Abstract: A constant-current circuit includes a circuit current source for generating a circuit current necessary for the constant-current circuit, and a light-emitting element current source for passing a first current which is part of the circuit current through a light-emitting element in response to an input signal. A current control circuit detects a change in the first current and generates a control signal based on the change in the first current. A controlled current source passes a second current which is part of the circuit current and changes the amount of the second current on the basis of the control signal so that the circuit current has a fixed amount.

Abstract: A semiconductor laser driving circuit is arranged so that a predetermined number of transistors can be selected from a group of transistors disposed in parallel according to the characteristics of a semiconductor laser and the selected transistors are connected in parallel between the semiconductor laser and a current source. An optimum current is allowed to flow through the semiconductor laser to cause it to generate a required light output. Moreover, a collector current which provides the optimum characteristics flows through each of the selected transistors. Therefore, an optimum high-speed response can always be realized irrespective of the magnitude of the driving pulse current of the semiconductor laser.