Patent ID: 7598479

Claim:
A gain switching circuit configured to switch a conversion gain of a preamplifier, the preamplifier being configured to output a voltage signal by amplifying an output current of a photo-detecting element configured to convert a burst optical signal into an electrical signal, the preamplifier being configured with a first series circuit formed with a first resistor and a first switching element and a second series circuit formed with a second resistor and a second switching element respectively connected in parallel with a feedback resistor, the gain switching circuit inputting a first gain switching signal defining a period for switching to a first conversion gain and a second gain switching signal defining a period for switching to a second conversion gain, said first gain switching signal and said second gain switching signal originating from outside of the gain switching circuit, the gain switching circuit comprising: a first operating unit configured to generate a first switching element operating signal to close the first switching element in response to the voltage signal output by the preamplifier being above a first threshold during the period defined by the first gain switching signal originating from outside the gain switching circuit; and a second operating unit configured to generate a second switching element operating signal to close the second switching element in response to the voltage signal output by the preamplifier being above a second threshold during the period defined by the second gain switching signal originating from outside the gain switching circuit.