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The paper presents a novel shunt voltage regulation architecture that uses an RF power detector, based on Pulse Width Modulation (PWM) technique, aiming at magnetic coupled Radio-Frequency Identification (RFID) transponders. The proposed regulator architecture implements two feedback loops: a voltage clamping circuit that composes a fast correction loop and a slow but accurate power detector loop. The first clamping loop guarantees fast over-voltage protection, mainly in high RF power condition (short reading distance), but degrades power efficiency when in low power condition (large reading distance). The second loop corrects the imprecision of the first loop according to the detected RF input power. Therefore, it increases power efficiency and reading distance when in low RF power condition. The RF power detector architecture, based on a PWM technique, and the shunt regulator design are presented and discussed in detail. RFID power regulation issues are also discussed, followed by a brief review of RF power detector circuits. The complete regulator architecture was implemented in a 180nm CMOS low cost process and prototyped as part of a commercial low-frequency (134 kHz) RFID transponder. The complete transponder area is 870×870 μm2, being 130×230 μm2 related to the regulator circuit only. Both resonant and supply capacitors are implemented on chip. The analog and digital functional blocks of the RFID system consume 4.5 μW. Performance of the transponder is measured with the shunt regulator enabled and compared with the power limiting approach using only clamping diodes. Measured results show an improvement of 16.7% in the maximum communication distance between the transponder and the reader because of the proposed regulation strategy.

This work presents the design of a low voltage dynamic comparator for low-power ADC applications. The dynamic comparator uses a pre-amplifier powered by a floating reservoir capacitor and a positive feedback bulk structure. The output stage comprises a simple circuit to reduce the total voltage overhead necessary to define the logic levels. The powering scheme of the pre-amplifier, with a floating reservoir capacitor, contributes to reduce the impacts of global process variability. The positive feedback bulk structure lowers the threshold voltages of the pre-amplifier transistors at the sampling phase. Such structure also provides positive feedback signal during the comparison phase to provide extra transconductance. The proposed dynamic comparator is designed and simulated in a 28 nm CMOS technology and reaches an IRN below of the quantization noise of a 10 bits differential ADCs working with 600 mV power supply. The dynamic comparator achieves 237 μV input-referred noise, while consuming only 38.8 fJ per comparison and having a nominal delay of 5.77 ns.

In this paper a self-biased current reference based on Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) Zero Temperature Coefficient (ZTC) condition is proposed. It can be implemented in any Complementary Metal-Oxide-Semiconductor (CMOS) fabrication process and provides another alternative to design current references. In order to support the circuit design, ZTC condition is analyzed using a MOSFET model that is continuous from weak to strong inversion, showing that this condition always occurs from moderate to strong inversion in any CMOS process. The proposed topology was designed in a 180 nm process, operates with a supply voltage from 1.4V to 1.8 V and occupies around 0.010mm2 of silicon area. From circuit simulations our reference showed a temperature coefficient (TC) of 15 ppm/oC from -40 to +85oC, and a fabrication process sensitivity of σ/μ = 4.5% for the current reference, including average process and local mismatch variability analysis. The simulated power supply sensitivity is estimated around 1%/V.

This paper presents a power-efficient Ultra Low Voltage (ULV) Fully-Differential (FD) Digital-Based Operational Transconductance Amplifier for Biomedical signal processing (BioDIGOTA), which digitally processes biological analog signals using CMOS standard-cells. Post-layout simulations, including parasitic effects in 180nm CMOS technology, show that BioDIG- OTA consumes only 144 nW at 300 mV of supply voltage while driving a 20 pF capacitive load, with a power efficiency factor (PEF) lower than 1. The layout occupies 0.03 mm2 total silicon area, excluding I/0 pads. The proposed BioDIGOTA proves that digital-based analog design can be adopted in biomedical signal amplifiers, lowering the total silicon area by 2.3X times compared to the current state of the art landscape while keeping reasonable power and system performance.

