Wideband Dynamic Biasing of Power Amplifiers for Wireless Handheld Applications

Abstract

The objective of the proposed research is to extend the battery life in cellular handsets by improving the transmitter efficiency. Bandwidth efficient modulation formats, such as W-CDMA, encode much of the information in amplitude modulation. Therefore, linear transmitters must be used so as not to increase transmission errors, and cause interference in adjacent bands. Various engineering trade-offs were examined to find a suitable transmitter architecture for W-CDMA. Dynamic biasing of the transmitter power amplifier (PA) provides a simple way to improve efficiency for applications that require highly linear amplification. The envelope elimination and restoration (EER) PA or EER-based polar-modulated PA is an attractive solution since it has potential to achieve very high efficiency with high linearity. However, the major impediment to EER implementation has been the lack of power-efficient dynamic power supply circuits that can operate with sufficient modulation bandwidth, and simultaneously achieve the required modulation linearity. This work proposes several solutions to this problem. First, a dynamic supply circuit using delta modulation was designed and implemented. An open-loop EER PA with 48% peak efficiency was constructed and tested with a cellular band IS-95 CDMA signal with a bandwidth of 1.25 MHz. The low switching loss by using a delta modulator made the implementation of a wideband dynamic biasing circuit possible. Second, a dynamic supply circuit using dual-phase PWM was designed and implemented to achieve wider bandwidth, lower noise, and higher efficiency. An open-loop EER PA was implemented with the dynamic supply IC. A digital gain compensation scheme was developed to further increase bandwidth and linearity. This enables a dynamic supply circuit with lower switching frequency to have larger usable bandwidth with little increased power consumption. A cellular band W-CDMA voice signal was used to evaluate the performance of the overall PA. The PA achieved 50% efficiency while passing all required spectral specifications of W-CDMA standard. To increase the inherent low dynamic range of an EER PA, a dual-mode power amplifier combining an EER PA and power-level tracking PA was proposed. This work will contribute to the development of high efficiency, small-sized multi-mode linear PAs for battery-operated wireless handheld devices.