Measuring with High impedance probes

Hi,
Can I use an active probe to measure the power gain of an LNA with 50 ohms input and output impedances. How valid is it or should I apply some corrections.
BRM

Theoretically is possible but with some (or a lot of) limitations.
First High impedance probe measure voltage and not power, this should be considered. So you hace to terminate properly (50 Ohm) the amplifier. The reading normally is done on a Spectrum analyzer which by default express the measured voltage in dBm equivalent, so if you're perfectly matched you read exactly the output power.
Second you should avoid to add another uncontrolled load to the amplifier (the probe tip): this means that your parasitic cap should be negligible. In practice I think that for high frequency (500 MHz, 1 GHz ?) this method gives you a great inaccuracy.
Mazz