# A Self-Powered UHF Passive Tag for Biomedical Temperature Monitoring
## 2 Architecture of Proposed Self-Powered Passive Tag  
The block diagram of the proposed self-powered passive tag prototype for biomedical temperature monitoring is presented in Figure 1. This battery-free passive temperature sensor is composed of three basic building blocks: a High Frequency (HF) section, a Power Management (PM) section, and a Low Frequency (LF) section. This prototype is included in the non-intermittent or RFID-like category because no communication protocol is implemented. The HF section collects the electromagnetic energy available in the surrounding environment using a dipole antenna, a matching network, and a differential power harvester. The HF section transmits the temperature information using an Amplitude-Shift-Keying (ASK) backscatter modulator. Finally, the PM section regulates all the energy gathered by the HF section and generates the voltage and current to supply and bias the LF section; these tasks are done with the aim of a shunt regulator, a voltage/current reference, and a Low Drop Out (LDO) voltage regulator. The LF section measures the temperature and produces a proportional deviation on the frequency of the ring oscillator. Since the proposed passive tag is battery-free and the energy collected for its operation is harvested from an RF link, the design methodology of the proposed RF passive temperature sensor must follow a low-power, low-voltage philosophy as well as a bottom-up circuit design strategy. In order to comply with the low-voltage/power design philosophy, the transistors of all the tag blocks must operate in the subthreshold, linear, or saturation region. Under sub-threshold region, the \(I_{D_{n}}\) drain current of the \(n-th\) transistor is determined by Equation (1), where \(k_{n}\) is the transistor aspect ratio \(W_{n}/L_{n}\), \(V_{GS_{n}}\) is the gate-source voltage, \(V_{TH_{n}}\) is the threshold voltage, \(V_{DS_{n}}\) is the drain-source voltage, and \(\eta\) is the subthreshold slope. Furthermore, \(V_{T}=kT/q\) is the thermal voltage and \(I_{o}=\mu C_{ox}(\eta-1)V_{T}^{2}\) is the saturation current where \(k\) is the Boltzmann coefficient, \(T\) is the absolute temperature, \(q\) is the electron charge, \(\mu\) is the electron or hole mobility, and \(C_{ox}\) is the silicon dioxide capacitance. For \(V_{DS_{n}}>4V_{T}\), the second exponential of Equation (1) can be neglected to obtain Equation (2). \[I_{D_{n}}=k_{n}\cdot I_{o}\cdot e^{\frac{V_{GS_{n}}-V_{TH_{n}}}{\eta V_{T}}} \cdot\left(1-e^{-\frac{V_{DS_{n}}}{V_{T}}}\right) \tag{1}\]  
\[I_{D_{n}}\approx k_{n}\cdot I_{o}\cdot e^{\frac{V_{CS_{n}}-V_{TH_{n}}}{\eta V_{ T}}} \tag{2}\]  
Figure 1: Block diagram of the proposed tag. If the transistor is biased to operate in the linear region, its drain current can be calculated using Equation (3), where \(\lambda\) is the transistor channel length modulation. By neglecting \(\lambda\) and assuming a very small \(V_{DS_{n}}\), Equation (3) can be simplified into Equation (4). \[I_{D_{n}}=\mu\cdot C_{ox}\cdot k_{n}\bigg{[}(V_{GS_{n}}-V_{TH_{n}})V_{DS_{n}}- \frac{V_{DS_{n}}^{2}}{2}\bigg{]}\cdot(1+\lambda V_{DS_{n}}) \tag{3}\]  
\[I_{D_{n}}\approx\mu\cdot C_{ox}\cdot k_{n}\cdot(V_{GS_{n}}-V_{TH_{n}})\cdot V_{ DS_{n}} \tag{4}\]  
Likewise, the \(I_{D_{n}}\) drain current of a transistor biased in saturation regime is given by Equation (5), which can be simplified into Equation (6) if the effect of the terms \(\lambda\) and \(V_{DS_{n}}\) are neglected. \[I_{D_{n}}=\frac{1}{2}\cdot\mu\cdot C_{ox}\cdot k_{n}\cdot(V_{GS_{n}}-V_{TH_{n}} )^{2}\cdot(1+\lambda V_{DS_{n}}) \tag{5}\]  
\[I_{D_{n}}=\frac{1}{2}\cdot\mu\cdot C_{ox}\cdot k_{n}\cdot(V_{GS_{n}}-V_{TH_{n} })^{2} \tag{6}\]  
Based on a bottom-up strategy, the building blocks of the proposed temperature sensor are described as follows. First, the LF section is designed to provide a temperature dependent oscillation, followed by the PM section where the minimum supply and all the bias voltages and currents are set. Finally, the HF section is designed to harvest all the power consumed by the PM and LF sections.