Improved Design Liquid Helium Temperature Regulator Using Operational Amplifier Circuits

We describe a liquid helium temperature regulator of unusually simple design, operation, and construction utilizing miniature solid state operational amplifiers. The combination of a unique saturating amplifier and an analog multiplier type of cross-correlation phase detection circuit eliminates fast risetime signals and switching transients, enabling the regulator to be used in a low noise shielded environment. The circuit is of the conventional ac Wheatstone bridge type using a carbon resistance thermometer for the sensing element and providing a proportional heater feedback of >1 W to the bath. The balanced input circuitry has been optimized for thermometer resistances between 500 Ω and 5 kΩ, and the regulator is capable of detecting a temperature change of 30 μK at a thermometer power dissipation of <3×10−11 W. Regulation of a superfluid 4He bath to better than ±4 μK for periods greater than 30 min has been achieved at a thermometer power of <5×10−9 W, with a response time of 0.1 sec. Owing to the high gain and broad bandwidth of this regulator, it is adaptable to a wide variety of other situations, such as a sample mounted on a copper block outside the bath, without encountering the usual problems of excessive power dissipation, instability, and thermal oscillations.