PA support APT(Average power tracking) & ET (Envelope tracking)

[Mini_Turbo]

I saw some PA's datasheets saying support APT or ET, is there any differencc between them from the design perspective? Thanks!

 

[mtwieg]

I believe the difference is that ET implements high level modulation by modulating the bias voltage to match the shape of the complex envelope. APT, on the other hand, low level modulation is used, and the variable bias voltage has a bandwidth much lower than the complex envelope. So APT is more like adjusting the small signal parameters of the amplifier (gain and dynamic range) while ET directly modulates the output signal.

 

[Mini_Turbo]

mtwieg, Yes, you are right.

The major difference is the modulator bandwidth, however, for PA designer, what should we take into account? Does the PA designed for APT can also support ET application?

 

[vfone]

Even APT provides less efficiency improvement than ET, it has more supporters due to issues that ET have during its implementation.
Especially it is a problem with the wide band noise generated by the ET DC-DC modulator when use LTE Intra-Band Carrier Aggregation, or using FDD with low TX/RX separation (Band 17, Band 13, Band 12, etc).

 

[mtwieg]

The major difference is the modulator bandwidth, however, for PA designer, what should we take into account? Does the PA designed for APT can also support ET application?

ET is necessary for highly nonlinear amplifier classes (C, D, E, F, etc), which should show a more linear response to drain voltage modulation. Linear topologies (A, AB) are compatible with both APT and ET. I suppose nowadays it's becoming desirable to design the PA to operate in class C or E for high power output conditions, but then back off into a linear mode at lower output power, in order to get the best dynamic range.

My approach would be to focus on obtaining good PA performance at the highest average output power. The real challenge comes when you have to characterize the PA as it backs off into APT mode, and using that info to get an overall linear response. Without knowing the specific application and device technology it's hard to say much else.