# Zin of a circuit with independent sources

1. Jan 8, 2014

### anhnha

Hi,
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.

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2. Jan 8, 2014

### Ryoko

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.

3. Jan 9, 2014

### sophiecentaur

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.