  A measurement system for determining the gain distributions of avalanche
photodiodes (APDs) in a low gain range is presented. The system is based on an
ultralow-noise charge--sensitive amplifier and detects the output carriers from
an APD. The noise of the charge--sensitive amplifier is as low as 4.2 electrons
at a sampling rate of 200 Hz. The gain distribution of a commercial Si APD with
low average gains are presented, demonstrating the McIntyre theory in the low
gain range.
