| 摘要 |
PURPOSE:To detect a light output cut stably and accurately with respect to the light signal mark rate variation by a simple and low-cost structure. CONSTITUTION:In comparing the light output from a light emitting element with the refernce value, the reference value is not set at a fixed voltage but it is made variable corresponding to the drive voltage or the drive current of the light emitting element. To a conventional detector circuit are added newly current switching circuit 21, including a pair of transistors Q3, Q4, DC amplifier 22 and CR circuit 23. A reference value to be fed to comparator circuit 15 is formed by means of these circuits. Circuit 21 is of the same structure as circuit 12 and it takes out from Q4 a current which is exactly equivalent to the drive current to be fed to light emitting element 11 via Q2. The variation in this equivalent current is DC amplified (22), its average value is taken by circuit 23, and it is made as a variable reference value. CR circuit 14 is provided corresponding to circuit 23. When one of poor frequency response characteristics is used for monitoring light receiving element 13, circuit 14 is not needed, and the output of element 13 shows the average value. At this time, the time constant of circuit 23 is adjusted at about 100kHz. By this structure, erroneous judgment can be eliminated. |
