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

This paper presents an on-chip low-pass filter (LPF) for cognitive radio 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 transistor (CMOS) composite pair. Leveraging the inherent advantages of both HBT and CMOS technologies, this OPAMP elevates the gain-bandwidth product (GBW) from the 0.834 Grad/
$s$
of a CMOS-only topology to an impressive 8.33 Grad/
$s$
. 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 its wide-tuning range, the proposed LPF achieves a state-of-the-art figure of merit (FoM
T
) of 1939, underscoring its suitability for cognitive radio applications.