| 摘要 |
<p>PURPOSE:To prevent a driving current larger than required from flowing to a light emitting element and to improve the deterioration of the light emitting element and reduce its power consumption by providing a negative feedback control circuit which makes the output terminal voltage of a light receiving element constant. CONSTITUTION:A variable impedance circuit Q3 is connected to light emitting elements L1-Ln and the negative feedback control circuit 9 which inputs output terminal voltages of the light receiving elements PT1-PTn receiving light signals and adjusts the variable impedance circuit Q3 is provided to form a negative feedback circuit, so driving currents of the light emitting elements L1-Ln are adjusted to make the output terminal voltages of the light receiving elements PT1-PTn constant. A storage circuit 13 which stores the feedback quantities of the negative feedback control circuit by the couples of light emitting elements L and light receiving elements PT and a switching control circuit 10 which turns on or off the negative feedback circuit are provided to operate the negative feedback circuit by the starting scan in the beginning of the operation and store the feedback quantity in the storage circuit 13; when a subsequent scan wherein a light signal is possibly cut off by a coordinate indication is made, the variable impedance circuit is adjusted with a feedback quantity read out of the storage circuit and the light emitting elements L1-Ln are so driven as to obtain constant output terminal voltages from the light receiving elements PT1-PTn.</p> |
