# A Self-Powered UHF Passive Tag for Biomedical Temperature Monitoring
## 3 Design Methodology, Layout and Circuit Simulation
### Layout and Circuit Simulation
#### 3.2.5 Power Harvester, Matching Network, and ASK Backscatter Modulator.  
Figure 20 shows the location of the implemented power harvester, matching network, and ASK backscatter modulator inside the microphotography of the whole self-powered tag. Moreover, a zoomed-in view of its layout is included. The required silicon area is 775 um \(\times\) 550 um.  
The transistors dimensions and elements values that compose the power harvester and the matching network are listed in Table 7. These sizes were obtained considering some design constraints. First, the power harvester is designed to provide a DC output voltage \(V_{REC}\) and a \(I_{L}\) current of 1.1 V and 5.5 uA. Second, the bonding wire shows an average parasitic inductance and resistance of 1.5 nH and 1 \(\Omega\); the parasitic capacitance of the bonding pad is 0.5 pF. Both elements cannot be modified as they are dependent on the used technology. Third, the values of the \(L\) on-chip inductors for RF purposes are set by the fabrication process, and the \(C_{L}\) is fixed to 150 pF because it serves as a reservoir capacitor.  
Figure 21a,b report the harmonic balance simulations used to tune the power harvester while Figure 21c shows its transient post-layout simulation. All simulations were carried using a 910 MHz frequency. Figure 21a shows the swept of \(W_{1}\) and \(W_{2}\) transistor widths to achieve an output voltage \(V_{REC}\) = 1.2 V by means of a 250 mV, 275 mV and 300 mV \(v_{ask}\) RF input voltage. The number of fingers of the NMOS and PMOS were fixed to four and eight, respectively. This simulation shows that a minimum \(v_{ask}=300\) mV is required to reach \(V_{REC}\) = 1.2 V as long as \(W_{1}\) and \(W_{2}\) are set to 1.5 um. Figure 21b depicts a sweep  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Element** & **Value** & **Element** & **Value** & **Element** & **Value** & **Element** & **Value** \\ \hline \(m_{1}\) & \((1.5\,\mathrm{\SIUnitSymbolMicro m}/0.18\,\mathrm{\SIUnitSymbolMicro m})\times 8\) & \(L_{bond}\) & 1.5 nH@1 \(\Omega\) & \(L\) & 4.18 nH & \(C_{L}\) & 150 pF \\ \(m_{2}\) & \((1.5\,\mathrm{\SIUnitSymbolMicro m}/0.18\,\mathrm{\SIUnitSymbolMicro m})\times 4\) & \(C_{pad}\) & 0.5 pF & \(C_{C}\) & 2.1 pF & \\ \hline \hline \end{tabular}
\end{table}
Table 7: Elements and transistor sizes of the power harvester circuit.  
Figure 20: Location of the HF section inside the microphotography of the whole self-powered tag and a zoomed-in view of its layout.  
Figure 19: Shunt regulator post-layout simulations. (**a**) Output voltage. (**b**) Delivered current.  
in the impedance port that excites the harvester. This simulation was used to find out the value of the real \(R_{i}\) and imaginary \(X_{i}\) part that ensures the maximum power transfer. From Figure 21b, it can be observed that the input impedance of the differential rectifier at \(-8.96\) dBm of input power is \(Z_{REC}=175.8\)\(\Omega+j236\)\(\Omega\). Figure 21c presents the post-layout transient response of the differential rectifier exhibiting a start-up time of \(30\)\(\upmu\)s. Figure 21d reports the Power Conversion Efficiency (PCE) of the RF power harvester, where three rectifier stages are used (N = 3) and the sizes of \(m_{1}\) and \(m_{2}\) are (1.5 \(\upmu\)m/0.18 \(\upmu\)m) \(\times\) 8 and (1.5 \(\upmu\)m/0.18 \(\upmu\)m) \(\times\) 4, respectively.  
The sizes of the transistors that compose the ASK backscatter modulator are \((W/L)_{m3}=4\,\upmu\)m/0.5 \(\upmu\)m with a number of fingers equal to \(nf=4\). Figure 22 shows how the backscatter modulator reflects \(-1.5\) dB of input power when a digital \"1\" is applied to its input. This unmatched power is reflected back into the antenna and it is further radiated back to the RF source.