A 30 W Cartesian Feedback Transmitter with 40 % Efficiency Incorporating an Inverted Doherty Amplifier

We present a 30 W Cartesian feedback transmitter with an efficiency of 40 % and an adjacent channel leakage power ratio (ACLR) of -51 dBc for 859 MHz band pi/4-DQPSK applications. The high efficiency of the system was achieved by a significant efficiency improvement for the final amplifier stage using a load modulation technique. The implemented Cartesian feedback transmitter, using a Doherty power amplifier, exhibited a 3
^rd
order inter-modulation distortion (IMD3) improvement of 27 dB for a two-tone signal at an output power level of 45 dBm. It also had ACLR improvement of 16.2 dB for the pi/4-DQPSK signal at an output power level of 45 dBm. The system performed with an efficiency as high as 40 % which was an improved value by about 7 % points as a consequence of the high efficiency characteristics of the inverted Doherty amplifier.