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

Area of Science:

• Electrical Engineering
• Analog Circuit Design
• Fractional-Order Circuits

Background:

• Traditional passive elements lack tunability, limiting filter and oscillator design.
• Fractional-order capacitors offer unique phase shift characteristics (-45°).
• Electronically adjustable pseudocapacitance is key for dynamic circuit parameter control.

Purpose of the Study:

• To present two discrete circuit solutions for passive, electronically adjustable constant-phase elements.
• To enable continuous tuning of pseudocapacitance via DC voltage for filters and oscillators.
• To compare the performance of MOSFET-based and varicap-based designs.

Main Methods:

• Development of two distinct circuit architectures for fractional-order capacitors.
• Utilizing DC voltage to control pseudocapacitance.
• PSpice simulations and laboratory measurements for validation.

Main Results:

• Both circuits maintain a constant phase over a decade frequency range with minimal ripple (±2°).
• Tuning ranges extend from hundreds of Hz to several MHz.
• Varicap-based circuit: 5:1 tuning ratio, 6% error, handles hundreds of mV signals.
• MOSFET-based circuit: 7:1 tuning ratio, 19% deviation, limited to 20 mV peak-to-peak signals due to nonlinearity.

Conclusions:

• Two viable circuit solutions for tunable fractional-order capacitors are demonstrated.
• The varicap-based design offers superior accuracy and signal handling for tunable circuits.
• These circuits advance the design of adaptive electronic systems like filters and oscillators.