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
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.
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.