Common Collector Colpitts Oscillator

Discussion in 'Homework Help' started by gamfo, Mar 5, 2014.

  1. gamfo

    Thread Starter New Member

    Mar 5, 2014
    3
    0
    I am trying to design a common collector colpitts oscillator, it looks more like a unity gain voltage amplifier with step up voltage transformer in the feedback!.
    The following stuff is known:
    β=300
    corner frequency(fc)=8k Hz
    Ic=0.1mA
    Vcc=10V
    L=30nH
    Cr1=50pF
    Cr2=30pF

    - I need to find the values of Rc, Re, Rbb to bias the oscillator to Ic=0.1mA, the part confusing me here is that I can't neglect the transistor input impedance looking into the base. Cb can be assumed to be very large.
    - How can I calculate the white noise level of the amplifier?
    - Calculating the center freq and Q of the oscillator, I think that freq can come from \omega=\frac{(Cr1+Cr2)}{Cr1*Cr2*L} is this correct??
    - How do I calculate the Lesson corner freq. ?
    - Whats the difference between the noise spectrum of the oscillator and that of the amplifier?
     
  2. Efron

    Member

    Oct 10, 2010
    81
    15
    Gamfo,

    I think you should spend some more hours to deeply understand the basics of amplifiers of this kind.

    Only afterwards you'll undertand how an oscillator can work in this configuration.

    To start with, your schematic does not correspond to a common collector amplifier, but to a common base amplifier.

    Then, let's start with the first point. What are your ideas to find the values of Rc, Re, Rbb to bias the oscillator to Ic=0.1mA.

    Do not pay attention to the other points for the moment.
     
  3. LvW

    Active Member

    Jun 13, 2013
    674
    100
    I think, it is indeed a common collector configuration because the feedback signal is derived from the emitter node (which is the output for CC configurations). And the frequency-determining network is between emitter and base (which is not grounded).

    Question: What do you mean with the term "Lesson corner freq."?

    This link gves a good treatment of your oscillator:
    http://www.google.de/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0CDAQFjAA&url=http%3A%2F%2Fseit.unsw.adfa.edu.au%2Fstaff%2Fsites%2Fhrp%2Fteaching%2FElectronics4%2Fdocs%2FPLL%2Fcolpitts.pdf&ei=ZDEYU_vgD8bVtQb0rIHwBw&usg=AFQjCNFg-aWNyC3bPuLPTuphnnyQRrfCOA&bvm=bv.62577051,d.Yms
     
    Last edited: Mar 6, 2014
  4. gamfo

    Thread Starter New Member

    Mar 5, 2014
    3
    0
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