[SOLVED] Questions about doherty GaN mmic PA

[maxsidou]

Hello everyone, currently i'm simulating one GaN doherty mmic PA, since i'm new to doherty and mmic GaN PA is a new research in our lab. So hope to get some advice here.

1) as for the OMN of the main amplifier, do i just match for the maximum power(match Zopt to 50ohm)? Or at the same time, the maximum PAE point should be matched to the 2*50ohm to get the best backoff efficiency? I tried to match them all but found difficult and the maximum PAE point always lines in the left part of smith chart. So is matching maximum PAE point to 2*50ohm a must-do step?

2) To get more obvious backoff efficiency enhancement, one driver stage is added before the main&auxiliary PA. It is interesting that backoff efficiency enhancement can only be observed
at center frequency but not in other frequencies. I changed a wide-band driver but in vain. So is there some noteworthy tricks in driver stage design in doherty mmic?

Any response, idea and papers recommendation is appreciated!

 

[BigBoss]

For the Main Power Amplifier, you've got three options. Max. Delivered Power OR Max. Efficiency OR Max. Gain.
Nor of them is overlaps in practice so you have to choose one of them which is more beneficial for your requirements.The others will automatically aligned according to the principal one.

 

[ktr]

In theory, auxillary amplifier corrects errors of main amplifier; however in practice, that means aux amplifier should be better than the main amplifier. However, if that were the case, you would use that aux amp as your main amp.

One thing I use is using S-parameters of the transistors(or amplifiers if you use ready-modules) to detect which one works better as peaking amplifier, to use it as correction amplifier. You can check noise circles and gain circles of two transistors(or amplifiers) and decide their roles accordingly.

 

[maxsidou]

Hello ktr, what do you mean by saying "correction"? and what are the errors of main amplifier? Look forward to your more specific response

 

[ktr]

Hello ktr, what do you mean by saying "correction"? and what are the errors of main amplifier? Look forward to your more specific response

When u say Doherty PA, I thought you know about the topology of Doherty PA which look something like this depending on how complex you wanna go.


Peaking amplifier only works for high input levels, which is in short conduction angles. However, opening the peaking amplifier adds a certain "load-pull" effect which creates "errors" in matching. Hence, arms should be in-phase with each other for proper combining. When we start talking about phase, combiner and input phase shifter becomes critical, hence some impedance matching and transformation needs to be done on those parts; however, that also makes the system "narrow-band" by nature. It's not an easy task.