(A) large-signal model of heterojunction bipolar transistor and Its application to an efficient power amplifier for mobile communication

Abstract

In this thesis, a novel power amplifier for wireless handheld terminals is proposed to enable a statistically efficient operation under a time-varying RF power for a WCDMA reverse link. A systematic design approach using the nonlinear large-signal models of heterojunction bipolar transistors is adopted to achieve a first-pass design success. The validity of the large-signal model is thus confirmed by analyzing the measurement results of the fabricated power amplifier. As a first step of design, a new extraction procedure for large-signal model of heterojunction bipolar transistor is described. To characterize the thermal behavior of the device, the pulsed measurement system is extensively used to extract the thermal resistance and the time constant of the device. The physical relationship between the thermal resistance and the active area of the device is explored to reveal that the value scales inversely with the area. Moreover, the parameter extraction procedure of Gummel-Poon large-signal model using isothermal measurements is developed for high power devices operating under high current density. The temperature-dependent characteristics of HBT, which are self-heating and ambient temperature effects, are successfully incorporated into the large-signal model by exploring a physical relationship between the device current and the temperature dependence of the built-in potential. As an example, the large-signal model for an in-situ output device from a two-stage power amplifier circuit is extracted for an accurate simulation of temperature effects and nonlinear behaviors under various conditions. With the extracted large-signal models, the asymmetrical behaviors of intermodulation distortion (IMD) in HBTs are analyzed when the terminating impedance at baseband frequency contains a reactive component. This phenomenon sometimes induces misunderstanding of distortion performance unless the baseband impedance effects are properly accounted for. The distorti...