PURPOSE: To obtain a rise time constant and a fall time constant optionally by connecting a resistance to a condenser in parallel through a diode that turns on at the time of discharge.

CONSTITUTION: When a constant voltage source ei rises up, a condenser C is charged through a resistance R1 to a voltage divided by resistances R1 and R2. The rise time constant at this time is determined by resistances R1 and R2. When the constant voltage source ei falls, the condenser C discharges through the diode and through resistances R3 and R4, since internal resistance of the constant voltage source ei is small. That is, the resistances R1∼R4 are connected in parallel, and fall time constant is determined by resistances R1∼R4.