# Output Impedance?

When you're trying to find the output impedance of say, a simple alternating current RC circuit, like a low or high pass filter, what value of frequency do you use? The whole point of these circuits is that their impedance changes based on frequency, so how can there be a fixed value for output impedance? Do you use cutoff frequency?

I'm fairly frustrated trying to find the answer to this, the text I'm reading doesn't bother to explain it and I can't seem to find an answer online.

• What about giving the complex number Z_r+j Z_j? – mikuszefski Jan 31 '17 at 7:58
• To add to this, what led me to this issue was a problem in my text that asked for a design of a filter circuit that had an impedance of 200 ohms. it's frustrating to me, because what does that mean? It seems to imply that impedance is a fixed value when it clearly isn't, which leads me to believe that there's some convention I'm missing that my text didn't bother to explain. – Bookie Jan 31 '17 at 8:14
• 1) instead of commenting on your own question, use the Edit function to add the extra info. 2) include a circuit diagram 3) a simple alternating current RC circuit The circuit does not alternate, it's the currents and voltages that do. 4) an output impedance does not have to be constant. It can and often will change over frequency. Often an output impedance has a Real part (resistors) and an Imaginary part (Capacitors and inductors). – Bimpelrekkie Jan 31 '17 at 8:54
• @Bookie: Imagine you have a problem text that says the friction of a driving car is 1kN. That does not mean that friction doesn't depend on speed. They expect you to know that it depends on speed and that you are clever enough to understand that the given information assumes some context. That's one way to test wheter a student understand a certain concept or not. – Curd Jan 31 '17 at 9:58