Low noise fast bipolar transistors for detector preamplifiers

The noise parameters of several species of microwave bipolar transistors were measured using a common base amplifier. The base spread resistance r_bb of the transistors was determined from the measured noise parameters. Some species were found to have r_bb of less than 30 Ω. For low-noise preamplifier applications in high-energy physics, the series noise contribution from these values of r_bb is negligibly small compared with that due to the emitter resistance. Using 2SC3585 as the input transistor, an input noise voltage of 0.98 nV/√Hz and input noise current of 1.81 pA/√Hz were obtained. A simple relation between the structure of the transistors and their r _bb was found