Generally output of VCO or I/Q modulator is differential.
So driving PA(Power Amplifier) by differential signal is natural.
Then output signal of PA is also differential.
But TX load is generally single ended unless we do use loop antenna.
So we have to interface differeitial output to single-ended output.
Here I pick up the following important characteristics of PA.
(1) Availble Maximum Output Power Level
(2) Power Added Efficiency
(3) Second Order Harmonic Level
If we can use fairly good balun which is broad band enough to suppress Second Order Harmonic,
Differential output PA is superior than Single-Ended PA regarding the above three characteristics.
However if outputs of PA are open drain of differential pair
and we can only use very poor balun which is composed from discrete L and C,
I think there is no advantage in Differential output PA compared to Single-Ended output PA.
For example, we often use a balun of Figure-3 in the following.
http://www.zen118213.zen.co.uk/RFMicrowave_Circuits_Files/Balun%20Design.pdfThis balun is not broad band, so Second Order Harmonic can not be suppressed at all.
And power can not be doubled even if we double cosumption current.
My opinions are:
If we can't use good balun, Differential Output PA is
- Less efficiency regarding available maximum power level and consumption current
- Second Order Harmonic Suppression is not good at all
So if provided same consumption current, I think Single-Ended output PA is superior than Differential one regarding available maximum power level.
Here consumption currents include not only final stage of PA but also driver stages which might have differential input/output.
I know differential LNA inputs is preferable for RX sensitivity.
But I don't think differential output can be valuable for good performance for TX.
How do you think ?
Is there any advantage in Differential Output PA ?
But I have to share TX pins with RX differential LNA inputs for low cost transceiver.
I'm always in very trouble to get best solution to accomodate best performance for both RX and TX.