Efficient 915 MHz amp?
It depends what modulation are you going to use. Some modulations require linear amplification and minimum phase distortion.
I am attaching info about PA efficiency (%) vs PA classes.
Good point! Saturated FSK modulation.
OK.
Because FSK is constant-envelope you can use a nonlinear PA.
For 300mW output power theoretically would be ok class C or E.
I am curious. You have a class E design that works at 915 MHz?
At the beginning the class-E was available only up to VHF frequencies…but not anymore.
http://scholar.lib.vt.edu/theses/ava...cted/chap5.PDF
http://scholar.lib.vt.edu/theses/ava...cted/Chap6.PDF
That is all very interesting. But I was hoping someone had an off the shelf part number chip that provided 55% efficiency.
Hi ,biff44.
Can you give me some suggestion on modulation vs PA classes? such as ,GMSK, QPSK, 8PSK, AM, PM, FM......
Just in a general sense. Some modulation schemes allow a fully compressed PA to be used.
Some modulation schemes, like QAM have half of the modulation "information" in the amplitude part of the signal. So, obviously, if you compress the amplitude of the largest signals by 1 or 2 dB due to saturation, you can not send the data at all. Those types of modulation require you to "back off" the output power. I think I recall that 16QAM requires around 10 dB of backoff. i.e. you can only run the output power to the P1db - 10 dB level.
Some modulation schemes that you think are constant envelope, really arent. in QPSK, for instance, if you go from the 0 to 180 degree state, the signal actually goes thru the origin, ie winks out in amplitude inbetween states. You amplifier has to be able to handle that without lingering am to pm conversion, or you will have poorer bit error rate because the phase state does not arrive at 180 degrees, but something a little different until the PA settles out.
Some times you can send the data thru just fine, but for emissions mask reasons you have to back off a little to please the government. Overdriven amplifiers can generate 3rd order, 5th order, etc. adjacent spurious emissions.
Hope that helps some.