DEPARTMENT OF TECHNOLOGY Design Of A Power Amplifier Based On Si-LDMOS For WiMAX At 3.5GHz

The aim of this thesis work was to design a power amplifier for WiMAX applications at 3.5GHz based on cheap Si-LDMOS technology and to analyze the behavior of the design regarding nonlinear distortions and memory effects. A MW6S004NT1 Si-LDMOS RF high power transistor, from Freescale, with an ADS MET model was used in the process. This transistor is specified for Class A-AB base station applications with frequencies up to 2GHz. However, we will investigate the capability of using it at WiMAX frequency. The first step in the work consisted of designing a highly linear Class A power amplifier which achieves the highest output power, efficiency and gain. Load-pull analysis and Pout-PAE optimizations were used in the ADS simulations. The implemented design showed a different behavior than the ADS one, with mismatch problem at the output, a shifted DC bias and a high self heating. After adjustments, the PA showed an acceptable gain of 6 dB, but the achieved output power and efficiency were limited in value for base station use. The main reasons behind this behavior were the non ideality of the used ADS model, the big size of the transistor and the internal feedback in the package which, at the work frequency, caused a positive feedback in the performance, and a high mismatch certainly at the output. The next step consisted on redesigning the PA using ADS, despite the non ideality of the model. Several adjustments have been added to the matching and bias networks of this design to improve his performance. The implemented design showed an improved gain of 10.5 dB near the P1dB compression point; however the achieved output power and efficiency at that point were much lower. The final step in this work consisted of analyzing the behavior of the first design regarding nonlinearity and memory effects for different classes of operation. ADS simulations and measured results were used in the process. The design showed a low performance regarding intermodulation distortions for class A. However by changing the bias to a class AB operation, the intermodulation distortion rejection was in the range of 39 dBc which is a good value regarding the WiMAX standards (32 dBc). Regarding the memory, the design showed low memory effects or a memoryless behavior with a maximum effect of 1.3 dB. Despite the output mismatch problem, this cheap transistor has shown a good power performance comparable with the GaAs technology. Extracting …