Colpitts oscillator. [FM transmitter]

Discussion in 'Radio and Communications' started by kazafken, Mar 25, 2009.

  1. kazafken

    kazafken Thread Starter New Member

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    [​IMG]

    BJTs works as an amplifier in the amplifier part of the FM transmitter.

    I'm wondering what's the function of the BJT in Colpitts oscillator part?
    All i know is Colpitts oscillator consists of the the 2 cap parallel with the inductor.

    I'm guessing its a buffer and a further amplification of the output,am i right?

    why the C5 is placed before the antenna? is it to dc coupled? i'm saying this because the spectrum analyzer must measure the frequency spectrum only at positions after the C5..

    And i don't understand why C8 is placed the in the center of the 2 capacitors and to the emitter part? Can i juz put C8 from collector to emitter?

    anyway,really apprecitate a lot if you can help.
  2. Audioguru

    Audioguru New Member

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    It is the 100MHz oscillator.

    No.
    The capacitors are in series. They are not the oscillator, they tune the coil.

    No.
    It is the 100MHz oscillator. You can add an RF amplifier transistor to it if you want.

    My Colpitts FM oscillator works the same but has one less capacitor.

    C5 prevents a short circuit if the antenna accidently touches 0V.

    Of course. C7 is not needed.

    Attached Files:

  3. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Something you have to understand is the oscillator is using a common base amplifier for Q2. The emitter is the input of the amplifier, and the collector is the output, with C8 providing the necessary feedback for the amplifier to oscillate. C6, C7, and L1 form a tank circuit, which selects the oscillators frequency.
  4. kazafken

    kazafken Thread Starter New Member

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    Thanks a lot,Bill and Audioguru.

    So, the transistor Q2 of the oscillator is the 100M Hz oscillator itself,which is also a common base amplifier? so,it amplifies output from collector of Q1?

    audioguru, tuning the coil means setting inductor value constant but the capacitor can be varied? Varied to get the oscillation frequency which is also the transmission frequency,right? and can you give me an example of RF amplifer? i'm not sure how to set it in my circuit.
    i saw this starter kit i bought once, there are 3 transistors and my friend told me one of them is a buffer to filter the noise. izit the correct function of the buffer/RF amplifier?

    Bill,what do u mean by neccessary feedback for the amplifier to oscillate? so the signal goes in the amplifer and amplified signal goes in to the oscillator, and the output from the oscillator should be a negative feedback or a positve one?
  5. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    The amplifier configuration of Q2 is common base. It is the amplifier that is the core of the oscillator.

    From the audio side, where the audio signal is injected, the same transistor Q2 is treated like a common emitter. Small changes in capacitance in Q2 from the audio signal is what FM modulates the RF.

    Many times a circuits function is based on how you look at it.
  6. kazafken

    kazafken Thread Starter New Member

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    what's C4 ?

    i've done an experiment where i put a 22n F (given by lecturer) and the output of the amplifier is not amplified to 2Vp. but,when i change it to 2.2nF..it amplifies perfectly to 2Vp, i wondering why?

    And R6 is the emitter resistance,right? My lecturer gave me the value which is 390ohm and the amplifier fails to amplify to 2 Vp. And so,i trust myself and change it to a higher value at 1.17k ohm and it is too amplify perfectly to 2Vp. why?
  7. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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    Not completely sure, but my best guess is to provide a good RF ground for the RF oscillator (a common base is grounded at the base). The capacitance is so small that it is negligible at audio frequencies.
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