Voltage Rectifier Circuit with Voltage Doubler Using New Active Diode

In this paper, voltage rectifier circuit with voltage doubler using new Active Diode (AD) is proposed for battery less biological signal measurement system using smartphone. Firstly, we propose the new AD which consists of AD core circuit and Bulk Regulation Transistor (BRT). The advantage of proposed AD (ADB) is insensitive to the threshold voltage of MOSFETs and very useful for low power and low voltage operation. Next, the voltage rectifier circuit with voltage doubler using the ADB is presented. Thanks to the good performance of the ADB, the designed rectifier could operate as theory by only the sin waves from the earphone terminal of smartphone. The ADB were fabricated actually using 1-P 3-M 0.6m CMOS process. The detailed simulation and measurement results are reported in this paper.


Introduction
Recently, healthcare systems using mobile devices receive attention to maintain personal health condition.In the case of realizing this kind of systems, additional sensor to measure biological signals for mobile devices are necessary.However, it is difficult to use just connecting sensor to mobile devices because the electrical characteristics of sensors are different depending on sensors.In order to overcome the problem, we have proposed the sensor conditioner IC (SCIS) for mobile devices such as smartphone.Fig. 1 shows the architecture of the proposed SCIC.In SCIC, we focus on the earphone-mic terminal, which is used for input-output interface and power supply to SCIC.SCIC consists of amplifier which is to amplify the signal from the additional sensor, filter which eliminates noises and AC/DC converter for supplying DC power to the amplifier and filter.In many cases, this kind of circuits, analog-to-digital (A/D) converter is often necessary; however, A/D converter built in mobile devices for analog input from earphone-mic terminal is used in the proposed SCIC.The battery less SCIC is very important for mobile devices because of light weight and small package.Therefore, in the proposed SCIC, we employed the AC/DC converter as mentioned previously.The AC/DC converter converts from AC signal, which is generated by application program on the mobile devices, to DC voltage.The DC voltage is used for supplying the DC sources for operating the amplifier and filter in SCIC.In the past, some AC/DC converters were proposed.In these circuits, some diodes were used for voltage rectifiers.However, a threshold voltage of the diode cannot be ignored because amplitude of the AC signal is very small.In order to overcome this problem, voltage rectifier using Active Diode (AD), which has ideal diode characteristic, were proposed [1] [2].The circuit P -536 is used a Bulk Regulation Transistor (BRT) for eliminating the dead zone.However, because BRT has a timing at which the back-gate potential becomes indefinite, there is the problem that the AD does not correctly operate at low input voltage.
In this paper, we propose a new AD.The proposed AD is very simple circuit which adds an inverter to conventional AD.Furthermore, we propose new voltage rectifier circuit with voltage doubler using the proposed AD.The proposed AD is evaluated using HSPICE and actual fabrication based on 0.6 m standard CMOS process.We report the detail simulation and experimental results in this paper.

Proposed AD with BRT
The schematic of the proposed AD with BRT is shown in Fig. 3.The difference between the proposed ADB and conventional circuit is the control signal for BRT.In the conventional circuit, Vin, which is a sine wave and gradually increase or decrease signal, is directly used as the controlling the BRT, therefore, the BRT has dead band.In the proposed ADB, the output signal of the comparator, which is digital signal such as Hi (VDD) or Low (GND), is used as the control signal for BRT.Therefore, the dead band can be omitted, so that we can overcome the problem.

Simulation and Experiment Result
The conventional and proposed ADB were fabricated using 0.6 m standard CMOS process.Fig. 4 shows both the simulation and experimental results of the conventional ADB.From Fig. 4 it can be confirmed that the back-gate voltage of M1 is an indefinite value.

Application to AC-DC Converter Using ADB
In this chapter, we explain AC-DC converter using the ADB.Fig. 6 shows the voltage doubler half-wave rectifier circuit, which is widely used.However, this circuits includes diodes, therefore, the problem described in the previous (problem of the threshold voltage and dead band of diode) is remained.Fig. 7 shows the proposed AC-DC converter, which is replaced by the proposed ADB instead of diodes and expanded to bipolar (plus and minus) supply voltage.In order to confirm the usefulness of the proposed AC-DC converter, the circuits were evaluated through HSPICE with 0.6 m CMOS device parameters.
Fig. 8 shows the output voltage at each output load.From Fig. 8, the desired voltage (2.6V) can be obtained from about 10 k .Fig. 9 shows the power efficiency at each output load.As the output resistance increase, the impedance of the comparator apparently decrease and the total amount of current consumed by the comparator becomes larger than the amount of current consumed by the load.Therefore, the power efficiency decreases from 50 k and the value is approximately 40%.

Conclusion
In this paper, we have proposed a novel active diode (ADB) firstly.As a result of pointing out the problem about the indefinite value of the back-gate potential in the conventional active diode were improved.The improvement was confirmed by HSPICE simulation and experiment using the actual chip fabricated with 0.6um CMOS process.Next, AC-DC converter using the proposed ADB for SCIC using mobile devices has been proposed and designed.The circuits were evaluated through HSPICE.From the simulation results, we could obtain approximately 70% efficiency up to the load of 40 kΩ.For future work, the actual chip fabrication of the sensor conditioner ICs and its evaluation.

Fig. 5
shows simulation and experimental results of the proposed ADB.When the change of the back-gate terminal of M1 was confirmed in the range of Vin from -2 [V] to 2 [V], a desired voltage characteristic was obtained.Therefore we could confirm that the proposed ADB operates as the AD without dead band.

Fig. 4
Fig. 4 Simulation and experiment results of the conventional ADB Fig. 6 General Voltage doubler half-wave rectifier