Tunability, reconfigurability, and adaptability for RF and microwave circuits are highly desirable because they not only enhance the functionality and performance but also reduce the circuit size and cost. This thesis studies impedance matching circuits that adapt themselves based on either transmit power level or operating environment, targeting the realization of high performance intelligent RF front-ends. Specifically, this study is divided into two distinct topics: (a) linear and efficient power amplifiers (PAs) using tunable matching networks (TMNs) that are dynamically controlled according to the instantaneous power level, and (b) adaptive matching systems based on impedance tuning units to automatically compensate the impedance variation of an antenna. The tuning elements used for implementing the adaptive impedance matching circuits include both semiconductor and ferroelectric varactors. To use the intrinsically nonlinear ferroelectric materials in wireless transceivers with stringent linearity requirements, a technique for improving the linearity of the ferroelectric varactors is proposed and implemented. Up to 16 dB improvement on IIP3 is demonstrated. A study of the tradeoff between the quality factor and tuning speed for the linearized varactors is conducted. Subsequently, two specific applications are investigated. First, a PA with a diode-based TMN is designed, fabricated, and tested. The TMN is dynamically controlled according to the instantaneous power level such that not only optimum load is provided but the AM-AM and AM-PM distortions of the PA are reduced, for the first time demonstrating both efficiency enhancement and linearity improvement using TMNs. Measurement results show that a 13% reduction in DC power consumption is achieved under the same linearity constraint. Second, a closed-loop system is proposed for adaptively performing impedance matching to an unknown load. The adaptive matching system is composed of an impedance tuner, an impedance sensor, and peripheral control circuitries. The impedance tuner, consisting of a phase shifter and a variable transformer, is a novel implementation using an all-pass network topology. Design equations for the phase shifter and variable transformer are derived. Compared to stub-based MEMS tuners, the lumped-element based tuner is preferable for cellular frequency bands because of its compact size
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