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|SOI operational amplifier applications in 300°C operating environment|
|Keywords: sensor electronics, silicon on insulator, amplifier front-end circuits|
|We have seen a steady development in high temperature electronics where bulk silicon electronics have been designed and characterized for 200°C operation. Many Silicon-on-insulator (SOI) components are also available and rated up to 225°C. Still, there are some industrial applications in aviation, power generation, and oil and gas that require temperatures up to 300°C and beyond, as well as operating life time measured in years rather than weeks. In this paper, we will highlight our recent results of using a custom designed SOI operational amplifier (opamp) in two application circuits characterized up to 300°C: 1. An instrumentation amplifier (in-amp) intended to provide a differential voltage gain of 100, and 2. A transimpedance amplifier (TIA) with a gain of 1.2 gigaohm, used to sense picoamp level signals. The opamp operates with a 5V supply, and has rail-to-rail inputs and outputs. The open loop gain of the opamp is about 100 dB at room temperature and stays above 60 dB at 300°C. The in-amp uses the classic three operational amplifier (opamp) architecture with off-chip feedback resistors. We compared the temperature drift of resistance of on-chip and off-chip resistors and found the off-chip option to be more stable, especially at the higher temperature end. The closed loop gain of the in-amp is set to 100 (10 per stage) with proper feedback resistor ratios. The closed loop gain of the in-amp remains stable up to 250°C, and drops to 95 at 300°C. Temperature dwell test shows the in-amp maintaining stable functionality at 300°C for at least 1000 hours. Both the in-amp and the TIA takes advantage of the low leakage MOSFET inputs of the opamp to enable sensing of small signals at high temperatures. The TIA also incorporates a silicon carbide diode to achieve gain compression by lowering the feedback gain when the input signal is higher, thus allowing the circuit to remain before saturation for a much wider input signal range. The picoamp level input signal necessitates the use of silicon carbide diode because the diode’s reverse leakage current remains small at high temperatures. The TIA circuit was also temperature stressed at 300°C for at least 2000 hours showing stable operation.|
|Cheng-Po Chen, Electrical Engineer