5
The output of the ramp signal generating circuit is the electric potential of a node B shown in FIG. 5A. The electric potential of the node B varies as folloWs. A capacitor Cx is charged by a constant current source While the sWitch S3 is in an OFF state. Accordingly, during this time period, the electric potential of the node B almost linearly rises as time elapses. This is What is called a ramp-up. When the sWitch S3 is ON, the capacitor Cx is discharged. If the resistance of the path reaching from the node B to a ground via the sWitch S3 is 0 at this time, the electric potential of the node B is immediately dropped to the ground level When the sWitch S3 is ON. If the path has a certain resistance, the electric potential of the node B almost linearly drops to the ground level at the speed depending on the resistance value of the path as time elapses, after the sWitch S3 is turned from the OFF state to the ON state.