A Comparative Study on a SOI-CMOS Capacitive Feedback Op-Amp Using different Bias Circuits for High Temperature Application Jeongwook Koh  , Muthukumaraswamy Annamalai Arasu and Minkyu Je Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research) kohj@ime.a-star.edu.sg Abstract. In this paper, we present comparative study on implementation of 1.0 um SOI-CMOS capacitive feedback op-amps using two different bias circuits, constant-gm bias and constant- current bias circuits for down-hole drilling application among high temperature electronics from 0 °C to 225 °C. Temperature dependence of leakage current and output resistance are investigated for high temperature operation of the op-amp and bias circuits. And the temperature dependence of open-loop ac characteristics of op-amps with two different bias circuits and the temperature dependence of closed-loop ac characteristics of capacitive feedback op-amp using these bias circuits are analyzed in this paper. Keywords: SOI-CMOS, Op-amp, capacitive feedback, high temperature electronics, constant-gm bias, constant current bias 1. Introduction High temperature electronics finds many applications like automotive, aerospace, nuclear and down-hole drilling. Those applications require a reliable operation at elevated temperature. Electronics used in down- hole-drilling application is to explore modern oil/gas wells through collecting, logging and/or processing data such as heading and inclination, temperature and pressure of the strata. The biggest obstacle for electronic data acquisition systems in deep wells is the high temperature (>170°C) encountered at great depth. Reliable high-temperature performance (up to 225°C, ambient) is a requirement due to the extreme temperatures encountered in deep wells typically over 2 km depth below the surface of the earth/sea bed [1]. Conventional integrated circuit (IC) technology like a bulk-CMOS technology is not capable of operating at these high temperatures. As temperature increases, several phenomena arise in bulk-CMOS technology - mobility degradation, threshold voltage down-shift and anomalous increase of leakage current with increasing temperature. Among them, anomalous increase of leakage current is the most serious problem limiting analog circuit performance and more seriously destroy the chip due to latch-up. SOI-CMOS technology is more attractive than bulk-CMOS technology thanks to reduction of this “excess” leakage current at high temperature regime [2]. In addition to technology consideration, analog circuit design consideration need to be considered for high temperature electronics. An important concept for an operational amplifier is to maintain a stable dc gain, speed and stability over operating temperature ranges. Generally those parameters have a direct dependency on a dc bias circuit. Literature study shows two popular techniques for high temperature operation, a constant-gm bias circuit and constant-current bias circuit [3] [4]. In this paper we make a comparative study on a 5V1.0 um SOI-CMOS (XI10 by X-FAB Semiconductor Foundry AG) capacitive feedback op-amp using these two bias schemes for high temperature operation. A  Corresponding author. Tel.: + 65 6770 5591 fax: +65 6774 5754. E-mail address: kohj@ime.a-star.edu.sg 2012 International Conference on Solid-State and Integrated Circuit (ICSIC 2012) IPCSIT vol. 32 (2012) © (2012) IACSIT Press, Singapore