A 0.1–3.2 GHz Reconfigurable LPF with Peaking Reducing and Selectivity Enhancement Using Adaptive Impedance Transformation Technique

This article presents an on-chip low-pass filter (LPF) for cognitive radio (CR) in a 130-nm SiGe BiCMOS technology. The LPF achieves a broad tuning range of 0.1–3.2 GHz with a 100-MHz step. To address gain variation caused by the frequency-peaking effect, we propose an adaptive impedance transformation (AIT) technique, achieving a remarkable ripple of less than 3.01 dB across the entire 0.1–3.2 GHz frequency band. This technique also enhances passband selectivity. Furthermore, we introduce a novel operational amplifier (OPAMP) featuring a four-stage heterojunction bipolar transistor (HBT)–complementary metal–oxide–semiconductor (CMOS) transistor composite pair. Leveraging the inherent advantages of both HBT and CMOS technologies, this OPAMP elevates the gain-bandwidth (GBW) product from 0.834 GHz of a CMOS-only topology to an impressive 8.33 GHz. The LPF requires only 8 mA in the low-power mode at 1.5 V and 17.5 mA in the high-power mode at 2 V. With such wide-tuning range, the proposed LPF is suitable for CR applications.