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FM Transmitter explanation

Discussion in 'Electronic Basics' started by Default Name, Jul 16, 2004.

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  1. Default Name

    Default Name Guest

    Hi everyone,

    I have been breaking my head over some simple circuit for a couple of
    days now. The circuit can be found at
    http://www.zen22142.zen.co.uk/Circuits/rf/2bjttx.htm.

    It is a simple, two-stage FM transmitter. What I would like is some
    simple explanation on how the circuit works in the oscillating section
    (2nd stage). I have 2 years of study in EE but I can't figure it out
    by myself (teachers aren't too keen on actual real-life circuits so I
    feel like a beginner here).

    Here's what I understand (correct me if I'm wrong):

    The first stage is simply a common emitter amplifier. The second stage
    is a Colpitts oscillator. I understand how the inductance and
    capacitors oscillate to the modulation frequency but I have trouble
    seeing how the frequency will change in proportion to the audio signal
    applied at the base of the transistor.

    I read it is because of variable base-emitter capacitance at high
    frequencies but I would like more precise explanations.

    Could anyone help?

    Thanks!

    Richard
     
  2. The audio stage is directly coupled to the base of the oscillator stage.
    Hence the bias of the oscillator is varied at an audio rate. Since any
    shift of the bias on that oscillator will shift it's frequency, this means
    that you get FM when you talk into the microphone.

    Michael
     
  3. To clarify.

    There is an internal base emitter diffusion capacitance. This
    capacitance is a function of the emitter current, i.e. Cbe =
    40.Ic/2.pi.ft. This capacitance directly effects the oscillation
    frequency.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
  4. Er, you mean, 40 . Ic / (2 . pi . ft) ????
     
  5. Well, yes. It needs the brackets.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
     
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