Effectiveness of parallel diode linearizers on bipolar junction transistor and its use in dynamic linearization
Student thesis: Doctoral Thesis
Related Research Unit(s)
Bipolar Junction Transistors (BJT) has been widely used in many modern electronic devices or products. With the introduction of Heterojunction bipolar transistor (HBT), the maximum operating frequency has gone up to several tens or hundred Giga Hertz. However, non-linear characteristics of Bipolar Junction Transistor will generate spectral regrowth in communication systems. The spectral re-growth when radiated causes co-channel interference to adjacent channels in the same cell site as well as co-channel interference to adjacent channels in neighboring cell sites. Any means of reducing this spectral re-growth is thus of great interest in maximizing network performance and increasing the mobile handset operating time. Pre-distortion linearizer based on parallel diode should be a good candidate to linearize transistor amplifier in mobile applications due to its lower cost and compact size. To further enhance the power added efficiency (PAE), dynamic collector current control of transistor amplifier for different output power levels may well be used. With pre-distortion linearizer and dynamic collector current control scheme, the power added efficiency of transistor amplifier should be ideally optimized. However, the non-linear characteristic of BJT and even HBT is changing with operating conditions . Such as the base-emitter exponential characteristic which dominates at low collector current conditions and the base-collector junction capacitance which dominates at low base-collector voltage or high collector current conditions. Hence, static linearization methods may not be sufficient to linearize transistor amplifier when dynamic collector current control scheme is used. For this reason, a novel Dynamic Linearization approach is proposed to dynamic switch different linearizers for different operation conditions. In order to achieve this, the linearization effectiveness of pre-distortion type diode linearizers is needed to be well understood. Effectiveness of linearization means under what operating conditions the linearizer performance is optimum. However, there is no previous works on the study the linearization effectiveness of parallel diode linearizers. In this thesis, the effectiveness of the base-emitter diode linearizer and the base-collector diode linearizer is studied. It was found that the base-emitter diode linearizer is effective in linearizing the non-linear base-emitter junction at low collector currents while the base-collector diode linearizer is effective in linearizing the non-linear base-collector capacitance at high collector currents. From these findings, a novel Dynamic Controlled Amplifier using Complementary Linearizers (DCACL) and Dynamic Controlled Single Diode Linearized Amplifier for PHS handset applications are proposed to dynamically control the amplifier to achieve both higher linearity and power added efficiency. Experimental results shows significant improvement on linearity and power added efficiency over wider collector current operating region.
- Junction transistors, Power amplifiers, Bipolar transistors, Linear integrated circuits, Mobile communication systems