OIP3 amplifier measurement question

[kavea]

Hello,
I'have a question about the OIP3 (output third order interception point) measurement of an amplifier. The question is quite simple, does the OIP3  of an amplifier depends on the impedance of the device connected at it's input ? i.e. if i connect generators which are not 50 ohms to an amplifier which has 50 ohms input impedance, will i measure the same OIP3 as that of the data-sheet ?

[1design]

It should, since OIP3 is the output 3rd order intercept point. Watch not to override it with the IIP3.

BR

[dmills]

Be careful here, some amplifier designs have very poor reverse isolation, thinking of that transformer feedback common base thing with the name I can never remember.
In these case source and load do interact, and can cause changes to the gain which can impact this measurement.

Just stick a big pad in front of the thing before making your measurements, pads have really nice properties this way.

Regards, Dan.

[kavea]

Thank you for the answers, i'he done some measurements on MAR6 and indeed, the OIP3 was different in the 50 ohm case and not 50 ohm (capacitive input impedance). However an other amplifier qbit 300 gave the same results, so it should depend on amplifiers and probably on its isolation.