Large-signal modeling of SiGe HBT for PA applications

Accurate modeling of large-signal behavior of power amplifiers (PAs) is key in minimizing the number of design spins and design cycle time. This paper presents the 1- and 2-tone large-signal behavior of SiGe HBT with respect to power, frequency, bias, and transistor geometry. While the transistor's weak non-linearity is largely determined by the trans-conductance (Gm) and the quasi-static (QS) charge-storage of the transistor, its high-power large-signal behavior heavily depends on the high-current gain roll-off characteristics and supply clamping. It is discussed from a PA-design standpoint how the SiGe bandgap engineering impacts the large-signal behavior through a steeper gain compression at high currents and high power levels. It is also shown that the 2-tone inter-modulation products of a transistor are closely related to its 1-tone distortion characteristics. Finally an accurate prediction of critical parameters of a practical WCDMA PA was demonstrated with careful accounting of the high-current effects in the SiGe HBT transistor.