A Two-Stage Amplifier for Power-Fluctuation-Tolerant Portable Pulse Oximeters

The pulse oximeter is a medical device that utilizes photoplethysmography (PPG) technology to measure blood oxygen saturation (SpO2) in the human body. One of its critical electronic components is the Operational Transconductance Amplifier (OTA). With the increasing demand for higher precision and improved noise immunity in medical instruments, medical-grade integrated circuits are striving for enhanced performance. Complementary Metal-Oxide-Semiconductor (CMOS) technology, as the dominant process in integrated circuit manufacturing, plays a pivotal role in the sampling, amplification, and filtering of biological signals. It is widely applied in the design of various medical device chips. This paper presents the design of a two-stage amplifier for pulse oximeters that operate under power supply fluctuations, utilizing 0.18m CMOS technology and designed with Cadence software. It achieves high gain and high slew rate while maintaining strong tolerance to power supply voltage fluctuations, ensuring stable performance in the oximeter. The simulation results demonstrate that under a 1.8V power supply, the amplifier designed in this paper exhibits a differential-mode gain of 83.9dB, a unity-gain bandwidth of 54.4MHz, and a common-mode rejection ratio (CMRR) of 83.6dB. Additionally, it achieves a slew rate of 42.6V/s, with a power consumption of 0.965mW. The low-frequency noise is measured at 4.29V/sqrt (Hz), and the power supply rejection ratio (PSRR) reaches 97.1dB. These results indicate that the amplifier possesses excellent tolerance to power supply fluctuations.