On-Chip Tunable Active Inductor Circuit for Radio Frequency ICs

RAJNI PRASHAR, GARIMA KAPUR

Department of Electronics and Communication,

Jaypee Institute of Information and technology,

A-10 Sector 62, Noida, Uttar Pradesh,

INDIA

Abstract: - The work presents a design of a gyrator-based on-chip tunable active inductor (AI) which can

moderately high-quality factor of about 120. The layout of the proposed circuit has been designed on Cadence

Virtuoso and fabricated at ON-Semiconductor, C5 CMOS process foundry of MOSIS fabrication service, USA.

The complete design has a chip area of 18x30µm2. The fabricated results illustrate the on-chip tuning ability of

the proposed AI from 3nH to 7nH with the help of externally applied Tunnelling and Injection voltages,

respectively. The proposed design also simulates a tunable bandpass filter and a tunable Low Noise Amplifier.

Key-Words: - Tunable Active Inductor, Floating Gate, Monolithic Microwave ICs, Radio-Frequency ICs

Received: April 29, 2022. Revised: August 25, 2023. Accepted: November 21, 2023. Published: December 31, 2023.

expand its wings into the near future. The current

trend in technology has moved towards employing

advanced semiconductor techniques, including

silicon germanium (SiGe) and silicon-on-insulator

(SOI) technologies, for the integration of on-chip

adjustable active inductors. These processes offer

improved performance, higher integration levels,

and lower power consumption. Modern tunable

active inductors aim to provide a wide range of

tunability to cover a broad spectrum of frequencies.

products, ensuring high signal quality. State-of-the-

art designs often feature reconfigurable active

inductors, allowing for dynamic adaptation to

changing operational requirements. This

reconfigurability can be achieved through digitally

controlled tuning circuits. The integration of digital

control interfaces (e.g., SPI or I2C) allows for

precise and flexible control of tunable active

inductors. This digital control facilitates system-

level optimization and calibration. Power efficiency

inductance value. Noise performance is essential in

RF and analog applications. Advanced designs

incorporate techniques to minimize noise figures

and achieve excellent noise performance in addition

to high linearity. Tunable active inductors are

designed to be more stable over a wide temperature

range. The trend is towards monolithic integration

of tunable active inductors with other RF and analog

circuitry, enabling more compact and highly

integrated RF front-end modules. With the

designs have been developed so far like in 1994

presenting a monolithic Narrow-band filter that uses

the concept of TAI(Tunable Active Inductor) but

with a moderate Q value and higher losses, [1].

Subsequently, a fully integrated GaAs MESFET-AI

the field of RFIC domain. So, to overcome the

drawback AI which works at a 50 GHz mm-wave

circuit provides a good Q-factor of 400 at the same

frequency, [3]. The current controlled technique to

tune the value of inductance is proposed by, [4]. A

design of AI and negative capacitance that gives the

linear impedance properties, which is useful in

broad frequency bands is given by, [5]. The major

design issues like the design process and inherent

noise sources were reported are elaborated in, [6].

medical science are proposed in, [7].

most integral part of RF/microwave circuit design,

this is because quality factor (Q) can be increased.

The wide tunability of an active inductor can also be

maintained with the advantage of a large inductance

value and small chip area, [8]. To achieve tunability

various techniques can be used like a current mirror,

digital resistive ladder switching, or floating gate

approach. One such application of active inductor

increased output voltage swing explained by, [10].

The tuning of the frequency is done with the help of

a varactor capacitor. The design uses the current

reuse technique to reduce the power consumption of

the design and reported the high value of noise

figure. Hence in the paper, a CMOS-based AI is

designed whose inductance and operating frequency

can be tuned using FGMOS technology. The

proposed inductor produces inductance ranging

from 3nH to 7nH and operates at 800MHz to 2GHz

and precise tuning ability. The first section of the

paper proposes AI design, its simulation results,

fabricated IC, and fabricated results. The second

section of the paper illustrates the vital role of AI

RFIC/MMIC systems necessitate components like

oscillators, filters, phase shifters, low noise

amplifiers, impedance matching circuits, biasing

circuits, etc., designed to function within the radio

frequency range. An enhanced version of a

traditional Gyrator-C active inductor, which

incorporates both common-emitter and common-

collector configurations with resistance feedback.

