JPS55138935A - Light transmitting and receiving system - Google Patents

Abstract

PURPOSE:Not only to make it hardly influenced by the secularcharge, variations in the characteristic, etc. of the light emitting and detecting elements but also to improve the transient characteristic, by feeding back an error signal related to the difference between an output level and a reference level of the light detecting element, to an input of the transmission circuit by which the light emitting element is driven. CONSTITUTION:At the time t0 when a signal (a) rises from V0 level to V1 level, an output of the light detecting element 2 does not rise yet, therefore an error signal (b) from the light quantity detecting circuit 10 is on -X3 level, an output signal (c) of the analog adding and subtracting circuit 11 becomes V3=V1+(X0-X3) level, and a driving current of the light emitting element 1 rises swiftly. At the time t1, the error signal (b) becomes -X2 level, and the signal (c) becomes V2= V1+ (X0-X2) level. At the time t2, an output signal of the light detecting element 2 exceeds a threshold level of the receiving circuit 4, the error signal (b) becomes X0 level, and the signal (c) is lowered to V1=V2+(X0-X2) level.