What is the Output impedance on the AX5043 ANT pins, and how can they be used for high efficiency power output?
While radio transceivers using power amplifiers with topologies such as class-A or class-B may have integrated amplifiers, AX5043 utilizes a class-E amplifier. Because of this, high efficiency power output can be achieved, though external components are required.
AX5043 pins ANTP, ANTN, and ANTP1 are not the PA outputj; rather, they are the transition between the internal switching transistor and the external resonant components. In the various AX5043 reference designs, LC, CC, CT, LT, and CM actually form the amplifier. For this reason, the output impedance of the ANT pins is not relevant to antenna matching. When the class-E power amplifier has been properly designed, the output impedance will be found at the balun (LB, CB) components, and will be 100 ohms differential, and 50 ohms after the balun. Between the balun and the antenna (or SMA port), filtering and Antenna matching networks can be inserted in series with the RF path.
Another important point is that the output impedance of the internal PA transistor also depends on the output power setting.
For more information on how the matching networks for the AX5043 should be designed check out this post FAQ: How to operate AX5043 at 27 MHz - (custom matching networks for AX5043)