The impact of the Q-factor of the parasitic capacitances of RF transistors on their load modulation capabilities

With the high number of users the wireless spectrum has become a valuable resource. In order to use it efficiently sophisticated coding schemes with high crest factors are used. As a result the power amplifier is operated far below maximum output power for most of the time, which results in rather low efficiency of traditional designs. Therefore efficiency enhancement methods such as the Doherty or Chireix combiner are experiencing a revival. Their performance heavily depends on the load modulation properties of the used RF transistors and unfortunately often falls behind the predicted theory, especially for high frequency operation. This paper analyses the impact of the different loss mechanisms of RF power transistors on their load modulation capabilities. Special attention was paid to the frequency dependent losses due to the limited Q-factor of the parasitic capacitance. Based on the example of a Doherty amplifier the importance of the load modulation properties for the efficiency in back off is highlighted.