Practical design of variable fractional-order capacitors with a single tuning feature using field effect transistors and variable capacitance diodes
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Date
2025-07-01
Authors
Šotner, Roman
Black, Chloe
Jeřábek, Jan
Freeborn, Todd
Colburn, Simon
Svoboda, Marek
0000-0002-2430-1815Advisor
Referee
Mark
Journal Title
Journal ISSN
Volume Title
Publisher
Springer Nature
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Abstract
This paper presents two discrete circuit solutions for realizing passive, electronically adjustable constant-phase elements, specifically half-order capacitors with a -45 degrees phase shift. Fractional-order capacitors with electronically adjustable pseudocapacitance are especially useful for designing tunable filters and oscillators. The ability to adjust pseudocapacitance electronically and continuously is a major improvement over traditional passive solutions. Their pseudocapacitance can be controlled by a DC voltage, allowing key parameters like the cut-off or oscillation frequency to be tuned. Two presented design approaches differ in accuracy, tuning range, and signal-handling capability. Both solutions maintain a constant phase over one frequency decade, with a phase ripple within +/- 2 degrees. The tuning range spans from hundreds of Hz to several MHz. Presented solutions allow pseudocapacitance tuning in range of hundreds of nano F/sec0.5 (with varicaps) and tens of micro F/sec0.5 (with MOSFETs). The MOS-based circuit offers a tuning ratio of 7 but shows a 19% deviation between simulation and measurement. It also suffers from notable nonlinearity, with undistorted operation limited to signal levels up to 20 mV peak-to-peak. The varicap-based solution achieves a tuning ratio of 5, with high accuracy (up to 6% error), and handles input signals in the hundreds of mV with acceptable distortion. PSpice simulations and laboratory measurements confirm the performance of both designs.
Description
Citation
Scientific Reports. 2025, vol. 15, issue 7, p. 1-17.
https://www.nature.com/articles/s41598-025-07319-5
https://www.nature.com/articles/s41598-025-07319-5
Document type
Peer-reviewed
Document version
Published version
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Language of document
en