar npn.
     
  3. asoljazworld

    Thread Starter New Member

    May 4, 2013
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    OK,
    So, can someone explain to me what the BJT is doing in this situation?
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    It is acting as an oscillator and is modulated by the AC singal coming from the microphone.
     
  5. asoljazworld

    Thread Starter New Member

    May 4, 2013
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    There is no microphone in the circuit, but do you mean the ac sigmal coming from teh antenna. And could you explain to me what the capacitor is for? The one that runs from drain to source?
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    There is a microphone on the left side of the schematic, called ELECTRET MIC. The capacitor that runs form EMITER to COLLECTOR is part of the oscillator, not sure how is that topology called.
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    ITS NOT A FET!!! so stop saying drain to source, its Collector to Emitter.

    the audio signal comes from the microphone on the left and is modulated onto the RF signal generated by the NPN transistor, connected as a Colpitts oscillator, to produce an AM signal that is transmitted up the antenna.


    Look up Colpitts Oscillator
     
    Last edited: May 4, 2013
  8. asoljazworld

    Thread Starter New Member

    May 4, 2013
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    Sorry
    I accidently uploaded the wrong picture from file, please take a look at the one attached
     
  9. Dodgydave

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    Jun 22, 2012
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  10. asoljazworld

    Thread Starter New Member

    May 4, 2013
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    Ok,
    I am just trying to understand the circuit. The part that I am having a hard time grasping in the FET and why there is a capacitor between the drain and souce. I am just trying to understand its functionality in the ciruit.

    Thanks
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The JFET is the active element in a common gate Colpitts oscillator. The capacitor in question provides the positive feedback necessary for oscillation.
    My best guess is that this is a superregenerative receiver.
     
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