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Zin of a circuit with independent sources 
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#1
Jan814, 08:35 PM

P: 170

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. 


#2
Jan814, 11:55 PM

P: 102

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
Jan914, 11:12 AM

Sci Advisor
Thanks
PF Gold
P: 12,271

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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