# Considering the power amplifier for complex modulate signal

#### caoquyen164

##### Junior Member level 2
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 ?

#### Marcinstein

##### Newbie level 6
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

#### Coores

##### Newbie level 1
Are there any guidelines on how to define the peak power of the signal properly?

#### FvM

##### Super Moderator
Staff member
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.