CMOS operational amplifier performance at cryogenic temperatures

The performance of three different two-stage CMOS operational amplifiers (op-amps) at temperatures below 100 K was investigated. The open-loop gain and unity-gain frequency increased by at least a factor of 2 and 1.3, respectively, when cooled from room temperature to 50 K. However, because of current kinks in the n-channel output transistors, all of the op-amps developed severe non-linearities in their output characteristics at temperatures below 50 K with ±5 V supplies. Reducing the supply voltages extended the op-amps useful cold temperature operating range. One op-amp was found to operate quite well down to 30 K with the supplies reduced to ±2.5 V. Methods of improving the cold temperature operation of CMOS op-amps are discussed.