Optimum bias conditions for linear broad-band InGaP/GaAs HBT power amplifiers

A design strategy for a linear broad-band InGaP/GaAs heterojunction bipolar transistor (HBT) power amplifier is presented. This design is based on the bias dependence of the nonlinear base-collector charge, as expressed in the C_BC versus V_CE and τ_C versus I_C characteristics of the device. Using this technique, it is shown that the second- and third-order distortions have separate optimum bias conditions and, furthermore, there is an inherent tradeoff in optimizing the second- and third-order distortions. The strong bias dependence of the nonlinear base-collector charge and the tradeoff between the different orders of distortion are verified on two 24-dBm 0.5-11-GHz distributed power amplifiers optimized for second and third-order distortions, respectively. The experimental results show that the harmonic and intermodulation levels are sensitive to the different order derivatives of the f_t versus I_C curve. Specifically, second-order distortion is related to the slope of the f_t versus I_C curve and third-order distortion is related to both the slope and curvature of the f_t versus I_C curve. This design technique suggests the importance of HBT device design to minimize distortion in high-frequency broad-band amplifier designs. Furthermore, to minimize high-frequency distortion in HBT amplifiers across a wide range of bias, it is desirable to linearize the base-collector charge, where flat C_BC versus V_CE and f_t versus I_C characteristics are ideally desired.