An ultra-low voltage and ultra-low power Digital-Based Operational Transconductance Amplifier (DB-OTA) is presented and demonstrated on silicon in 180 nm CMOS. The DB-OTA is designed using digital standard cells, hence benefitting from technology scaling as much as digital circuits, while also being technology- and design-portable, and requiring minimal design and integration effort compared to conventional analog-intensive OTAs. The fabricated DB-OTA testchip occupies a compact area of 1,426 $\mu \text{m}^{2}$ , operates at supply voltages down to 300 mV, and consumes only 590 pW while driving a capacitive load of 80pF. Its measured Total Harmonic Distortion (THD) is lower than 5% at a 100-mV input signal swing. Based on these results, the proposed DB-OTA achieves 2,101 V−1 small-signal figure of merit (FOMS) and 1,070 large-signal figure of merit (FOML). To the best of the authors’ knowledge, the power is the lowest reported to date in an OTA, and the achieved figures of merit are the best in sub-500 mV OTAs reported to date. The low cost, the low design effort and the high power efficiency of DB-OTA make it well suited for purely harvested low-frequency analog interfaces in sensor nodes.

This paper presents a non-linear shunt regulator based on a PWM RF power detector for RFID applications. The shunt regulator is composed of two feedback loops. The first is a fast loop based on a simple RF voltage clamp that guarantees overvoltage protection for the internal circuits. The second is an accurate loop based on a power detector circuit that slowly corrects the first loop errors according to the input RF power. The theoretical formulation of the RF-to-DC conversion in the power detector is presented and compared to a high-level implementation. A small-signal model of the complete shunt regulator was developed to assist in the CMOS circuit design. The regulator is part of a commercial low-frequency RFID transponder. It shows an improvement of 16.7 % in the maximum communication distance of the transponder with the shunt regulator enabled when compared to the previous power limiting approach using only clamping diodes.

This work presents the analysis, design, and performance evaluation of three usual CMOS proportional to absolute temperature (PTAT) voltage generators, with emphasis on variability effects. Minimization or compensation of the main error sources, such as fabrication variability and intrinsic non-linearities, is an important design challenge required to increase the precision and robustness of a voltage reference. The CMOS PTAT topologies are analytically described and design methodologies for PTAT circuits in subthreshold are presented. The compromises between design conditions and resulting performance are evaluated through simulation for these three PTAT generators, including linearity with temperature, temperature coefficient (TC), and variability impact. Monte-Carlo simulations demonstrated the sensitivity of each topology to fabrication variability, showing that the self-cascode MOSFET structure presents the best accuracy of TC, nominal PTAT voltage, and linearity with temperature.

In this paper, a self-biased temperature-compensated CMOS voltage reference operating at picowatt-level power consumption is presented. The core of the proposed circuit is the self-cascode MOSFET (SCM) and two variants are explored: a self-biased SCM (SBSCM) and a self-biased NMOS (SBNMOS) voltage reference. Power consumption and silicon area are remarkably reduced by combining subthreshold operation with a self-biased scheme. Trimming techniques for both circuits are discussed aiming at the reduction of the process variations impact. The proposed circuits were fabricated in a standard 0.18- $\mu \text{m}$ CMOS process. Measurement results from 24 samples of the same batch show that both circuits herein proposed can operate at 0.45/0.6 V minimum supply voltage, consuming merely 55/184 pW at room temperature. Temperature coefficient (TC) around 104/495 ppm/°C across a temperature range from 0 to 120 °C was measured. Employment of a trimming scheme allows a reduction of the average TC to 72.4/11.6 ppm/°C for the same temperature range. Both variants of the proposed circuit achieve a line sensitivity of 0.15/0.11 %/V and a power supply rejection better than −44/−45 dB from 10 to 10 kHz. In addition, SBSCM and SBNMOS prototypes occupy a silicon area of 0.002 and 0.0017 mm2, respectively.

