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Zin of a circuit with independent sources

  1. Jan 8, 2014 #1
    This is not a homework question. I need to calculate input impedance for a LNA and this is part of it.
    How do you calculate input impedance, Zin, for the circuit below?
    I tried by applying an input voltage, V1, and calculate Iin.
    Input impedance will be V1/Iin. However, the expression still contains V1 and I can't figure out how to
    make it independent on V1.

    There is another method that I am not sure.
    Input impedance will be equal to the impedance as all independent sources are turned off.
    This means that Zin in the picture will be equal to s(Lg + Ls) + 1/(gg + sCgs)

    gg is a conductance.


    Attached Files:

    • Zin.JPG
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  2. jcsd
  3. Jan 8, 2014 #2
    Your equation (s(Lg + Ls) + 1/(gg + sCgs)) is the impedance. Someone please confirm, but all you need to do is replace 's' with 2*pi*f*i where i is the imaginary number sqrt(-1) and f is the frequency in hertz. Then solve the resulting complex equation into real and imaginary parts.
  4. Jan 9, 2014 #3


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    I don't know what the application could be but, assuming the two sources (V1 I1) are coherent, the input impedance (V12/i(in)) seen by V1 will depend upon the phase of the current I1. Have I read the circuit diagram right?

    You could solve this using Kirchoff (as always) but there may be an easier way.
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