PURPOSE: To obtain a variable-constant filter block suitable to IC-implementation with good mass-producibility by providing a control part which changes filter characteristics by setting the switching period of each switching element.
CONSTITUTION: Switches S and capacitors C constitute switched capacitors S1C1", S2C1', S5C2"', S3C2", S4C2', S7C3', and S8C0', and operational amplifiers OP1∼OP4, integral capacitors C0∼C3 connected across them, and resetting switches S6 and S9 constitute a switched capacitor filter. Amplifier OP1 holds voltage V1' N5C0'/ C0 times input voltage Vi until S9 is shorted. Amplifiers OP2 and OP3 are applied with voltage V1' by way of S1C1' and S3C2" and output voltage V0 via S2C1' and S2C2' respectively, and amplifier OP3 with the output of OP1 via S5C2"', so that the voltages will be integrated by C1 and C2 respectively. Amplifier OP4 is applied with the output of OP3 and voltage V1' and holds the sum voltage by integraing. Then, control circuit CNT exercises control by setting switching periods of S1∼S9 to fixed values, so that a filter widh desired characteristics can be obtained.
KATOU SEIJI
UENO NORIO
