Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
      File size:
      6.8 KB
  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


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    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.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook