Single-Op-Amp Differential Elliptic Filter for Steep Roll-Off
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Solution Overview
Problem
Existing analog filters, such as Bessel and Butterworth, have broad transition regions between pass and stop bands, which limits their ability to achieve steep roll-off rates without increasing complexity and component count, whereas Elliptic filters provide the steepest roll-off but are typically more complex and require multiple components.
Innovation Solution
A differential elliptic filter circuit is implemented using a single op-amp with inverting and non-inverting feedback paths and feedforward paths to provide complex conjugate poles and zeros, allowing for a steeper roll-off without the need for additional components, effectively converting prior art filters into Elliptic filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the filter order is increased to achieve steeper roll-off, then the transition region becomes narrower, but the number of components and circuit complexity increases
Solution Approach 1:
The patent combines multiple filter functions (low-pass and high-pass) into a single elliptic filter circuit using one operational amplifier. The feedback paths and feedforward paths are merged to simultaneously provide the necessary poles and zeros, achieving steep roll-off without requiring multiple separate filter stages or additional components.
Solution Approach 2:
The single operational amplifier in the circuit performs multiple functions: it provides amplification, establishes virtual grounds at its inputs, and enables the configuration of both feedback and feedforward paths. This multi-functionality allows the circuit to achieve elliptic filter characteristics with minimal components, resolving the contradiction between performance and complexity.
2Speed
If Elliptic filter configuration is used to achieve narrow transition region, then the roll-off rate increases, but the number of components increases
Solution Approach 1:
The patent merges the functions of providing poles and zeros into a single operational amplifier circuit. The feedback paths provide the necessary poles while the feedforward paths provide the zeros, all within one op-amp configuration. This eliminates the need for multiple separate components or stages that would traditionally be required to achieve elliptic filter characteristics.
Solution Approach 2:
The patent uses both inverting and non-inverting feedforward paths in addition to the feedback paths, adding a dimensional aspect to the circuit configuration. This allows the simultaneous establishment of virtual grounds and provision of zeros without requiring additional operational amplifiers or increasing the overall component count.
3Device complexity
If Bessel or Butterworth filter types are used, then the component count is reduced, but the transition region becomes broader
Solution Approach 1:
The patent employs feedback paths with specific impedance configurations (combining resistive and capacitive elements) to establish complex conjugate poles. The feedback mechanism, combined with the feedforward paths, enables the circuit to achieve elliptic filter characteristics with the steep roll-off rate, overcoming the limitation of simpler filter types while maintaining low component count.
Data Source
AI summary
A differential elliptic filter circuit includes: a differential amplifier, feedback and feedforward paths. An upper pair and a lower pair of inverting feedback paths couple a corresponding one the differential signal outputs of the amplifier to an inverting one of a pair of inputs of the amplifier, to provide two complex conjugate poles, and establish upper and lower virtual grounds at the amplifier inputs. Upper and lower inverting feedforward paths couple corresponding passive nodes of the upper and lower pairs of inverting feedback paths to respectively the lower and upper virtual grounds to provide two zeros of the circuit. The upper and lower non-inverting feedforward paths couple an upper and lower one of a pair of differential signal inputs of the circuit to respectively the upper and lower virtual grounds to enable positioning of the two zeros of the circuit on an imaginary axis of a pole-zero plot.