Patent ID: 7962047

Claim:
A preamplifier, comprising: an input port that inputs a current signal according to the intensity of an optical signal; a first amplifier that converts the current signal that is input from the input port into a voltage signal; a second amplifier that inputs the voltage signal that is output from the first amplifier and amplifies the voltage signal; an output port that outputs the voltage signal that is output from the second amplifier; and a counter that counts the number of voltage signals and outputs a counted result, wherein the gain of the first amplifier is changed over on the basis of the count value that is output from the counter, wherein the first amplifier includes a main transistor, a cascode transistor that is connected to an upper stage of the main transistor, and a bias terminal that is disposed at a connection point between the main transistor and the cascode transistor, and wherein a current injection into the main transistor and a current drawing from the main transistor are controlled through the bias terminal.