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 <inline-formula> <tex-math notation="LaTeX">$R$ </tex-math></inline-formula>-<inline-formula> <tex-math notation="LaTeX">$C$ </tex-math></inline-formula> 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 mm² 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 <inline-formula> <tex-math notation="LaTeX">$7.4~ \mu \text{A}$ </tex-math></inline-formula> 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/<inline-formula> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula>, when the proposed amplifier is wired in unity-gain configuration to drive a 500-pF load capacitor.