Tunable Impedance Switching With Low Distortion and Low On-Resistance
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Solution Overview
Problem
Tunable impedance circuits face distortion issues in both on and off states due to the presence of selection switches, which affect the accuracy of impedance selection, particularly in oscillator circuits where frequency control is critical.
Innovation Solution
A low-distortion tunable impedance circuit design that includes a fixed impedance and a plurality of impedance selection circuits with series-connected switch transistors, where the terminals of these transistors are driven by drive impedance networks to reduce off-state distortion and maintain low on-state resistance, effectively minimizing distortion by reducing parasitic capacitances and evenly distributing voltage across the transistors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single selection switch transistor is used to select impedance branches, then the circuit complexity is low, but the distortion performance deteriorates due to on-resistance and parasitic capacitances
Solution Approach 1:
The single selection switch transistor is segmented into multiple series-connected transistor stages (first transistor stage, second transistor stage, etc.). Each stage contributes to voltage division across parasitic capacitances, reducing the voltage across each individual transistor and thereby reducing distortion currents. This segmentation maintains manageable circuit complexity while significantly improving distortion performance.
2Manufacturing precision
If the width-to-length ratio of the switch transistor is increased to reduce on-resistance, then the on-state distortion is reduced, but the off-state parasitic capacitances increase causing increased off-state distortion
Solution Approach 1:
The selection switch is divided into multiple transistor stages connected in series. Each transistor can be optimized with appropriate width-to-length ratios to achieve low on-resistance while maintaining manageable parasitic capacitances. The series connection causes voltage division across the parasitic capacitances of each transistor, reducing the total distortion current even if individual transistors have moderate capacitance values.
Solution Approach 2:
Impedance networks are introduced as intermediary elements connected to the terminals of each transistor stage. These impedance networks control the voltages at the terminals, ensuring optimal voltage distribution across the transistors and their parasitic capacitances. This mediation allows the transistors to operate with reduced distortion while maintaining proper switching functionality.
3Manufacturing precision
If multiple series-connected transistor stages are used to reduce distortion, then the distortion performance is improved by 1/N2 factor, but the device complexity increases
Solution Approach 1:
The selection switch is systematically segmented into N transistor stages connected in series between the selected impedance branch and ground. This segmentation provides a structured approach to distortion reduction with predictable performance improvement (1/N2 factor). The modular nature of the segmentation allows for scalable design where the number of stages can be adjusted based on performance requirements.
Solution Approach 2:
Impedance networks serve as intermediary control elements that manage the increased complexity by providing systematic voltage control across each transistor stage. These networks ensure that the additional transistors contribute effectively to distortion reduction without creating unmanageable complexity in the control logic or voltage distribution.
Data Source
AI summary
A tunable impedance circuit can include a fixed impedance and one or more impedance selection circuits. Each impedance selection circuit can include a first impedance connected to a first interface terminal, a second impedance connected to a second interface terminal, and a plurality of series-connected transistors connected between the first and second impedances. Each impedance selection circuit can also include a plurality of drive impedance networks connected to gates, sources, drains, bodies, and isolation regions of the series-connected transistors, and a control circuit to provide a plurality of control signals to the drive impedance networks to turn on and turn off the series-connected transistors. For each impedance selection circuit, turning on and turning off the respective plurality of series-connected transistors can bring the series combination of the respective first and second impedances into and out of electrical communication with, e.g., into and out of parallel with, the fixed impedance.