Active Inductor LPF Topology for High-Bandwidth Low-Power TDD
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Solution Overview
Problem
Existing low-pass filters in wireless communication systems face challenges in achieving high bandwidth while minimizing current consumption, particularly in high signal bandwidth applications, and struggle with stability issues due to the need for complex poles and feedback loops.
Innovation Solution
A filter circuitry utilizing an active inductor with a current mirror topology and switchable components, allowing for bidirectional operation and scalable bandwidth, eliminating the need for feedback loops and reducing current consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If op-amps with feedback elements are used to generate complex poles, then the required attenuation at stop band is achieved, but current consumption increases significantly
Solution Approach 1:
The patent extracts the feedback loop from the filter design by using a feedforward topology. The complex poles are generated through the interaction of the feedforward path with capacitive and resistive elements, eliminating the need for op-amps with feedback while maintaining stop band attenuation performance.
Solution Approach 2:
The patent replaces the traditional op-amp feedback mechanism with an alternative electrical topology using feedforward amplification combined with RC networks. This substitution achieves the same filtering function without requiring high-bandwidth op-amps, thereby reducing current consumption.
2Speed
If higher bandwidth is required for 5G applications, then the signal processing capability is improved, but the unity gain bandwidth of op-amps must be increased which further increases current consumption
Solution Approach 1:
The patent removes the feedback loop dependency that constrains bandwidth to be limited by op-amp unity gain bandwidth. By using feedforward topology, the filter can operate at bandwidths determined by the RC time constants rather than op-amp limitations, enabling higher signal bandwidths without proportionally increasing current consumption.
Solution Approach 2:
The patent changes the design parameters from op-amp dominated specifications (unity gain bandwidth) to passive component dominated specifications (RC time constants). This allows independent optimization of bandwidth and current consumption by selecting appropriate R and C values without being constrained by op-amp performance limits.
3Reliability
If real poles are used for filtering, then the filter order can be increased, but significant gain droop occurs in the passband
Solution Approach 1:
The patent uses asymmetric feedforward paths with different capacitive and resistive values to create complex pole locations that provide both high-order filtering and passband gain preservation. The asymmetric RC networks generate frequency-dependent phase shifts that create complex poles without the gain droop associated with real poles.
Solution Approach 2:
The patent creates dynamically interacting RC networks in the feedforward path that produce frequency-selective behavior. The interaction between multiple RC stages with different time constants creates complex pole pairs that provide steep roll-off while maintaining passband gain, unlike static real pole configurations.
Data Source
AI summary
A filter circuitry (200) using an active inductor is disclosed. The filter circuitry (200) has a first terminal (In1/Out1) and a second terminal (In2/Out2). The filter circuitry (200) comprises a first transistor (M1) and a second transistor (M2). The filter circuitry (200) further comprises a first switch (S1), a second switch (S2), a first capacitor (C1), a second capacitor (C2) and a resistor (R). The first and second transistors (M1/M2) together with the resistor (R) and the first and second switches (S1/S2) are connected in a current mirror topology. The first and second capacitors (C1/C2) are connected at the first and second terminals of the filter circuitry (200) respectively. The filter circuitry (200) is configurable to either have the first terminal (In1/Out1) as input and the second terminal (In2/Out2) as output or have the first terminal (In1/Out1) as output and the second terminal (In2/Out2) as input by changing on-off states of the first and second switches. The transistors are interconnected in a current-mirror fashion. Depending on the switch position one of the transistors also acts as part of an active inductor such that the circuit functions as a low pass filter with a complex pole pair and a real pole. Depending on the switch position the LPF allows signal flow in either direction. For use in a TDD environment in combination with a passive mixer (420).