Electromagnetic interference (EMI) disturbances coupled in the power supply of voltage and current references can severely degrade their performance, due to its finite power supply rejection (PSR). The design of a 90-dB PSR 4-dBm EMI-resistant NMOS-only voltage reference is herein presented. The voltage reference is designed based on the zero-temperature-coefficient transistor operating point. The high PSR is obtained using zero- $V_{T}$ transistors as active loads in the open and feedback loop of the circuit. Two versions, using standard $V_{T}$ and low-power $V_{T}$ transistors, were designed in a 130-nm CMOS process. Both are designed using the same thermal compensation principle. The circuits occupy 0.014 and 0.006 mm $^{2}$ of silicon area while consuming around 1.15 and 0.156 $\mu$ W at 27 $^\circ$ C, respectively. Postlayout simulations present a reference voltage of 206 and 450 mV with an average temperature coefficient of 321 and 86 ppm/ $^\circ$ C (1000 samples), under a temperature range from $-$ 55 to 125 $^\circ$ C. An EMI source of 4 dBm (1 $\text{V}_{\text{pp}}$ ) injected in the power supply, according to the direct power injection standard, yields $-$ 0.17% and $-$ 0.1 ${\%}$ of the maximum dc shift and 822 and 950 $\mu \text{V}_{\text{pp}}$ of the maximum peak-to-peak ripple for the standard $V_{T}$ and low-power $V_{T}$ implementations, respectively.

This paper presents a fully-integrated 32.7 kHz relaxation oscillator designed in TSMC 40 nm CMOS for IoT applications. Simulation results show that the proposed relaxation oscillator consumes 40 nW at room temperature from 0.6 V power supply. The architecture achieves a temperature coefficient (TC) as low as 12.5 ppm/°C (μ = 35.5 ppm/°C for 400 samples) from −40°C to 125°C. Also, more than 50% of samples have a TC less than 30 ppm/°C. The oscillator also has a fast start-up time, achieving a steady-state of the operating frequency within 3 cycles. The energy efficiency is 1.22 nW/kHz, and the entire circuit occupies an active area of 0.127 mm2.

The Relaxation oscillators (ROSCs) offer a better trade-off between power consumption, physical area and accuracy. This paper presents a study of recently published ultra-low-power relaxation oscillators with low-temperature coefficient, identifying its characteristics, performance parameters, and main challenges. A brief review show as state-of-the-art circuits handle with PVT (Process, voltage, and temperature) variations. By the end of this analysis, some trends in relaxation oscillators design are discussed.

The calibration of the effects of process variations and device mismatch in Ultra Low Voltage (ULV) Digital-Based Operational Transconductance Amplifiers (DB-OTAs) is addressed in this paper. For this purpose, two dynamic calibration techniques, intended to dynamically vary the effective strength of critical gates by different modulation strategies, i.e., Digital Pulse Width Modulation (DPWM) and Dyadic Digital Pulse Modulation (DDPM), are explored and compared to classic static calibration. The effectiveness of the calibration approaches as a mean to recover acceptable performance in non-functional samples is verified by Monte-Carlo (MC) post-layout simulations performed on a 300 mV power supply, nW-power DB-OTA in 180 nm CMOS. Based on the same MC post-layout simulations, the impact of each calibration strategy on silicon area, power consumption, and OTA performance is discussed.

This work presents a resistorless self-biased and small area sub-bandgap voltage reference that works in the nano-ampere consumption range with 0.75 V of power supply. The circuit applies a curvature compensation technique that allows an extended temperature range without compromising the temperature stability. The behavior of the circuit is analytically described, and a design methodology is proposed which allows the separate adjustment of the bipolar junction transistor bias current and its curvature compensation. Simulation results are presented for a 180 nm CMOS process, where a reference voltage of 469 mV is designed, with a temperature coefficient of 5 ppm/°C for the ź40 to 125 °C extended temperature range. The power consumption of the whole circuit is 16.3 nW under a 0.75 V power supply at 27 °C. The estimated silicon area is 0.0053 mm2.

This paper presents a power-efficient Ultra Low Voltage (ULV) Digital-Based Operational Transconductance Amplifier (DB-OTA), which uses static logic gates and processes digitally the analog input signal. Post-layout simulations in 180nm CMOS technology show that at 300mV supply voltage the circuit consumes just 2nW while driving a capacitive load of 80pF with Total Harmonic Distortion lower than 5% at 100mV input signal swing. The total silicon area is $1,426\ \mu \mathrm{m}^{2}$ . The maximum energy efficiency supply for the DB-OTA and its scalability to 40nm CMOS technology node are also demonstrated.

