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Difficult, understanding Input and output impedance

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robsims:
Seems like everyone has a different understanding about calculating input and output impedance's. It doesn't matter how the circuit is drawn. There are always two input terminals and two output terminals. Even for a single resistor (see attached picture). Do you decouple the source and the load when calculating input and output impedance's. What is the right answer?

fourfathom:

--- Quote from: robsims on August 18, 2019, 08:34:00 pm ---Seems like everyone has a different understanding about calculating input and output impedance's. It doesn't matter how the circuit is drawn. There are always two input terminals and two output terminals. Even for a single resistor (see attached picture). Do you decouple the source and the load when calculating input and output impedance's. What is the right answer?

--- End quote ---

Rinput = R + RL
Routput = R + Ri

Just that simple.

GerryR:
If you draw a box around R1 and R2 in your initial drawing, you have your answer.  When trying to load-match the output impedance to the load,  you can't include the load in the calculation of the output impedance.  Look at it this way, many instruments have a 50 ohm output impedance and tell you to match that with your load.  The load isn't connected to the instrument until you put it there, so it must have a 50 ohm impedance before you connect the load.  (That is how you get maximum power transfer for an output device (frequency generator, etc.) and the way they spec it to give outputs to a particular value.)  If your load is different than the output impedance of the instrument, say in this case a frequency gen, the output will vary from spec.  (Of course, there are cases where you want the "load" impedance to be much higher than the output of the device, e.g. a voltmeter.)

robsims:
Many thanks GerryR. That's the same rstofer mentioned earlier and that's what i also thought. Again, the professor in the vid i posted earlier is right. So i think this will be the end of discussion (EoD). Everything is crystal clear now.

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