Patent ID: 8531237

Claim:
A low-pass filter that filters an input signal input to a filter input terminal to output a filtered output signal to a filter output terminal, the low-pass filter comprising: a capacitor connected between the filter output terminal and ground; a first field-effect transistor having a gate terminal, a first conduction terminal connected to the filter input terminal, and a second conduction terminal connected to the filter output terminal; a first resistor connected between the gate and first conduction terminal of the first transistor; and a first current source connected to the first resistor to supply a first current to the first resistor, the first resistor generating a first voltage thereacross based on the supplied first current for electrically biasing the gate terminal of the first transistor, wherein the first current source includes: a second current source to supply a second current; a second field-effect transistor having a gate terminal, a first conduction terminal connected to the gate terminal thereof, and a second conduction terminal connected to the second current source, the second transistor generating a second voltage at the second conduction terminal thereof based on the second current supplied from the second current source; a second resistor; a third field-effect transistor having a gate terminal, a first conduction terminal connected to the first resistor, and a second conduction terminal connected to the second resistor, the third transistor conducting the first current between the first and second conduction terminals thereof for supply to the first resistor in response to a control signal applied to the gate terminal thereof, the second resistor generating a third voltage at the second conduction terminal of the third transistor conducting the first current; and an operational amplifier having a first input connected to the second conduction terminal of the second transistor, a second input connected to the second conduction terminal of the third transistor, and an output connected to the gate terminal of the third transistor to output the control signal to the gate terminal of the third transistor so as to maintain the third voltage substantially equal to the second voltage.