Low noise voltage and charge preamplifiers for phonon and ionization detectors at very low temperature

We have developed voltage and charge sensitive FET preamplifiers which feature ultra low noise, convenience and flexibility for phonon and ionization detectors operated at 20 mK. With an NJ132L J-FET, the white noise of the voltage amplifier is 1.1 nV/√Hz at room temperature and 0.9 nV/√Hz when the FET is cooled to ≈120 K; the {1}/{f} knee is below 100 Hz. The power dissipation of the FET, about 7 mW in our application, allows it to be used in a 4 K environment. With the same cooled FET, the charge amplifier has shown a noise of 120 e ^- rms with a total input capacitance of 45 pF.