A voltage reference based on MOSFETs operated under Zero Temperature Coefficient (ZTC) bias is proposed. The circuit operates in a power supply voltage range from 0.3 V up to 1.2 V and outputs three different reference voltages using Standard- $V_{T}$ (SVT), Low- $V_{T}$ (LVT), and Zero- $V_{T}$ (ZVT) MOS transistors biased near their ZTC point by a single PTAT current reference. Measurements on 15 circuit samples fabricated in a standard 0.13- $\mu \text{m}$ CMOS process show a worst-case normalized standard deviation $(\sigma /\mu)$ of 3% (SVT), 5.1% (LVT) and 10.8% (ZVT) respectively with a 75% of confidence level. At the nominal supply voltage of 0.45 V, the measured effective temperature coefficients (TCeff) range from 140 to 200 ppm/°C over the full commercial temperature range. At room temperature (25 °C), line sensitivity in the ZVT VR is just 1.3%/100 mV, over the whole supply range. The proposed reference draws around 5 $\mu \text{W}$ and occupies 0.014 mm2 of silicon area.

In this work a resistorless sub-bandgap voltage reference topology is presented. It is a self-biased and small area circuit that works in the nano-ampere consumption range, and under 1 V of power supply. The behavior of the circuit is analytically described, a design methodology is proposed and simulation results are presented for two CMOS processes, XFAB 0.18 μm and IBM 0.13 μm. Experimental results from one fabrication run demonstrate a reference voltage of 570 mV, with a temperature coefficient as low as 11 ppm/°C for the 0---125 °C range, while the power consumption of the whole circuit is 5 nW under a 0.9 V supply at 27 °C. The occupied silicon area is 0.0022 mm$$^2$$2.

A 50 MHz---1 GHz low-noise amplifier circuit with high linearity for IEEE 802.22 wireless regional area network is presented. It was implemented without any inductor and offers a differential output for balun use. Noise canceling and linearity boosting techniques were combined to improve the amplifier performance in such a way that they can be separately optimized. Linearity was improved using diode-connected transistors. The amplifier was implemented in a 130 nm CMOS process in a compact 136 μm × 71 μm area. Simulations are presented for post-layout schematics for two classes of design: one for best linearity, another for best noise figure (NF). When optimized for best linearity, simulation results achieve a voltage gain >23.7 dB (power gain >19.1 dB), a NF 3.3 dBm (7.6 dBm max.) and an input power reflection coefficient S11 24.7 dB (power gain >19.8 dB), a NF ?0.3 dBm and an S11 11 dB. Monte Carlo simulation results confirm low sensitivity to process variations. Also a low sensitivity to temperature within the range ?55 to 125 °C was observed for Gain, NF and S11. Power consumption is 18 mW under a 1.2 V supply.

This work presents an ultra-low power low-voltage high-order temperature-compensated voltage reference. The proposed circuit is based on the self-cascode MOSFET (SCM) and explores the dependence of the threshold voltage (V T ) with the transistor dimensions. The SCM is biased by the leakage current of a zero-V T transistor for PSRR improvement. The proposed circuit is composed only of 3 transistors. The high-order temperature compensation is achieved through a bulk-driven scheme. Additionally, the proposed high-order compensation also attenuates the mismatch variability of the voltage reference. Post-layout simulation results for a standard 130 nm CMOS process are presented. A voltage reference of 90.5 mV with a 1 ppm/°C temperature coefficient (TC) is achieved at typical corner. The circuit can operate at a minimum supply voltage as low as 0.3 V while consuming 43.7 pW at room temperature.

This work presents a fully integrated single-stage 2.4 GHz power amplifier (PA) that was designed in a 180nm CMOS process using a power combining technique to achieve higher output power. A series combining transformer (SCT) combines two cascode differential amplifiers and makes extensive use of virtual AC ground nodes to reduce the impact of bond-wire parasitics. The center-taps of the output transformers are used for 2nd harmonic impedance tuning inside the chip. In post-layout simulation the PA uses a 3.3 V power supply and achieves 24dBm maximum output power at a peak drain efficiency of 23.4%. The test chip returned recently from fabrication and it will undergo electrical tests soon.