## 3 The State-of-the-Art of AC-DC Converters
As an example, some works introduced architectures where auxiliary delay cells are inserted with the aim to adjust the times when the MOSFET have to switch from cut-off to conduction, and vice-versa [36]. Similar effect can be obtained by acting on the effective threshold of the comparator. This can be done adding a voltage generator in series with one of the input terminals or designing the inner differential couple in an asymmetrical way. Among the topologies that exploit these design strategies, there are those presented in [37; 38; 39]. On the other hand, modulation of the channel conductivity can be an effective strategy to pursuit for those applications whose input signal amplitude is lower than a conventional threshold voltage (about 600 mV). Bootstrapping of the gate terminal and/or body biasing techniques are often exploited to control the channel conductivity of interested MOSFETs. Both strategies act similarly on the electrical features of the MOSFET, and they can be  
Figure 10: Rectifier proposed in [35]: **(a)** simplified block diagram; **(b)** control strategy; **(c)** detailed schematic of the real-time on/off calibrations for one side of rectifier. Reproduced with permission from [PERSON], IEEE Journal of Solid-State Circuits; published by IEEE, 2016.  
used to improve the conduction and/or to reduce reverse losses [40; 41; 42]. Simplified block diagrams of the architectural solutions just mentioned, are gathered in Figure 11.  
In [31], it is shown as AC-DC charge pump circuits can be optimally designed to have the minimum circuit area for small form factor vibration energy harvesting. The proposed system (Figure 12) was composed by a passive cross-coupled AC-DC converter without filter, whose rectified output signal feeds a current-controlled ring oscillator and a Dickson CP, whose diodes were implemented by composite CMOS diodes. The frequency of the clock signals was higher than that of the input signal, in such manner pumping capacitances can be reduced and output current strongly increased as respect to the CPs for AC-DC conversion present in the literature. [PERSON] proposed a technique that combines a passive rectifier and a linear regulator without using a comparator (named _prectulator_) [43]. Since there is no speed limitation due to the comparator, the proposed technique can operate with high frequency input signals. As can be seen in Figure 13, the output transistor used for regulating the output voltage in a linear regulator is also used as a passive rectifier. To achieve rectification without turning on the parasitic bipolar transistor of the output transistor, an auxiliary rectifier is used for biasing the bulk terminal of the output transistor. Figure 11: Simplified block diagrams of modified active diodes: conventional (**a**), with improved turn-on instant and conduction angle by means of delay cell (**b**) and threshold voltage shifting (**c**), bootstrapped (**d**) and body biased (**e**). Figure 12: System diagram of the rectifier proposed in [31]. An implantable system for powering and data transmission is proposed in [44] (Figure 14). This system employs two rectifier paths, namely the main and the auxiliary path. In the main path there are the main rectifier (Figure 15a), DC-DC cross coupled Dickson charge pump (Figure 15b) and a regulator (Figure 15c). The main rectifier is realized by cross coupled NMOS and active PMOS diode with a simple comparator. The auxiliary path is used to supply a ring oscillator and a bias generator circuit. The use of a ring oscillator allows working at high frequency thus reducing the size of the capacitors. The rectifier of the auxiliary path is a push-pull voltage doubler (Figure 15d). For high available input power, \(PCE\) for this architecture increases with \(P_{out,dc}\). For low available input power, \(P_{out,dc}\) is low, resulting in lower rectification efficiency because \(PCE\) is limited by the quiescent power dissipation in the LDO and other circuit blocks.