# Protection Circuit Design for Ultrasound Transducers
## 3 Design Topology and Analysis of the Developed Protection Circuits
The transducer is connected to the positive side of the receiver, and a common-mode DC voltage (V\({}_{\text{CM}}\)) of 1.5 V is connected to the negative side of the receiver. During the transmission period, a high-voltage switch (S\({}_{1}\)) is switched on. During the receiving period, the MOSFET shunt device (S\({}_{2}\)) is turned on. The measured discharged pulse voltage was approximately 1.1 V\({}_{\text{P-P}}\) when a transmit pulse of 10 V\({}_{\text{P-P}}\) with a pulse period of 250 ns was applied. Figure 10 shows the protection circuit based on N-channel MOSFET devices (N\({}_{1}\) and N\({}_{2}\)) with a parallel cross-coupled diode pair (D\({}_{3}\) and D\({}_{4}\)) for the ultrasound pulse-echo system. Owing to the gate-source-connected configuration, MOSFET devices have no external bias voltages, eliminating a large gate-source parasitic capacitance and leaving a small parasitic drain-source capacitance [44]. The measured bandwidth, the DC power consumption, the IL, equivalent input noise, recovery time, and total harmonic distortion (THD) were 60 MHz, 26 mW, \(-\)5 dB, 2.5 nV\(/\)Hz, 0.2 \(\upmu\)S, \(-\)93 dB, respectively. This protection circuit can provide a lower IL value, lower equivalent input noise, and lower DC power consumption than conventional protection circuits based on a diode limiter configuration. Figure 10: Schematic design of the protection circuit based on gate-source connected MOSFET devices. Adapted with permission from [PERSON] [44]. Copyright 2008, IEEE. Figure 9: Schematic representation of the protection circuit using a common-mode DC voltage (only one connection is shown to simplify the analysis). Adapted with permission from [PERSON] et al. [43]. Copyright 2007, IEEE. Figure 11 shows a protection circuit based on a dual-MOSFET device for ultrasound imaging applications. The MOSFET devices (N\({}_{1}\) and N\({}_{2}\)) have a high-breakdown voltage tolerance of up to 250 V [45]. In the limiter circuit, there is a gate-source connected to MOSFET with a parallel cross-coupled pair at the outputs of the transmitter and receiver electronics. With this configuration, the signal-to-noise (SNR) was improved from 10 dB to 20 dB. The measured bandwidth, referred-to-input noise, and recovery time with the protection circuit and receiver were 41 MHz, 2 nV/Hz, and less than 1 uS, respectively. Figure 12 shows the protection circuit based on a high-voltage NMOS switch for 50 MHz capacitive micromachined ultrasonic transducer (CMUT) applications [46]. The protection circuit was fabricated using a 0.18 um complementary MOS (CMOS)/double-diffusion MOS (DMOS) process. During the transmission period, the expanders (D\({}_{1}\) and D\({}_{2}\)) are conducted and the switch (N\({}_{1}\)) made by the NMOS transistor is turned off because the S\({}_{1}\) clock signal may provide a zero or negative voltage. During the receiving period, the switch is turned on because the S\({}_{1}\) clock signal may provide a positive voltage. This configuration is useful for unipolar pulse transmissions so the chip area can be reduced for multichannel CMUT applications. Figure 11: Schematic design of the protection circuit using dual gate-source connected MOSFET device and a parallel cross-coupled pair. Adapted with permission from [PERSON] et al. [45]. Copyright 2009, IEEE. Figure 12: Schematic design of a protection circuit based on a high-voltage switch. Adapted with permission from [PERSON] et al. [46]. Copyright 2011, IEEE. Figure 13 shows the protection circuit based on the silicon carbide diode bridge for ultrasound system applications requiring very high voltages above 400 V [47]. Even if the diode bridges (D\({}_{3}\), D\({}_{4}\), D\({}_{5}\), and D\({}_{6}\)) do not completely suppress the discharged pulse, the parallel diode pair (D\({}_{7}\) and D\({}_{8}\)) can also suppress the pulse. Compared to pin diodes, silicon carbide (SiC) diodes have a faster reverse recovery time, which is usually less than 15 ns [47].