Hi,

I don't really design PAs, but the answer is probably get a good estimate by hand and then verify and modify by computer simulation. Also when you say "the PA of WLAN802.11.a", you seem to imply that there is only one PA used for WLAN802.11a. Your transistor size depends on what kind of PA you are designing too.


cheers,
Aaron

You will start with hand calculations to determine what the peak current requirements are for the output device.

If you assume that there is a matching network, then the peak current requirement of the differential pair will be defined by the peak load current transformed by the matching network. So, if you have a required average power delivered to the load then the peak power will be the average plus the peak to average ratio of the signal (lets say 10dB PAR for 802.11 OFDM).

Once the peak load current is defined then you can try to size a MOSFET to deliver that current without going into triode. Once you analyze the device you will find you need to impedance match it to the load impedance and then the device current requirements will change according to the impedance transformation ratio.

Typically the higher the required peak output power, the lower the drain impedance will be (this also depends on the available power supply voltage, the lower the voltage the higher the current).

Another way to estimate the required device current is to assume some target peak efficiency and then calculate what the device current would be to meet that efficiency and attempt to design the MOSFET for the target dissapation and power delivered to the load.

My point here is that the required load current is one of the driving requirements for determining output stage size and current.