The static passive resistor is substituted with a

tunable active resistor (TAR) to augment the

operating frequency can be tuned from 800MHz to

2GHz. On-chip active inductors are typically

smaller in size compared to their passive

counterparts, which can be crucial for miniaturizing

amplifiers, oscillators, or filters, enabling the

creation of compact and highly functional RF front-

end circuits. In this work, we showcase a small,

adjustable active inductor that we simulated using

an FGMOS simulation model and BSIM level MOS

models, within a 0.35μm CMOS process

environment in Virtuoso (also compatible with T-

Spice). This design successfully incorporates nearly

all the specifications mentioned earlier while

employing an efficient and compact circuit

Furthermore, the adjustable FGPMOS resistor

within the feedback path allows for the tuning of the

inductance, as illustrated in Figure 1(a). The current

sources have also been substituted with MOSFETs

operating in saturation mode to maintain a steady

current under specific sizing (W/L) and biasing

(gate voltage) conditions. This facilitates the

derivation of the impedance/admittance and the

creation of an equivalent passive model for this

circuit, as depicted in Figure 1(b).

efficiency in battery-powered devices and reducing

heat dissipation. Tunable active inductors can be

employed in filter and amplifier circuits to achieve

precise frequency selectivity and gain control. This

ICs. Tunable active inductors can be customized to

meet specific circuit requirements, making them

adaptable to various applications and allowing

circuit designers to fine-tune their designs for

optimal performance. Integration of active

components reduces the need for external discrete

components, leading to potential cost savings in

terms of component procurement, assembly, and

testing. Based on the Gyrator-based single-ended

grounded Active Inductor and on-chip programming

been substituted with MOSFETs operating in

saturation mode. These MOSFETs ensure a

consistent current under specific sizing (W/L) and

biasing (gate voltage) conditions.

On-chip tuning Ability: With the help of three

voltages at the tunneling junction, drain, and source

for the programmer FGPMOS can be used to change

the value of inductance extracted from the single

end. The resistance at the feedback path value

changes with the help of the changing charge at the

operating frequency can also be programmed with

on-chip programming of the feedback resistor value.

The programming is also very precise, thus fine-

tuning of inductance value can be obtained. The

model is designed using BSIM 3 level 49 MOS

models, using 0.35um CMOS process, as shown in

Figure 3.

reveal the inductive behavior of this single-port

device, as depicted in Figure 4. It illustrates the

(Vtun=7.2 to 8.2V), resulting in a self-oscillation

frequency range from 700MHz to 1.5GHz.

Similarly, with injection (Vinj=6V to -6.5V), the

self-oscillation frequencies vary from 600MHz to

1.5GHz, respectively.

Fig. 4: Frequency response (GHz) of voltage at I/O

port with the input signal of 1V ac signal at

800MHz

Fig. 5(b): transient response of floating-gate voltage,

I/O port

elements within its corresponding passive circuit

have been theoretically computed. Consequently,

variations in the floating gate voltage (Vfg) result in

alterations to the inductor, leading to changes in the

feedback resistor (Rf), inductance value,

transconductance, and its associated quality factor.

These calculations and results have been compiled

and presented in Table 1.

3nH to 7nH with specific biasing and sizing

conditions of the active inductor design.

Consequently, impedance at the port has been

observed and is shown in Figure 6(a). Subsequently,

by applying equations 1 and 2, the theoretical

component value is computed. This enabled us to

determine how changes in the floating gate voltage

impacting the feedback resistor (Rf), and

consequently, influencing the inductance,

impedance at the port, and the results are depicted in

Figure 6(b)

Fig. 6: (a) Impedance Zin (real)

Fig. 6(b): Impedance Zin (imaginary)

Fig. 7: Noise performance at 0.25v 1Hz noise

signals at input port

However, the analysis of circuit sensitivity has

performance is 10.97sqV/Hz. The supply consumes

an average power of approximately 11.88mW.

