Improving the Linearity and Efficiency of RF Power Amplifiers

Agrowing number of semiconductor technologies are being applied to RF power transistor applications. These technologies include Si LDMOS FET, SiGe HBT, InGaP HBT, GaAs MESFET, AlGaAs pHEMT, SiC MESFET and AlGaN/GaN HEMT. The dependencies of linearity and efficiency of such technologies are often common, such as transconductance derivatives, capacitance variations, breakdown effects and parasitic resistances. This article overviews the work that has been achieved to date to maximize linearity and efficiency in the most promising technologies, as related specifically to infrastructure applications. The article also addresses the increasing number of device and circuit level techniques that are being used to enhance these two important parameters as required for IM3, ACPR and ACLR suppression in 3G systems such as W-CDMA/UMTS. This article focuses on high power (that is greater than 10 watt) RF transistor technologies where digital modulation techniques are demanding higher and higher peak-to-average ratios (PARs) and thus higher peak powers. Peak and average DC-to-RF efficiencies have become critical parameters, and much attention is being focused in decreasing multi-carrier intermodulation distortion, adjacent channel power ratios (ACPRs) and adjacent channel leakage ratios (ACLRs). Unfortunately, improving transistor linearity often leads to decreased efficiency which directly affects overall system efficiency, heat removal, size and cost.