According to ali_desidero
you are using ADS, right? you should use nf(2) to simulate NF of your PA. As mentioned already by the others, NFmin is the minimum noise figure and depends on your source impedance (this is 50 Ohm in your case, I suppose). I don't know exatly what nf(1) is, but I think nf(1) is just a feature of ADS and gives SNRout/SNRin instead of SNRin/SNRout.
If this is really the definition used in ADS, then we should use nf1 right?
I am having the same problem now.
But I am simulating NF for LNA.