The aim of this research is to study of detection sensitivity in optical preamplifier, which is used as a first stage in optical receivers. This subject is important because it is used in laser rangefinders. The parameters that affect the distance measurement using time of flight technique are studied in details. Then, noise analysis and signal to noise ratio calculation are performed in preamplifiers to detect a signal with pulse width of 30ns using Matlab. The study results are applied to some preamplifiers to determine the minimum detectable power.