Study On The Linear Performance Of SiGe HBT RF Power Amplifiers At Device Level

With the rapid development of 3G wireless communications, increasing wireless users, increasing wireless services and increasing requirements of data flow, conflict with the limited resources of spectrum. To provide services of higher quality within the limited extent of spectrum for more users, transmission technologies with higher spectrum efficiency must be utilized, which inevitably requires higher linear circuits.Power amplifier (PA) is the most important module in the radio frequency front-end, which to a large extent determines the linear performance of the entire system. Linear technology of the traditional methods mainly concentrated on the circuit level, such as the feed-forward, feedback and pre-distortion, etc. To a certain extent, these methods can improve the system linearity, but they also bring the disadvantages of more complex systems, larger area and greater energy consumption. This paper research the non-linear mechanism of the core of PAs, i.e. the HBTs, from the device level to find out the nonlinear mechanism within devices. Then we can simplify the circuits and improve power efficiency.To achieve higher output power, RF power HBT generally consists of many small units of the same structure, which connected together by a certain style of metal layers. The small units largely determine the performance of high power HBT, while the parasitic parameters of metal layers under RF conditions also have important influence on the performance of the devices.This article takes SiGe HBT for the study. In non-linear research of the small unit, the Volterra Series was utilized to do theoretical analysis, combined with ADS simulation to prove its results. Analysis showed that:(1) Under the small-signal condition, rπ,gm and Cπwere the most important non-linear factors in the SiGe HBTs, but non-linear effects of them were canceled partly which made the overall linear feature become better. (2) Due to the feedback effect, RE and RBi can reduce the non-linearity of Gm to a certain extent. (3) Input and output match networks had important impact on the PA's linear performance.In order to study the performances of the HBT composed of those small units, a full distributed large signal HBT model including the parasitic parameters induced by the metal layers was firstly established. The model is of great significance for device design and circuit design. Simulation found that, because of the parasitic elements, the Quasi-static work points of different unites within power device spread greatly, resulting in the deterioration of its performance. As the research result of this paper, a hierarchical and non-symmetrical design method was put forward. It can greatly improve the linear performance, power capacity and frequency characteristic of the power HBT.