Abstract
This paper presents a low noise amplifier for large arrays of silicon photomultipliers (SiPMs) operated in cryogenic environments, especially liquid argon (87 K) and liquid nitrogen (77 K) . The goal is for one amplifier to read out a total photosensitive surface of tens of cm2 while retaining the capability to resolve single photoelectron signals. Due to the large capacitance of SiPMs, typically a few nF per cm2, the main contributor to noise is the series (voltage) component. A silicon-germanium heterojunction bipolar transistor (HBT) was selected as the input device of the cryogenic amplifier, followed by a fully differential operational amplifier, operated in an unconventional feedback configuration. The input referred voltage noise of the circuit at 77 K is just below 0.4 nV/√Hz white (above 100 kHz) and 1 nV/√Hz at 10 kHz. The value of the base spreading resistance of the HBT at 77 K was determined from noise measurements at different bias currents. Power consumption of the full circuit is about 2.5 mW . The design gives the flexibility to optimally compensate the feedback loop for different values of the input capacitance, and obtain a gain-bandwidth product in the GHz range. The signal-to-noise ratio obtained in reading out SiPMs is discussed for the case of a 300 kHz low pass filter and compared with the upper limit that would derive from applying optimum filtering algorithms.
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