# Input impedance of a device under large AC signal

1. Feb 5, 2012

### ghost613

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.

2. Feb 5, 2012

### yungman

What impedance are you talking about, input or output? I have not studied this for a few years, I don't think load pull can give you that. Load pull only give you the region of most optimal power. When you draw the load pull, it is the region where you get above a certain power gain.

Small signal S-parameters will give you the input or output impedance but it is different from large signal S-parameters. Problem is the large signal S-parameter is hard to get. That's the reason people use load pull.

If you are working on RF power amp, get the book by Steven Cripps. It is a different world from small signal RF design and the RF books that I have ( a lot) don't cover any of that, not even polzar.

https://www.amazon.com/Amplifiers-Wireless-Communications-Microwave-Library/dp/0890069891/ref=sr_1_1?ie=UTF8&qid=1328424696&sr=8-1

3. Feb 5, 2012

### ghost613

Hi, yungman,

Thanks for your reply and your suggestions. I am talking about input impedance. I am not working on RF power amp. It's kind of like BJT but replace the base input with light illumination. I don't know too much about circuit so I have no idea what to do...

Basically, I want to know the voltage drop on this device by characterizing its impedance.

4. Feb 5, 2012

### yungman

I still don't know what you mean, give us more information. Do you mean the input is a filament like a light bulb? If so, I am not familiar with this but I am sure quite a few people here can help you. Please explain in more detail.

5. Feb 5, 2012

### ghost613

I simplify the device configuration in the following plots.
=====ITO electrode===
-----------------------

water

-----------------------
Nonlinear device
-----------------------
=====ITO electrode===

My goal is to get the electrical field distribution within the water. I want to use the equivalent circuit to solve the problem. If I know the input impedance of the nonlinear device, I can treat the circuit as the voltage divider. So I can get the voltage drop on the water layer to get the electric field. However, the voltage used between the two electrode is a large AC signal, say Vsinωt. I did transient simulation for the device to get the output current of the device - Io +I1sin(ωt+Φ1)+ I2sin(ωt+Φ2)+...

But I don't know how should I define the input impedance this case. Since V/I or dV/dI seems meaningless. Some of my friends tell me to define Z= Vsinωt/I1sin(ωt+Φ1), i.e., the impedance at a certain frequency. But I checked the other components, they are not so small compared with I1sin(ωt+Φ1). I guess I can not neglect them.

I am wondering whether should I treat the circuit as voltage divider. Since the signal is large AC and the device is nonlinear.

6. Feb 5, 2012