||Different types of dc-dc regulators are essential to power up various integrated circuits that give nowadays battery powered mobile systems with diverse functions. As trend of mobile systems is towards smaller size, more functions, and longer battery time, dc-dc regulators with high compactness and power efficiency are most wanted. In this thesis, different low voltage and low power circuit techniques are proposed to bring those dc-dc regulators commonly used in mobile systems with higher efficiency at a wide range of load condition and also smaller size. A low-quiescent current, high-slew rate, error amplifier is proposed to bring low-dropout regulators (LDRs) with high light-load efficiency. Transient responses of such LDRs are not degraded as slew-rate of the proposed design is no longer limited by its quiescent current. The proposed design improves light-load stability of LDRs applied to system-on-chips. This design was fabricated in a 0.18μm CMOS process. Measurement result shows that the LDR responds within ~200ns and is fully recovered within ~1μs at only 1.2μA quiescent current. A 0.9V input pulse-width modulator is proposed for a boost converter employed in systems with single-cell Nickel-metal hydride battery. This modulator enables the use of entire battery capacity to prolong battery time. A CMOS-control rectifier (CRR) is proposed to provide adaptive dead-time control, which improves efficiency by eliminating shoot-through current and minimizing both body-diode conduction loss and charge-sharing loss. The CCR enables synchronous boost converter to operate in discontinuous-conduction mode, which enables the use of small off-chip components at sub-MHz switching frequency to minimize switching related losses. This design was fabricated in a 0.35μm CMOS process. Experimental results prove that the converter can be directly powered from 0.9-V input with ~85% efficiency at 100-mA output. An auto-selectable-frequency pulse-width modulator is proposed for a buck converter. This modulator improves light-load efficiency by selecting converter switching frequency based on its load current from a set of pre-defined frequencies, which are designed to guarantee that the converter output spectrum is as predictable as the one with pulse-width modulation even the switching frequency is changed with load current. Very small off-chip components can be used by designing the maximum switching frequency in MHz-range. This design was fabricated in a 0.35μm CMOS process and a significant improvement at light-load efficiency is experimentally verified.