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Post subject: RF amplifier Maximum ratings and complex waveforms Posted: Tue Oct 07, 2008 10:45 pm
Joined: Tue Oct 07, 2008 10:19 pm
I have a question on how to correctly rate the maximum peak power of a RF device, given its maximum drain (collector) voltage and maximum current and/or power dissipation. My question rises as more complex and demanding waveforms are being used in RF power amplifiers nowadays: higher peak-to-average ratios (PAR). More specifically I am looking for a guide on how to derive voltages and currents being developed at the devices terminals (not at the 50-Ohm impedance nodes) under a known set of conditions: Average power, PAR, VDD. Is it possible for someone to point me in the right direction on how to find this information?
Post subject: Posted: Thu Oct 09, 2008 3:35 pm
It is very complex answer.. I don´t know if I can completely answer it..
When I design a power amplifier the first thing you have to analyze even before choosing any transistor is your input signal. It is not always that easy to know the PAR of the complex modulated signals, the best way is by measuring the CCDF curve of your signal (you can also simulate it). With the CCDF curve, you will have a record of the maximum peak power of the signal.
Then you select a transistor knowing the average power that you need and the peak power that your signal demands (of course that if your requirements allow it, you can chopp the output signal by reducing its PAR, selecting a smaller transistor). Let me clarify this with an example:
-If you have a signal with a PAR of 10dB and you need 10W of average power you will have to choose a 100W amplifier. In this case, the max peak power at the output will be 100W
-If you have a signal with a PAR of 10dB and you need 10W of average power and you choose a 50W amplifier. In this case, the max peak power at the output will be 50W and you will be chopping the signal to a PAR of 7dB.
-If you have a signal with a PAR of 5dB and you need 10W of average power and you choose a 100W amplifier. In this case, the max peak power at the output will be 31.6W and you will be using the amplifier with a lot of back off.
With the above information I was trying to explain how to rate the max output peak power of an amplifier.
Now if you want to know the peak voltage at the very output of the transistor you will need to know the impedance of that point. You can have that by measuring or asking to the manufacturer the loadpull data of the transistor.