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An all-transistor CMOS active inductor with a self-resonance frequency fR=5.7 GHz is presented. Large fR is achieved by forming an all-NMOS signal path. The measured quality factor, Q, is as high as 665, but Q can be infinite theoretically. Both fR and Q are tunable via biasing and on-chip varactors. As an example for using the active inductor, a high-Q bandpass filter for radio-frequency applications is designed. The inductor circuit was implemented in TSMC 0.18-μm standard digital CMOS technology and occupies an area of 26.6 μm×30 μm including double guardrings. For a supply voltage of 1.8 V, the circuit consumes 4.4 mW, and IIP3 is measured at Vpp=270 mV.