The voltage is large AC signal, not small AC. I never deal with this kind of biasing... I am doing some simulations to extract the impedance of the device. I heard about some ways to measure it, like load pull, S parameter. But I can't not do measurement now... The device is quit nonlinear, when I input sinωt signal, do transient response simulation and then Fourier transformation. The output current contains DC component +sin(ωt+Φ1)+ sin(ωt+Φ2)+... How do I know the impedance for this case ?? I want to know the device impedance since I want to calculate the voltage drop when I connect the device with other electronic devices. The final goal is to get the electric field distribution. Anyone know about this ? Thanks !!!