UC San Diego

Switching-mode amplifiers represent attractive possibilities for RF wireless communication systems because their high efficiency can potentially extend the battery life time for portable devices, lower the cost of heat-sinking equipment and increase the device reliability. The reason such circuits are not widely used at present is the difficulty in maintaining the signal fidelity required for the modern complex modulation formats such as EDGE (Enhanced Data dates for GSM Evolution), CDMA (Code Division Multiple Access) or OFDM (Orthogonal Frequency Division Multiplexing) signals. The objective of this dissertation is to investigate and analyze options for circuits and system configurations based on switching-mode amplifiers and as a result provide insight for use of switching amplifiers in modern wireless communication systems to achieve high linear amplification as well as high efficiency. First, a current-mode class-D amplifier which is designed to achieve high efficiency at RF frequencies was investigated and the loss mechanisms were analyzed. Two current-mode class-D amplifiers based on GaAs HBTs with different novel on-chip inductor implementations were designed, fabricated and measured. The results achieved efficiencies up to 78% (in nonlinear operation) at 700MHz were compared to demonstrate the circuit performance. The counterpart of current-mode class -D amplifier, the voltage-mode class-D amplifier was also analyzed. The loss mechanisms were expressed in formulations which can be used to estimate the amplifier efficiency based on circuit parameters. A voltage-mode class-D was built in CMOS technology. A measured drain efficiency of 62% was achieved at 800MHz. Based on the designed voltage-mode class-D amplifiers, several approaches for the implementation of linear amplifier were developed for CDMA signals, including digitally-driven outphasing amplifier systems and delta-sigma amplifier systems. The outphasing technique was shown to provide amplification with adequate linearity to achieve the ACPR specifications along with efficiency of 48% for CDMA signals. The limitation of the efficiency enhancement via using the Chireix structure was addressed based on the loss analysis results derived for voltage-mode class-D amplifiers. The delta-sigma modulation approach was also investigated using the voltage-mode class-D amplifiers. The delta-sigma modulated signals were first generated to provide good noise shaping and linearity with CDMA signals. The switching amplifier systems based on the delta-sigma signals was then built and measured. The results show the linearity specification was achieved along with efficiency of 33%. Tradeoffs of efficiency and signals fidelity were analyzed for this architecture. Finally, a new digital modulation scheme, digital polar modulation, was presented. The linearity of new modulation scheme was demonstrated by two-tone signals. The system was able to show high efficiency of 54% along with good linearity (with IM3 below -39dBc). The results illustrate the promise of this approach for future digital RF transmitter systems