Patent ID: 8222590

Claim:
A signal amplifier connected to a photodetector that converts a received optical signal to a current signal, said signal amplifier comprising: a preamplifier converting said current signal to voltage and supplying a resultant voltage signal; a main amplifier amplifying the voltage signal supplied from said preamplifier, wherein at least an input stage of said main amplifier is made up from a differential circuit; two or more follower circuits provided between said preamplifier and said main amplifier and supplied with said voltage signal; resistors having equal resistance provided for each follower circuit on paths between an output of said preamplifier and each input of said main amplifier, and a capacitor having one end connected to one of said resistors and the other end grounded; wherein a low-pass filter detecting an average potential of said voltage signal is formed by said capacitor and the resistor connected to said capacitor.