Abstract A custom designed SOI operational amplifier (opamp) is used in two application circuits and characterized up to 300°C: 1. An instrumentation amplifier (in-amp) with a differential voltage gain of 100, and 2. A transimpedance amplifier (TIA) with a gain of 1.2 giga Ohm, used to sense picoamp level signals. The opamp operates with a 5-volt 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. The closed loop gain of the in-amp remains stable at 100, 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. 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 a higher dynamic range input before the circuit output saturates.