Ripple filter circuit
View Patent ↗A ripple filter circuit for ensuring the driving of a driving subject while efficiently eliminating ripple even when the operational voltage margin is small. A first transistor, which is connected to a power supply voltage line, is connected to a ground line via a load of the driving subject. A second transistor, which has substantially the same characteristics as the first transistor, and a dummy load are arranged between the power supply voltage line and the ground line. An operational amplifier includes an inverting input terminal, connected between the second transistor and the dummy load, and a non-inverting terminal at which the voltage is decreased by a predetermined voltage from the power supply voltage. The operational amplifier also includes an output connected to the gate terminal of the second transistor and to the gate terminal of the first transistor via a lowpass filter.
1. A ripple filter circuit for eliminating ripple, the ripple filter circuit comprising:
a driving transistor, connected to a power supply line, for supplying output voltage to a driving subject connected to a ground line;
a reference transistor and a dummy load connected in series between the power supply line and the ground line, wherein the reference transistor functions as a replica of the driving transistor, and the dummy load corresponds to load of the driving subject;
a compensation circuit, connected to a gate terminal of the reference transistor, for canceling a characteristic variation in the reference transistor, wherein the compensation circuit provides its output to a gate terminal of the drive transistor via a low pass filter.
2. The ripple filter circuit according to claim 1 , wherein:
the compensation circuit includes a drop voltage setting power supply and an operational amplifier connected to the power supply line;
the operational amplifier includes input terminals connected to a connection node between the reference transistor and the dummy load and to one of connection nodes of the drop voltage setting power supply; and
the operational amplifier includes an output terminal connected to the gate terminal of the reference transistor.
3. The ripple filter circuit according to claim 2 , wherein the drive transistor and the reference transistor are depression type MOS transistors.
4. The ripple filter circuit according to claim 2 , wherein the drive transistor and the reference transistor are enhancement type MOS transistors, the ripple filter circuit further comprising:
a voltage adjustment power supply for applying voltage that is higher than or lower than the voltage at the power supply line to the power supply terminal of the operational amplifier in order to drive the driving transistor or the reference transistor.
5. The ripple filter circuit according to claim 1 , wherein the reference transistor is arranged near the driving transistor.
6. The ripple filter circuit according to claim 1 , wherein:
the reference transistor is smaller in size than the drive transistor; and
the dummy load and the load of the driving subject has a relationship equivalent to that of the drive transistor and the reference transistor.