Frequency Compensation of Three-Stage OTAs to Achieve Very Wide Capacitive Load Range

Abstract

This paper proposes an optimal design approach for three-stage amplifiers driving an ultra-wide range of load capacitor. To this end, efficient state-of-the-art solutions have been combined to develop a power-efficient frequency compensation solution. High-speed feedback pathways relying on Miller capacitors and current buffers are implemented within the amplifier scheme to push the non-dominant poles to high frequencies for small to medium load capacitors. A small resistor is also shared between the two pathways to improve the stability regardless of the load capacitor. A serial R-C branch is then added to extend the lower limit of load drive capability to small load capacitors. Gain margin is, for the first time in literature, analytically evaluated and included in the design phase. A prototype of the proposed amplifier is fabricated in 65-nm CMOS process with active area of 0.0017 mm 2 and 1.15 pF total compensation capacitance. It can drive the load capacitor range from 200 pF to 100 nF, while drawing a quiescent current of 7.4 µA from a 1.2-V input voltage supply. A unity-gain frequency of 1.67 MHz was measured with an average slew-rate of 1.31 V/µs, when the proposed amplifier is wired in unity-gain configuration to drive a 500-pF load capacitor.

A 29.5 dBm OOB IIP3 TIA Based on a Two-Stage Pseudo-Differential OTA With R-C Compensation and Cascode Negative Resistance

“…The series of R-C has been frequently exploited for frequency compensation in previous literature but with different names, including local impedance attenuation (LIA) [16], [17], impedance adapting compensation (IAC) [18], and capacitors with equivalent series resistance (ESR) [19]. Despite having the same circuit form, they differ slightly in terms of philosophy or implementation.…”

Ladder-Type Gm -C Filters With Improved Linearity

“…It is comprised of capacitors and transconductors [22], both of which can be realized efficiently in CMOS technology. Gm−C integrators are composed of Gm stages in an openloop formation, so their frequency response will not be compromised by the dominant poles of a frequency compensation network inserted for the stability of feedback amplifiers [23]- [27]. The bandwidth of a Gm−C filter can be tuned much more readily than their inductance-capacitor (LC) or active resistance-capacitor (RC) counterparts [28], [29], for the inductors and resistors cannot be realized efficiently in CMOS technology (with high quality factor and/or high accuracy).…”