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
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
-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
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.