Synthesis of High-Order Continuously Tunable Low-Pass Active-R Filters

This work discusses the synthesis and design methodology of high-order and frequency-tunable low-pass active-R filter architectures for multi-standard wireless applications. Active-R filters use the inherent integrator-like behavior of amplifiers to realize their frequency response. The main advantages of this type of filter are high-frequency performance and a low integrated area. Active-R filters only need the Miller capacitor used in internally compensated amplifiers. In this work, amplifiers with configurable unity-gain frequencies enable the continuous tuning of active-R filters. Three different filter architectures realize a fifth-order Butterworth prototype tunable in the 1-50 MHz frequency range. These filters are designed, fabricated, and tested using the TSMC 0.18 μm process. Each fifth-order filter consumes power in the range 7.45 mW to 9.38 mW from a 1.8 V supply and has an integrated area ≤ 0.33 mm
²
. Compared with state-of-the-art active filters, the filters presented in this work have the largest tuning range without linearity degradation and the smallest area per tuning range ratio. Also, the measured filters show a competitive FoM 153.3 dB/J when operating at high frequencies.