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Homework Help: Modelling a varactor

  1. Aug 5, 2011 #1


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    modelling a varactor [solved]

    1. The problem statement, all variables and given/known data
    basically i'm designing a fm transmitter, the master oscillator uses a varacter for modulation
    the varactor diode we are given is the BB201, i have attached the spec sheet in this post.
    a spice model is also given for this diode:
    [.MODEL BB201 D(RS=.23 CJO=133.21E-12 M=1.3251 VJ=3.4103)]

    I need to find a way to model the capacitance of the varactor diode in order to design my master oscillator.

    2. Relevant equations
    one of the lectures stated the relationship between the capcitance of the varactor is:
    C = K/sqrt(Vr)
    where C is the capacitance of the diode, Vr is the reverse bias voltage, and K is some constant of proportionality.
    From my search around the net i realize this is one of the simpler models and I assume we will settle for this model in this course.
    It is that constant K I'm trying to figure out. I can't find a way to extract this value from the data sheet.

    3. The attempt at a solution
    out of all the numbers at my disposal, using cj0 as that constant seems to make sense as the capacitance on that graph in the spec sheet are all at the same order of magnitude as cj0.
    except that doesn't make sense, because cj0 is the capacitance when there is no bias on the diode (right?) so cj0/sqrt(0) =/= cj0

    I have a feeling this is really simple but i'm just not seeing it.
    I mean, they are not going to ask me to run a non-linear regression using the model and numbers on the spec sheet. I don't think they teach non linear regression in second year electrical enginnering here...

    edit: ah nevermind, looked into the spice model parameters and found the relating equations on a text book

    Attached Files:

    Last edited: Aug 5, 2011
  2. jcsd
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