Operational amplifier design using GaAs MESFET for temperature applications up to 350 deg C
A GaAs-MESFET technology based on highly stable ohmic and Schottky contacts is used to develop a two stage operational amplifier circuit with stable operation up to 350 deg C for sensor signal conditioning. The open loop gain of the whole circuit achieves 60 dB at 20 deg C decreasing to 10 dB at 300 deg C. By compensation methods a gain-bandwidth product of 80 MHz with a phase margin of about 50 degrees is achieved. The MESFET device behavlor is described by an extended TOM model which includes additional elements modeling the gate and drain leakage currents. Special design measures such as exploiting temperature stable properties of the MESFET devices have to be applied to minimize temperature induced drifts of the circuit's bias points. At the moment, trim voltages are used to compensate process variations to achieve an offset compensated output voltage. In later versions of the process, InGaP etch stop layers together with material selective etching techniques will be used for bette r reproduceability of threshold voltage and saturation current.