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BJT Miller Effect

Discussion in 'General Electronics' started by Marco Ferra, Dec 14, 2003.

  1. Marco Ferra

    Marco Ferra Guest

    Hi. I will be brief.

    After testing the frequency response of the NPN BC337 in the lab I came
    across the Miller effect. The gain at high frequencies dropped -3dB at
    40.6 KHz. One of the questions of the report was to calculate the
    internal parasitic capacitances C-pi and C-miu but it seems that I'm
    only able, using Miller's theorem, to calculate the total CT parasitic
    capacitance.

    Is there a way, knowing the upper corner frequency, to obtain
    mathematically and individullay C-pi and C-miu knowing the gain
    K = (Vc / Vb) ? (btw this gain is negative because Vc(ac) < 0)

    And is there a way, not to estimate the value of Rin, that I am making
    Rin = Vb / Ib, but to know accuratly also the values of Rx and R-pi?

    If I'm seeing this (what I think I know) in the wrong way please do tell
    me.

    Sincere regards, Marco
     
  2. Robert Baer

    Robert Baer Guest

    Rin, if i remember correctly, is beta*re+rbb' (that is little Remitter
    and base spreading resistance); and re is a linear function of current
    (it has been 30 years but i think 27 ohms at 1mA - correct me if i am
    wrong).
     
  3. Marco Ferra

    Marco Ferra Guest

    Yes. You are right. In fact re = VT / IE.

    (being VT bjt's thermal voltage = 25 mV so 25m / 1m = 25 ohms).

    The report is ambiguous about this "Rin" but I guess if i make my point
    and prove it mathematically all will be fine.

    Thank you for the reply
    Marco
     
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