# Calculating complex input/ouput impedance for FET in Microwave office

#### Hawaslsh

##### Full Member level 2 Hello,

I realize this is a very specific problem for a specific software, but I thought I'd throw it out there. Thanks in advance. I had a general question about microwave office. I wanted to see the input/output impedance of the large signal models under certain bias conditions. The modified Gamma probes give me access to the reflection coeff looking to the FET. I can easily plot that on a smith chart and see the impedances, input and output However, I want to use those values in some other equations, so i put a voltage and current probe on both the gate and drain nodes in the schematic. I wrote very simple equations to calculate the impedances using the voltages and currents probes The top equations calculate the series RC input impedance which looks into the gate of the FET. I simply divide the Voltage of the gate by its current. The real part is the series R and the imag part series C. That part works fine, and I calculate the same resistance and capacitance as the gamma probe predicts. For the output impedance, since we know its a parallel RC, i calculate the output admittance and try and find Y = 1/R + jwC. However, all i calculate is the 50 ohm load with zero capacitance. Does anyone have any insight as to why this method worked on he input and not the output? Why didn't I calculate 50 ohms on the input but rather the input to the FET?

#### BigBoss

##### Advanced Member level 5 You measure the Port Impedance not Output Impedance.Because there is a signal at the Input but there is no signal at the Output.The simulator does not know which circuit you simulate.Instead, you have to drive the circuit in both ways.For Input Impedance, you should drive the circuit by its Input, for Output Impedance you should drive the circuit by its Output.So it simply will be transformed into Large Signal s-Parameters Simulation Problem.While Output is terminated, the Input Impedance will be measured, then while Input is terminated The Output Impedance will be measured.

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