Hello all.
I think I understood the problem.
Katykaty: I think you just wat to measure input and output impedances of an amplifier in normal operatin conditions, i.e., large signal.
Let's say, in normal test you are exciting this amplifier with a signal source that already has some quite high power. The output of the amplifier is connected to a load.
1-Between the preamplifier and the amplifier ( the one you are testeing) you put a coupler (for example 30db). This coupler should have the 2 coupled outputs available (A=direct and B=reverse).
2-Put another coupler between the output of the amplifier and the load. A similar coupler as I described in item 1 (C=direct and D=reverse) .
3- Now:
S11= B/A
S22=D/C
4- And:
S21=C/A
S12= A/C
To do this measurement you just bias the amplifier for normal operation, class A, AB, C, etc....
Apply the right input power.
Check that the output power is around the expected value and then follow the steps I described before.
You can also use a Network analyser to do the measurements for you. Just switch off the internal generator and put it into A/B mode.
You can calibrate it in low level, so that you don't burn the calibration kit!!!! Assuming that the only non-linear device that you have is the amplifier that you are testing, the calibration in low power will be also valid for high power.
Godd luck....
Greetings.
S.