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US 7,075,364 B2Granted
Active-RC filter with compensation to reduce Q enhancement
AssigneeQUALCOMM Incorporated
InventorsPrasad Gudem, Peter C. Gazzerro
Life of this patent
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Abstract
An active filter with compensation to reduce Q-enhancement is disclosed. The filter may include a first amplifier and a second amplifier coupled to the first amplifier. The second amplifier having a negative feedback loop including a buffer having a plurality of compensation resistors. The compensation resistors may each have a value that results in the biquad filter having substantially zero Q-enhancement. The biquads may cascaded to form higher order filters.
Claims25
A biquad filter, comprising:
- a first amplifier;
- and
- a second amplifier coupled to the first amplifier, the second amplifier having a negative feedback loop including a buffer having a plurality of compensation resistors, the compensation resistors each having a resistance value that results in the biquad filter having substantially zero Q-enhancement, wherein Q-enhancement represents an undesirable peak in amplitude around a pole frequency of an amplitude frequency response for the biquad filter,
- wherein an input of the first amplifier is coupled to an input to the biquad filter and an output of the second amplifier is coupled to an output for the biquad filter when an cutout of the first amplifier is coupled to an input of the second amplifier, and
- wherein the input of the second amplifier is coupled to the input to the biquad filter and the output of the first amplifier is coupled to the output for the biquad filter when the output of the second amplifier is coupled to the input of the first amplifier.
2 more elements
A biquad filter, comprising:
- a first differential amplifier;
- and
- a second differential amplifier coupled to the first differential amplifier, the second amplifier having at least one compensation resistor connected across its differential input, and a negative feedback loop including a buffer having a plurality of compensation resistors, different from the at least one compensation resistor, said at least one compensation resistor connected across the differential input of the second differential amplifier and the plurality of compensation resistors in the negative feedback loop each having a resistance value than results in the biquad filter having substantially zero Q-enhancement, wherein Q-enhancement represents an undesirable peak in amplitude around a pole frequency of an amplitude frequency response for the biquad filter,
- wherein an input of the first differential amplifier is coupled to an input to the biquad filter and an output of the second differential amplifier is coupled to an output for the biquad filter when an output of the first differential amplifier is coupled to an input of the second differential amplifier, and
- wherein the input of the second differential amplifier is coupled to the input to the biquad filter and the output of the first differential amplifier is coupled to the output for the biquad filter when the output of the second differential amplifier is coupled to the input of the first differential amplifier.
2 more elements
A biquad filter, comprising;
- a first differential amplifier having a pair of input resistors R1 and a pair of negative feedback resistors R2;
- a second differential amplifier having a pair of input resistors R3, the second differential amplifier further having a negative feedback loop comprising a buffer and a plurality of compensation resistors RC2 coupled to the buffer;
- a negative feedback loop between the first and second amplifiers, the negative feedback loop comprising a pair of negative feedback resistors R4;
- and
- a load resistor RL coupled to an output of the first amplifier when an output of the second amplifier is coupled to an input of the first amplifier, and coupled to the output of the second amplifier when an output of the first amplifier is coupled to an input of the second amplifier;
- wherein the input of the first amplifier is coupled to an input to the biquad filter and the output of the second amplifier is coupled to an output for the biquad filter when the output of the first amplifier is coupled to the input of the second amplifier,
- wherein the input of the second amplifier is coupled to the input to the biquad filter and the output of the first amplifier is coupled to the output for the biquad filter when the output of the second amplifier is counted to the input of the first amplifier;
- and
- wherein the resistors each have a value that satisfies the following equation:
- R
- out1
- R
- 1
-
-
- R
- 2
-
-
- R
- 3
-
-
- R
- 4
- =
- R
- out2
- R
- L
-
-
- R
- 4
-
-
- R
- c2
- =
- R
- out3
- R
- c2
40 more elements
A method of constructing a biquad filter, comprising:
- coupling a pair of input resistors R1 and a pair of negative feedback resistors R2 to a first differential amplifier;
- coupling a pair of input resistors R3 to a second differential amplifier;
- coupling a plurality of compensation resistors RC2 to a buffer and placing the buffer and plurality of compensation resistors into a negative feedback loop of the second differential amplifier;
- coupling a pair of negative feedback resistors R4 between the first and second differential amplifiers;
- coupling a load resistor RL to an output of the first differential amplifier when an output of the second differential amplifier is coupled to an input of the first differential amplifier, and to the output of the second differential amplifier when an output of the first differential amplifier is coupled to an input of the second differential amplifier;
- and
- selecting the resistor values for the load resistor RL using the following equation:
- R
- out1
- R
- 1
-
-
- R
- 2
-
-
- R
- 3
-
-
- R
- 4
- =
- R
- out2
- R
- L
-
-
- R
- 4
-
-
- R
- c2
- =
- R
- out3
- R
- c2
- wherein the input of the first differential amplifier is coupled to an input to the biquad filter and the output of the second differential amplifier is coupled to an output for the biquad filter when the output of the first differential amplifier is coupled to the input of the second differential amplifier, and
- wherein the input of the second differential amplifier is coupled to the input to the biquad filter and the output of the first differential amplifier is coupled to the output for the biquad filter when the output of the second differential amplifier is coupled to the input of the first differential amplifier.
40 more elements
An active filter, comprising:
- a first biquad including a first amplifier and a second amplifier coupled to the first amplifier, the second amplifier having a negative feedback loop including a first buffer having a plurality of first compensation resistors, the first compensation resistors each having a resistance value that results in the first biquad filter having substantially zero Q-enhancement, wherein Q-enhancement represents an undesirable peak in amplitude around a pale frequency of an amplitude frequency response for the first biquad filter;
- and
- a second biquad including a third amplifier and a fourth amplifier coupled to the third amplifier, the third amplifier having a negative feedback loop including a second buffer having a plurality of second compensation resistors, the second compensation resistors each having a resistance value that results in the second biquad filter having substantially zero Q-enhancement, wherein Q-enhancement represents an undesirable peak in amplitude around a pole frequency of an amplitude frequency response for the second biquad filter.
Dependencies are inferred from the claim text.