aaron_do
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Hi,
if I had to pick one of the equations, it would be the second one. The third looks like the 0.85 was found by experience, and the first is missing the effect of Vsat. However, you can't just assume Vsat is 2 V, since some PAs are designed with a 1.8-V supply. In fact, I think the value of Vsat is going to depend on the linearity required by your PA since there isn't exactly a well defined clipping of the signal due to Vsat...
For a first-pass design equation, equation 2 is fine...
Aaron
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