Active Inductor Design for RF Circuit Applications

Active inductors employ transistor-based networks to emulate the behaviour of passive inductors while offering tunability, reduced silicon area and integration into standard CMOS processes. By replacing large spiral coils with transistor circuits, designers can achieve high inductance values over a wide frequency range, adjustable via bias currents or feedback elements. Such structures enable compact voltage-controlled oscillators (VCOs), tunable bandpass filters and low-noise amplifiers, critical for wireless transceivers from sub-GHz to millimetre-wave bands. Key design challenges include maximising quality factor, ensuring stability across process-voltage-temperature corners and minimising noise and power consumption. Recent innovations have addressed these via multi-loop feedback, cascode configurations and multi-finger transistor topologies, delivering self-resonant frequencies beyond 4 GHz and quality factors exceeding 1 000 at midband frequencies. Integration with varactors and feedback resistors further extends tuning ranges above 100 %, enabling agile frequency hopping and multi-standard operation in 5G and Internet-of-Things applications.

Research from Nature Portfolio

No recent Nature Portfolio content available.

Research from all publishers

In 2024, a wideband CMOS VCO design achieved a 0.73–3.1 GHz tuning range in a 0.18 µm process by combining an active-inductor core with an active-resistor feedback element. This topology delivered a compact 0.046 mm² footprint, 6.7–10.1 mW power consumption and phase noise of –80.7 to –84.5 dBc/Hz at 1 MHz offset, demonstrating how resistor-based fine-tuning enhances inductance agility and noise performance. Another recent study (2025) presented a high-Q grounded tunable active inductor fabricated in 130 nm CMOS for 5G NR (FR1) front ends. Utilising a cascode stage, feedback resistor and multi-gate fingers, the design achieved an inductance range from 6.7 nH to 84.4 nH, a self-resonant frequency near 4 GHz and a peak quality factor above 1 586 at 2.38 GHz, all under a 1 V supply with only 2 mW power. The chip occupied 345 × 400 µm² and demonstrated excellent agreement between simulation and measurement, underlining the practicality of active inductors in high-performance transceiver modules.

Active Inductor Design for RF Circuit Applications publication trend

The graph below shows the total number of articles in active inductor design for rf circuit applications across all publications each year (not limited to Nature Index journals).

Technical terms

Active inductor: A circuit that uses transistors and feedback networks to emulate the impedance of a passive inductor, offering tunability and reduced area in integrated circuits.

Quality factor (Q): Ratio of inductive reactance to equivalent series resistance at resonance; a higher Q indicates lower energy loss and sharper frequency selectivity.

Tuning range: Span over which the effective inductance (or oscillation frequency) can be varied, expressed as a percentage of the centre value.

Self-resonant frequency (SRF): Frequency at which the active inductor’s parasitic capacitance resonates with its inductance, marking the upper usable limit.

Cascode configuration: A two-stage transistor arrangement that enhances output impedance and isolation, improving stability and Q in active-inductor circuits.

References

1. A Compact 0.73~3.1 GHz CMOS VCO Based on Active-Inductor and Active-Resistor Topology. Journal of Low Power Electronics and Applications (2024).
2. Design and Realization of a High-Q Grounded Tunable Active Inductor for 5G NR (FR1) Transceiver Front-End Applications. Sensors (2025).