Publication detail

Practical design of variable fractional-order capacitors with a single tuning feature using field effect transistors and variable capacitance diodes

ŠOTNER, R. BLACK, C. JEŘÁBEK, J. FREEBORN, T. COLBURN, S. SVOBODA, M.

Original Title

Practical design of variable fractional-order capacitors with a single tuning feature using field effect transistors and variable capacitance diodes

Type

journal article in Web of Science

Language

English

Original 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.

Keywords

Adjustability; Constant phase element; Fractional-order; MOSFET; Pseudocapacitance; Tunability; Varicap

Authors

ŠOTNER, R.; BLACK, C.; JEŘÁBEK, J.; FREEBORN, T.; COLBURN, S.; SVOBODA, M.

Released

1. 7. 2025

Publisher

Springer Nature

ISBN

2045-2322

Periodical

Scientific Reports

Year of study

15

Number

7

State

United Kingdom of Great Britain and Northern Ireland

Pages from

1

Pages to

17

Pages count

17

URL

https://www.nature.com/articles/s41598-025-07319-5

Full text in the Digital Library

http://hdl.handle.net/11012/255157

BibTex

@article{BUT198331,
  author="Roman {Šotner} and Chloe {Black} and Jan {Jeřábek} and Todd {Freeborn} and Simon {Colburn} and Marek {Svoboda}",
  title="Practical design of variable fractional-order capacitors with a single tuning feature using field effect transistors and variable capacitance diodes",
  journal="Scientific Reports",
  year="2025",
  volume="15",
  number="7",
  pages="17",
  doi="10.1038/s41598-025-07319-5",
  issn="2045-2322",
  url="https://www.nature.com/articles/s41598-025-07319-5"
}