If a network has only dependent sources, you need to use a test current source or test voltage source (1v, 1a, something simple) since Zth = Voc/Isc doesn't work.

Is it valid to use a DC source as a test source with reactive components like an inductor and a capacitor to solve for the port impedance?

I need to solve for Z at terminals 1-1'.

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2 Answers 2


No, it is not valid to use a DC source. The impedance will be a complex number with real and imaginary parts ( or magnitude and phase) and will be dependent on the frequency.

  • \$\begingroup\$ How should one go about solving for it then? \$\endgroup\$
    – jonnyd42
    Jan 8, 2016 at 1:16
  • \$\begingroup\$ Your original suggestion of applying a source should work. If you tried this and it did not work can you provide details of what went wrong? \$\endgroup\$ Jan 8, 2016 at 1:18
  • \$\begingroup\$ So should I apply a source with amplitude of 1 and the same phase as well as angular frequency? So it would be Vtest = 1cos(10^4*t)? \$\endgroup\$
    – jonnyd42
    Jan 8, 2016 at 5:20
  • \$\begingroup\$ It's an interesting one. As long as your network behaves in a linear way then you can represent it as a thevenin source at a particular frequency. Applying 2 different voltages will allow you to determine the impedance. I think your source will act as a passive load I.e. zero current at zero applied voltage, so the thevenin equivalent will only be the impedance, not a source. \$\endgroup\$ Jan 8, 2016 at 10:26

You cannot use a DC source to find the impedance, as the impedance depends on the frequency. You should use a source which has the same frequency of interest.


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