aaron_do
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Hi,
typically neither method is used.
Although there are negative feedback based power amplifiers, they're not very common because (also speculating) they represent a heck of a design challenge. In order to optimize performance, power amplifiers need to operate close to their stability and reliability limits. So you can imagine that adding a path from output to input could lead to problems if the circuit and layout modeling is inadequate.
Filtering only works if there is only one frequency channel being used as so far there aren't any good tunable RF filters on the market (good research project?). That rules out Wifi, Mobile and Bluetooth. Also, filtering removes the sidebands, but doesn't improve SNR, so its use is limited.
The most common way to reduce PA distortion is simply to back-off the PA (i.e. transmit at a lower power than it is capable of). But there are plenty of methods such as pre-distortion, cartesian feedback etc which have applications.
regards, Aaron
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