Solve for Input/Output impedance of the linear circuit

Hi Guys,

One question always confuses me:
When I see people solve for input/output impedance of the linear circuit. For example, for output impedance, sometime they SHORT input but sometime they OPEN input and they're all called "output impedance".

Questions is that:
In which conditions should I SHORT or OPEN the input?

Thanks!
SQ

Well, I think you are confusing things up a little. The output impedance of a circuit is the actual impednace when it is being used normally.

If you have an RF filer, that might mean when the input is matched to a 50 ohm load for instance. If you are talking about a RF amplifier, the output impedance is what you get when you have the input connected to its antenna. If you have an audio amplifier, is is the impedance when the input microphone is attached.

What you might be thinking about instead are circuit design parameters. There are Z parameters, T parameters, S parameters, etc. They are matrices with input, output, forwared transfer, and reverse transfer parameters measured over frequency range. They are used to cascade different complex circuits together, while treating them as simple "black boxes". Some require matched impedance on the untested port, some require open or short circuit on the untested port, etc.

You will need to find a good basic book on electronics theory and study them.

To get output impedance, you short the inputs, and inject an ac source at the outputs.
The key point is understanding what "short" is.
"Shorting" in this case, means connecting both inputs to an ac ground where bias conditions of the circuit are maintained.

checkmate, that`s true for (ideally) voltage driven circuits with Rsource=0 ohms only.
In general, bif44`s statement is correct: terminate the input as it is used during normal operation - and this can be 50 ohms or less or more.

Here is a pretty good introduction:

Two-port network - Wikipedia, the free encyclopedia