Filter circuit

Abstract

A filter circuit includes a voltage amplifier, a resistor, a capacitor, and an analog switch connected between the voltage amplifier and the capacitor. When the voltage amplifier is turned on, the analog switch is opened so that the capacitor is disconnected from the voltage amplifier. Thus, an output voltage of the voltage amplifier sharply increases to its steady state value, as soon as the voltage amplifier is turned on. When the output voltage of the voltage amplifier is fully stabilized, the analog switch is closed so that the capacitor is connected to the voltage amplifier. During the period of time when the analog switch is closed, the filter circuit is configured as an imperfect integrator circuit with filter characteristics that depend on a capacitance of the capacitor and a resistance of the resistor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:

FIG. 1 is a schematic diagram of a first-order low pass filter circuit according to a first embodiment of the present invention;

FIG. 2 is a timing chart of an operation of the first-order low pass filter circuit of FIG. 1;

FIG. 3 is a flow chart of the operation of the first-order low pass filter circuit of FIG. 1;

FIG. 4 is a timing chart of an operation of a conventional first-order low pass filter circuit;

FIG. 5A is a schematic diagram of a conventional first-order high pass filter circuit, and FIG. 5B is a schematic diagram of a first-order high pass filter circuit according to the second embodiment of the present invention;

FIG. 6A is a schematic diagram of a conventional second-order low pass filter circuit, and FIG. 6B is a schematic diagram of a second-order low pass filter circuit according to the third embodiment of the present invention;

FIG. 7A is a schematic diagram of a conventional second-order high pass filter circuit, and FIG. 7B is a schematic diagram of a second-order high pass filter circuit according to the fourth embodiment of the present invention;

FIG. 8A is a schematic diagram of a conventional first-order band pass filter circuit, and FIG. 8B is a schematic diagram of a first-order band pass filter circuit according to the fifth embodiment of the present invention;

FIG. 9A is a schematic diagram of a conventional first-order all pass filter circuit with a high-frequency phase shift of 180°, and FIG. 9B is a schematic diagram of a first-order all pass filter circuit with a high-frequency phase shift of 180° according to the sixth embodiment of the present invention;

FIG. 10A is a schematic diagram of a conventional first-order all pass filter circuit with a low-frequency phase shift of 180°, and FIG. 10B is a schematic diagram of a first-order all pass filter circuit with a low-frequency phase shift of 180° according to the seventh embodiment of the present invention;

FIG. 11A is a schematic diagram of a conventional second-order low pass filter circuit with a phase shift of 180°, and FIG. 11B is a schematic diagram of a second-order low pass filter circuit with a phase shift of 180° according to the eighth embodiment of the present invention;

FIG. 12A is a schematic diagram of a conventional a first-order band pass filter circuit with a phase shift of 180°, and FIG. 12B is a schematic diagram of a first-order band pass filter circuit with a phase shift of 180° according to the ninth embodiment of the present invention; and

FIG. 13 is a schematic diagram of a first-order low pass filter circuit according to the tenth embodiment of the present invention.

Claims

1. An active filter circuit comprising: an active circuit including a voltage amplifier and a resistor connected to the voltage amplifier;a capacitor having a first electrode and a second electrode;a first analog switch connected between the first electrode and the active circuit; anda second analog switch connected between the second electrode and the active circuit, whereineach of the first and second analog switches is opened when the voltage amplifier is turned on, closed when an output voltage of the voltage amplifier reaches a predetermined steady state value, and opened when the voltage amplifier is turned off.

2. The filter circuit according to claim 1, further comprising: a first clamp circuit for clamping a voltage at a node between the first electrode of the capacitor and the active circuit to a first predetermined voltage, anda second clamp circuit for clamping a voltage at a node between the second electrode of the capacitor and the active circuit to a second predetermined voltage.

3. The filter circuit according to claim 2, wherein each of the first and second clamp circuits is a zener diode, andeach of the first and second predetermined voltage is a zener voltage of the zener diode.

4. The filter circuit according to claim 1, wherein the voltage amplifier operates from a positive power supply, andeach of the first and second analog switches is a n-channel MOS transistor.

5. The filter circuit according to claim 1, wherein the voltage amplifier operates from a negative power supply, andeach of the first and second analog switches is a p-channel MOS transistor.

6. The filter circuit according to claim 1, wherein the voltage amplifier is an operational amplifier.

7. The filter circuit according to claim 1, wherein the filter circuit is one of a first-order low pass filter circuit, a first-order high pass filter circuit, a second-order low pass filter circuit, a second-order high pass filter circuit, a first-order band pass filter circuit, a first-order all pass filter circuit with a high-frequency phase shift of 180°, a first-order all pass filter circuit with a low-frequency phase shift of 180°, a second-order low pass filter circuit with a phase shift of 180°, a second-order high pass filter circuit with a phase shift of 180°, and a first-order band pass filter circuit with a phase shift of 180°.

8. The filter circuit according to claim 1, wherein the resistor, the voltage amplifier, the capacitor, and the first and second analog switches are integrated on one semiconductor chip.