Low pass filter
First Claim
1. A low frequency cutoff filter comprising:
- a filter circuit including a capacitor connected between an input terminal and an output terminal, and an active resistor connected to the output terminal, the active resistor having a very large resistance; and
a bias circuit having a negative feedback to set a biasing voltage of the active resistor to a predetermined value so that an output voltage of the output terminal has the predetermined value and an output resistance of the bias circuit is very large, wherein the active resistor comprises;
a first transistor having a drain connected to the output terminal and a source connected to a supply voltage; and
a second transistor having a drain connected to the output terminal and a source connected to a ground.
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Accused Products
Abstract
There is disclosed a low frequency filter. A low frequency cutoff filter includes a filter circuit having a capacitor connected between an input terminal and an output terminal and an active resistor connected to the output terminal, having a very large resistance, and a bias circuit having a negative feedback to set a biasing voltage of the active resistor to a desired value, thereby implementing the cutoff filter within a semiconductor chip as one set with the capacitor having a small capacitance. A low frequency pass filter includes an active resistor having a very large resistance, means for setting a biasing voltage of the active resistor to a desired value, and a capacitor connected between the output terminal and the ground. Therefore, the low pass filter can be integrated-circuited using even small capacitor.
104 Citations
13 Claims
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1. A low frequency cutoff filter comprising:
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a filter circuit including a capacitor connected between an input terminal and an output terminal, and an active resistor connected to the output terminal, the active resistor having a very large resistance; and a bias circuit having a negative feedback to set a biasing voltage of the active resistor to a predetermined value so that an output voltage of the output terminal has the predetermined value and an output resistance of the bias circuit is very large, wherein the active resistor comprises; a first transistor having a drain connected to the output terminal and a source connected to a supply voltage; and a second transistor having a drain connected to the output terminal and a source connected to a ground. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A low frequency pass filter comprising:
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a filter circuit including an active resistor connected to an output terminal; and a bias circuit connected between an input terminal and the filter circuit, for setting a biasing voltage of the active resistor to a predetermined value, wherein the active resistor comprises a first transistor having a drain connected to the output terminal and a source connected to a supply voltage; and a second transistor having a drain connected to the output terminal and a source connected to a ground; and wherein the bias circuit comprises a third transistor having a gate connected to the gate of the first transistor and a source connected to a supply voltage; a fourth transistor having a gate connected to the gate of the second transistor, a source connected to the ground, and a drain connected to the drain of the third transistor; an operational amplifier having an inverting input supplied with an input signal voltage, a non-inverting input connected to the drain of the third transistor, and an output connected to the gates of the second and fourth transistors; a noise rejecting capacitor connected between the non-inverting input of the operational amplifier and the ground; a fifth transistor having a gate connected to the gate of the third transistor, a drain connected to the gate of the fifth transistor, and a source connected to the supply voltage, the fifth transistor forming a current mirror circuit together with the first and third transistors; and a current source for supplying currents to the fifth transistor. - View Dependent Claims (12, 13)
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Specification