This dissertation focuses on three different efficiency enhancement methods that are applicable to handset applications. These proposed designs are based on three critical requirements for handset application: 1) Small form factor, 2) CMOS compatibility and 3) high power handling. The three presented methodologies are listed below:
1) A transformer-based power combiner architecture for out-phasing transmitters
2) A current steering DAC-based average power tracking circuit for on-chip power amplifiers (PA)
3) A CMOS-based driver stage for GaN-based switched-mode power amplifiers applicable to fully digital transmitters
This thesis highlights the trends in wireless handsets, the motivates the need for fully-integrated CMOS power amplifier solutions and presents the three novel techniques for reconfigurable and digital CMOS-based PAs. Chapter 3, presents the transformer-based power combiner for out-phasing transmitters. The simulation results reveal that this technique is able to shrink the power combiner area, which is one of the largest parts of the transmitter, by about 50% and as a result, enhances the output power density by 3dB.
The average power tracking technique (APT) integrated with an on-chip CMOS-based power amplifier is explained in Chapter 4. This system is able to achieve up to 32dBm saturated output power with a linear power gain of 20dB in a 45nm CMOS SOI process. The maximum efficiency improvement is about ∆η=15% compared to the same PA without APT. Measurement results show that the proposed method is able to amplify an enhanced-EDGE modulated input signal with a data rate of 70.83kb/sec and generate more than 27dBm of average output power with EVM<5%.
Although small form factor, high battery lifetime, and high volume integration motivate the need for fully digital CMOS transmitters, the output power generated by this type of transmitter is not high enough to satisfy the communication standards. As a result, compound materials such as GaN or GaAs are usually being used in handset applications to increase the output power. Chapter 5 focuses on the analysis and design of two CMOS based driver architectures (cascode and house of cards) for driving a GaN power amplifier. The presented results show that the drivers are able to generate ∆Vout=5V, which is required by the compound transistor, and operate up to 2GHz. Since the CMOS driver is expected to drive an off-chip capacitive load, the interface components, such as bond wires, and decoupling and pad capacitors, play a critical role in the output transient response. Therefore, extensive analysis and simulation results have been done on the interface circuits to investigate their effects on RF transmitter performance. The presented results show that the maximum operating frequency when the driver is connected to a 4pF capacitive load is about 2GHz, which is perfectly matched with the reported values in prior literature.