Effect of Printed Circuit Board Structures on Temperature-Dependent Gain Characteristics of RF Power Amplifier Chips

We fabricated and characterized heterojunction field effect transistor radio frequency (RF) power amplifier (PA) test fixtures, for wireless applications, with various printed circuit board (PCB) structures. The RF matching and bias circuits of the test fixtures were designed so that they had the same RF characteristics. The only source of the variation of the RF gain (S₂₁) was different thermal characteristics of each PCB. The values of the junction temperature (T_J) and the junction-to-ambient thermal resistance (R _{J A}) of each test fixture were shown to be changed as much as 80deg C and 30deg C / W, respectively, by the change of PCB structures. The change of Rja was shown to be originated from the change of the PCB thermal resistance, assuring that the structure of the PCB was the dominant factor in determining R _{J A} Finally, we obtained a universal relation between S₂₁ of the amplifier and Tj. This work suggests that thermal budget of PCB is as important as that of package in wireless RF equipments.