A 28nm bulk-CMOS 50MHz 18 dBm-IIP3 Active-RC Analog Filter based on 7 GHz UGB OTA

Abstract

This paper presents the design and the experimental validation of a 6th-order continuous-time low-pass filter in 28 nm bulk-CMOS, based on the cascade of 3 Rauch biquadratic cells. Each cell exploits a broad-bandwidth Operational Transconductance Amplifier (OTA), without Miller compensation scheme for differential Loop Gain stability. This maximizes the OTA unity gain bandwidth, with no power increase w.r.t classical compensation schemes, and improves both frequency response accuracy and linearity over the filter pass-band. This aggressive design choice is sustained by the higher 28 nm CMOS transistor’s transition frequency and by the intrinsic feature of the Rauch cell, whose R-C feedback/direct path nets introduce two poles and two zeros that self-compensate the differential loop-gain. On the other hand, the proposed OTA only exploits a compensation scheme for the common-mode signal stability, which does not affect the differential signal. The prototype synthesizes 50 MHz low-pass frequency response at 3.3 mA current consumption from a single 1.1 V supply and performs 18 dBm and 16.5 dBm Input IP3 for 10&11 MHz and 40&41 MHz input tones, equalizing the linearity performance over the filter pass-band, just thanks to the OTA wider bandwidth. This finally allows 153 dBJ-1 and 158 dBJ-1 Figure-of-Merit at 10&11 MHz and 40&41 MHz input tones.