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An all-transistor lowpass biquad based on an active inductor for operation up to several gigahertz is presented. A convenient parasitic notch near the passband edge sharpens the transition band. The filter can be tuned via biasing and on-chip varactors. Higher order filters can be constructed by simple direct cascading of the second-order blocks. A fifth-order filter example is designed and simulated in TSMC 0.18-μm CMOS technology, with cutoff frequency at 4.57 GHz and passband ripple less than 1 dB.