1 Low noise cryogenic electronics : preamplifier configurations with feedback on the bolometer

We developed preamplifier configurations used to readout resistive bolometers such as those based on Neutron Transmuted Doped Germanium thermometers (so called NTD Ge thermometers)[1] or NbSi thin film thermometers [2, 3]. We introduce the impedance regulating preamplifier configuration. This configuration, is compared to previously proposed readout configurations with feedback on the bolometer [11]. It is shown that the impedance regulating preamplifier allows to achieve Extreme Electrothermal Feedback without using transition edge thermometer and SQUIDs readout. Thus the detector and readout electronics are straighforwardly calibrated and can handle much larger event rate. Furthermore, looped configurations simplify the design and improve performance of detection such as bandwidth. Finally, we show that the effects on signal to noise ratio are marginal. Over the last ten years, low temperature detectors have undergone extensive development. They address detection problems which require high sensitivity, high energy resolution and/or low thresholds: X-ray and far infrared spectroscopy, double beta decay, Dark Matter searches. Their performance critically depends on the quality of the thermometer used, and of the readout electronics and wiring of the apparatus. This paper describes some of the electronics we have developped for the EDELWEISS and Planck Surveyor collaborations. We assume in the following that the thermometers used are resistive, of the order of 1 MΩ resistance, or more. The temperature increase of the bolometer induces a variation of the thermometer impedance. In this context, JFETs are used as front end electronics to achieve low noise performance. We introduce a new preamplifier configuration: the impedance regulating preamplifier and discuss the properties of previously proposed preamplifier configurations with feedback on the bolometer[11] and of the classical unlooped voltage amplifier readout. Preamplifiers have been developed or are under test for each of these configurations. We will use a simple adiabatic model of bolometer with standard parametrisation to compute the bolometer's thermal behaviour. It is shown that feedback on bolometers yields significant changes in the detection behaviour such as natural calibration of the detector, shorter risetime, reduced relaxation time. Thus, it simplifies readout electronics, avoiding the complex additional circuits otherwise needed to achieve the same behaviour. The effect on signal to noise ratio are only marginal. We will summarise and compare the merits of the different readout configurations in the discussion. Detailed explanation of designs, performances and implementations of electronic boards will be published later. Thermal behaviour of a bolometer: the adiabatic approximation. 3 Figure 1) shows a simplified …