FM transmitter based on bipolar 2N3904 transistor

Discussion in 'Homework Help' started by hamzaraziel, Feb 17, 2017.

  1. hamzaraziel

    Thread Starter New Member

    Feb 17, 2017
    1
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    Hi, can anyone explain to me how this oscillator works , i need it tomorow for school project thnx

    upload_2017-2-17_13-13-45.png
     
  2. wayneh

    Expert

    Sep 9, 2010
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    4,267
    L1 and C2+C3 form an LC circuit that will "ring" at a characteristic frequency when perturbed. Any current flowing across the transistor causes a perturbance to start ringing and affects the frequency of ringing.

    I don't see how the carrier frequency could be present without an audio signal, so it's not much of a transmitter.
     
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  3. Bordodynov

    Well-Known Member

    May 20, 2015
    1,043
    317
    This oscillator circuit. The oscillator frequency is determined by all capacities. Including the parasitic capacitance of the transistor. These non-linear capacity and depend on the DC transistor mode. Audio signal mode changes the transistor and thereby modulates the frequency (and amplitude) of the generator.
     
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  4. AlbertHall

    Distinguished Member

    Jun 4, 2014
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    C2 provides positive feedback around the transistor and this causes oscillations at the RF frequency.
     
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  5. wayneh

    Expert

    Sep 9, 2010
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    How does the feedback get to the base of the transistor? I'm not arguing – I'm befuddled by anything RF. Would the circuit oscillate if the transistor were replaced by a fixed resistor?
     
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  6. MrAl

    Distinguished Member

    Jun 17, 2014
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    754
    Hi,

    What is the transition frequency of this transistor? I dont think it is very high, maybe 200MHz. At 100MHz you wont have much gain left. It might work to some degree but the transmission power will be very low.
    To do better, get a transistor that is actually made for RF work. They go up much higher in frequency so there is more gain left when you are working at 100MHz. I dont remember the part numbers but you can look one up or else i'll take a look myself.
     
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  7. AlbertHall

    Distinguished Member

    Jun 4, 2014
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    It doesn't. The feedback is from the collector to the emitter. It is a common configuration of colpitts oscillator - common base.
     
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  8. BR-549

    Distinguished Member

    Sep 22, 2013
    2,998
    724
    No. No one can. You would have to understand how simple oscillators work.........before you can understand this one.

    And if you are in a class that expects you to answer this...........you need to take basic classes before this one.

    You can't jump over a mountain, you have to climb it.
     
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