A Doherty Amplifier with a Tunable Transformer for Load Modulation

This paper presents a load modulated Doherty Amplifier at 24GHz for FMCW Radar applications. In this design a conventional hybrid coupler is replaced with a CS-CG balun and an additional RC phase shifter in series with the input of the auxiliary power amplifier to provide the necessary phase shift. At the output, the transmission line is replaced, and the main and auxiliary power amplifiers are biased and designed such that their voltages are in phase, and the output is given to a single ended balun. The balun impedance is tuned with a variable capacitor to provide load modulation and improve the back-off efficiency and output power. The PA is designed in 65nm CMOS technology and provides a gain of 10.06dB with a Psat of 12.82dBm and peak P.A.E of 16.6%.