# Considering the power amplifier for complex modulate signal

1. ## Considering the power amplifier for complex modulate signal

Dear all,
I'm choosing a 1W power amplifier for QAM 256, 20MHz bandwidth signal.
as far I know, the QAM256. 20MHz signal is such like the below.

the the total power of 20mhz signal is 1W=30dbm, the power of a 1HZ bandwidth signal is 30-10log(20*10^6)= -43dbm. besides, the PAR of 256QAM ~ 7-> the peak power of a signal is -36dbm.
so, I just need a power amplifier which can stand only -36dbm (may be need some more back-off). Did I wrong ?

•

2. ## Re: Considering the power amplifier for complex modulate signal

Hi,
Peak power of a signal isn't -36 dBm. Its rather -36dBm/Hz which is power spectral density. First you must define peak power of your signal to properly fit amplifier.
Marcin

•

3. ## Re: Considering the power amplifier for complex modulate signal

Are there any guidelines on how to define the peak power of the signal properly?

•

4. ## Re: Considering the power amplifier for complex modulate signal

Crest factor of different QAM signals can be easily found in literature, just ask Google. It's 4.23 dB for 256QAM, means you need at least 2.7 W amplifier peak power for 1 W average. I presume it's a single carrier signal.

--[[ ]]--