Phase Locked Loop for automatic tuning of low-frequency Gm-C filters

Transfer functions implemented by Gm-C filters depend on absolute values of transconductance and capacitances, which are greatly affected by variations in the fabrication process. Hence, a design procedure that makes the filter cutoff frequency precisely defined, despite the influences of fabrication process variations is necessary. This work develops a Phase-Locked Loop (PLL) in order to detect such variations and enable the automatic adjustment of a Gm-C filter cutoff frequency. An important aspect of the proposed approach is the design of an operational transconductance amplifier (OTA) that presents a highly linear behavior in a wide range of input differential voltages. Spice simulations, including Monte Carlo analysis, using BSIM3v3 device models of a 0.35 μm CMOS technology were performed to verify the robustness of the proposed design approach.