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Low Noise Amplifiers Based on High TC SQUIDs

Doctoral thesis, 2001

This thesis describes the development, fabrication, performance and applications of liquid nitrogen cooled voltage amplifiers based on high temperature superconducting quantum interference devices (SQUIDs). The SQUID based voltage amplifier is intended for characterization of electrical and noise properties of devices with resistance around and below 1 Ohm. For different amplifier configurations, the best performance achieved in this work was a voltage noise en = 3.2 pV/sqrt(Hz), a currentnoise in = 0.28 pA/sqrt(Hz), an energy sensitivity enin = 1.7 * 10-22 J, a noise temperature Tn = 10 K, a bandwidth f3dB = 300 kHz and a slew rate ^V = 91 mV/s.
We investigated different schemes to magnetically couple an input coil efficiently to a SQUID. We developed a concept where a normal metal input coil was put on top of a magnetometer. The input coil could easily be adjusted to match source resistances from 1 mOhm to 1 kOhm. Within this range, the noise temperature of the amplifier can be as low as 10 K.
We used bias reversal to reduce the low frequency current noise of the voltage amplifier. We demonstrated that the voltage amplifier works for both liquid nitrogen cooled sources and for sources at room temperature.
The measured noise and the dynamic performance were in good agreement with predictions from the model of the SQUID-based voltage amplifier. The energy sensitivity was shown to be independent of the number of turns in the input coil for a given coil geometry, coil resistivity, coil position and for a given SQUID magnetometer.
We have used the voltage amplifiers to investigate the electrical characteristics and the noise properties of high-Tc SQUIDs and to investigate the noise properties of carbon fibre and Ag/AgCl electrodes in salt water, used for detecting motionally induced voltages. The concepts developed in this work have been used to construct voltage and current amplifiers for discharge and dielectric loss measurements in high voltage insulators.