Modern wireless systems improve spectrum efficiency by employing advanced modulation techniques such as OFDM, but result in a large peak-to-average power ratio (PAPR) of the transmitted signal that degrades the efficiency of the RF power amplifiers (PAs). Envelope tracking and polar RF PAs are potential solutions to maintain a good efficiency at a large PAPR, but critically rely on the performance of their supply modulators. Conventional voltage regulators, such as linear regulators and switching regulators, fail to offer high efficiency, high linearity, and small output voltage ripple at the same time. Hybrid supply modulator (HSM) that combines a linear regulator and a switching regulator emerges as a promising solution to achieve an optimized tradeoff between different desig...[ Read more ]

Modern wireless systems improve spectrum efficiency by employing advanced modulation techniques such as OFDM, but result in a large peak-to-average power ratio (PAPR) of the transmitted signal that degrades the efficiency of the RF power amplifiers (PAs). Envelope tracking and polar RF PAs are potential solutions to maintain a good efficiency at a large PAPR, but critically rely on the performance of their supply modulators. Conventional voltage regulators, such as linear regulators and switching regulators, fail to offer high efficiency, high linearity, and small output voltage ripple at the same time. Hybrid supply modulator (HSM) that combines a linear regulator and a switching regulator emerges as a promising solution to achieve an optimized tradeoff between different design parameters. Despite of many designs in the literature, some fundamental problems of HSMs, such as stability and design optimization, are not adequately investigated in previous works.

In this thesis, a systematic study on wideband very-high-frequency (VHF) HSMs is presented, which attempts to establish a more solid foundation for the analysis and design of HSMs. First of all, the stability problem of HSMs is studied. The rationale behind the stability condition of hysteretic-controlled HSMs clarified, and a heuristic large-signal stability condition for HSMs is proposed, which includes hysteretic control as a special case. Then, a framework for analyzing and optimizing HSMs is presented. It discusses the design optimization of HSMs in a quantitative way, and allows us to obtain more insights of the tradeoffs between different performance parameters.

Two design examples are constructed to validate the theoretical results. The first design is an efficiency-enhanced HSM with single-capacitor current-integration control that uses the charge instead of the output current of the linear amplifier for control. The second one is a 100MHz HSM with ripple-current-based PWM control that is able to overcome the bandwidth limitation of using an absolute current sensor. Measurement results of both designs agree well with the theoretical predictions. Switching at a peak frequency of 50MHz, the first design is able to track a 0.8V_pp sinusoidal signal with high fidelity up to 10MHz, has an output voltage ripple below 8mV, and achieves a peak efficiency of 88.3% at the maximum output power of 23dBm. The second design improves the tracking range over the first one to 13MHz without using any feed-forward path. At last, some future research directions are pointed out.