Additionally, the circuit exhibits a temperature

sensitivity of approximately 2.5mV/°C, as

illustrated in the Figure 8.

generates inductive impedance at one port through

the use of the gyrator concept. This concept

produces inductance (measured in nH) by utilizing

MOS transconductance and junction capacitances.

feedback path allows for the adjustment of this

inductance value within a specified range under

specific design conditions (ranging from 3nH to

9nH). Furthermore, the circuit exhibits excellent

2.2 Fabricated Active Inductor and its Results

Inspired by the simulated results of the proposed

active inductor design, we have created the circuit

layout, which is depicted in Figure 9. This tunable

AI CMOS circuit has been designed using Cadence

Virtuoso. The layout displayed in Figure 9 includes

connection points for input/output, and notably, for

the floating gate node. However, as illustrated,

instead of individual I/O connection points, the

necessary pins have been linked to the outermost

Fig. 9: Layout of the proposed circuit (The occupied

chip area is 235×114 μm2)

Design software was utilized in conjunction with an

automated cutting tool. The PCB was meticulously

crafted and polished using these tools. Images of the

PCB are presented in Figure 10(a) and Figure 10(b)

11 is employed for the measurement of various

components, devices, circuits, and sub-assemblies.

These analyzers include both a signal source and

multiple receivers, typically displaying amplitude

and phase information in terms of ratios while

performing frequency or power sweeps. It's

important to note that a network analyzer

continuously examines a known signal in terms of

Fig. 12: PCB design final mask provided to

automatic PCB designing machine (a) back side

where IC pins and wires will be soldered (b) back

side where red surface (Cu)

progress, the printed circuit board (PCB) for AI

devices was meticulously crafted using the Cadsoft

Eagle PCB Design software and precision-cut using

Fig. 18(a) Tuning of the Q factor of bandpass filter

An active inductor has various applications in

RFICs (Radio-Frequency Integrated Circuits) and

MMICs (Monolithic Microwave Integrated Circuits)

due to its ability to provide controllable inductance

and improve the performance of RF and microwave

circuits. Active inductors can be used in the input-

communication systems. Active inductors help

optimize loop bandwidth and phase noise in PLLs,

contributing to improved frequency synthesis

performance. Active inductors are employed in

VGAs to adjust the gain levels in RF systems.

Tunable active inductors enable dynamic gain

control. In RF and microwave circuits, impedance

matching is critical for efficient power transfer.

Active inductors can be used in matching networks

to achieve impedance transformation and adapt to

circuits, where precise control of harmonics and

subharmonics is required. In phased-array antenna

systems, active inductors help control the phase and

impedance of individual antenna elements, enabling

beamforming and steering. Active inductors are

integral to the transceiver architecture, where they

contribute to the performance of both the receiver

Active inductors are crucial in the development of

5G and beyond-5G communication systems, which

require agile frequency tuning and advanced RF

front-end designs. Active inductors are used in

RFICs and MMICs for space and satellite

communication systems, where reliability,

efficiency, and frequency flexibility are essential.

The areas of interest where AI plays a vital role

are listed below:

less circuits, wake-up circuits, RFIDs,

wireless circuits for low-power operation,

and near-field communication systems.

circuits are used for RF coupling.

In this day and age, the low-power RF transceiver

has become a desirable demand in medical,

scientific, and industrial operating bands. Because

of the development in wireless technology, the

performance of RF receiver is very important and

low noise amplifier is the first stage in an RF

receiver circuit. Several system requirements such

major advantage of using this technique is that it has

a low power consumption of LNA. The topology

was implemented at 180nm technology and has a

noise figure of 2.88dB, a gain of 10.1dB, and a

power consumption of 0.84 mW.