Active Filter Transfer Function Control With Passive Feedback
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Solution Overview
Problem
Active filters in integrated systems, such as wireless transceivers, face challenges in achieving power and area savings due to the need for high-order filtering and the power consumption of wideband operational amplifiers, which is incompatible with battery-operated and low-power applications.
Innovation Solution
A filter circuit with a differential amplifier and feedback circuit using a feedback resistor and capacitor to control the transfer function, allowing the coefficients to depend solely on passive elements and amplifier gain and poles, thereby reducing power consumption and area requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high-order filtering is implemented using traditional active filters, then filtering performance is improved, but power consumption and area increase
Solution Approach 1:
The patent changes the parameter dependency of the transfer function by forcing coefficients to depend solely on passive circuit elements (resistors and capacitors) rather than on the gain and poles of wideband operational amplifiers. This parameter transformation allows the use of low-power amplifiers while maintaining high-order filtering performance, directly resolving the contradiction between filtering performance and power consumption
Solution Approach 2:
The patent substitutes the reliance on active amplifier characteristics (gain and poles) with reliance on passive circuit element characteristics. By replacing the mechanical/electrical dependency on active amplifier parameters with dependency on passive element parameters, the system achieves high-order filtering without requiring power-hungry wideband amplifiers
2Reliability
If high-order filtering is implemented using traditional active filters, then filtering performance is improved, but area increases
Solution Approach 1:
The patent transforms the parameter dependency from active amplifier characteristics to passive circuit element characteristics. This allows the realization of high-order transfer functions with fewer active components, directly reducing the area required for the filter circuit while maintaining filtering performance
Solution Approach 2:
The patent creates a universal filter architecture where the transfer function coefficients are determined by passive elements that can be easily scaled and integrated. This multi-functional approach allows the same circuit topology to achieve high-order filtering with reduced component count and area
3Reliability
If wideband operational amplifiers are used, then filtering performance is improved, but power consumption increases
Solution Approach 1:
The patent extracts the dependency on wideband amplifier characteristics from the transfer function. By removing the reliance on amplifier gain and poles, the system can use low-power amplifiers while maintaining filtering performance, directly resolving the contradiction between performance and power consumption
Solution Approach 2:
The patent introduces passive circuit elements as intermediaries that mediate between the amplifier and the filtering function. These passive elements determine the transfer function coefficients, allowing the amplifier to operate at low power while the passive elements provide the filtering characteristics
Data Source
AI summary
A filter circuit includes a differential amplifier circuit to provide a number of poles including a dominant pole, and a feedback circuit to feed a portion of an output of the differential amplifier circuit to an input of the differential amplifier circuit. The feedback circuit includes a feedback resistor and a feedback capacitor to provide a controllable increase in an order of a transfer function of the filter circuit along with non-dominant poles of the differential amplifier circuit coupled in parallel with the feedback resistor. Coefficients of a transfer function of the differential amplifier circuit are forced to substantially depend solely on one or more of a plurality of passive circuit elements, the feedback resistor, and the feedback capacitor to control a dependence of the transfer function of the filter circuit on a gain of the differential amplifier circuit and poles of the differential amplifier circuit.