# Protection Circuit Design for Ultrasound Transducers
## 3 Design Topology and Analysis of the Developed Protection Circuits
This transformer can extend the pulse duration of the echo generated by a high-frequency ultrasound transducer owing to its nonlinear characteristics [38]. The resistor (R\({}_{1}\)) can be helpful to be matched to the receiver with a 50 \(\Omega\) termination and reduced high-voltage pulses. The ILs of the developed protection circuit were \(-4\) to \(-8\) dB at 20 MHz and \(-55\) to \(-66\) dB at 45 MHz. When using a 20 MHz transducer, a center frequency of 20.5 MHz and a bandwidth of 21.6 MHz were achieved. When using a 45 MHz transducer, a center frequency of 46.6 MHz and a bandwidth of 34.7 MHz were also achieved [38]. As shown in Figure 7, [PERSON] et al. have developed a protection circuit that is an expander and limiter consisting of a bridge diode connection with a transmission line to improve image resolution for high-frequency ultrasound skin-imaging applications. The impedances of the bridge diodes (D\({}_{3}\), D\({}_{4}\), D\({}_{5}\), and D\({}_{6}\)) are controlled by the resistance values (R\({}_{1}\) and R\({}_{2}\)), which must be selected accordingly. In addition, a suitable length of the transmission line could improve the impedance-matching condition. Therefore, the configuration can influence the IL and bandwidth of the protection circuit. When high-voltage pulses are transmitted to the transducer, the expander operates at a low impedance, but the limiter operates at a high impedance to protect the receiver. When low-voltage echoes are received from the transducer, the supply voltage (\(\pm\)V\({}_{\text{dd}}\)) can provide a DC  
Figure 5: Schematic design of the protection circuit based on a bridge diode and a transformer [35]. Figure 6: Schematic representation of the expander and transformer-based limiter. Adapted with permission from [PERSON] [38]. Copyright 1999, IEEE. voltage through the resistors (R\({}_{1}\) and R\({}_{2}\)) to conduct the diodes (D\({}_{3}\), D\({}_{4}\), D\({}_{5}\), and D\({}_{6}\)), which should have a low impedance. When positive transmitted pulses go to the bridge diode, diode (D\({}_{5}\)) is conducted and diode (D\({}_{6}\)) suppresses the positive pulses. When negatively transmitted pulses reach the bridge diode, the diode (D\({}_{3}\)) is conducted, and the diode (D\({}_{4}\)) suppresses the negative pulses. The measured ILs of the expander and limiter at 150 MHz were approximately \(-10\) dB and \(-15\) dB, respectively. The signal distortion improved by 19 dB. Figure 8 shows a protection circuit based on a double-bridge diode configuration for ultrasound catheter applications. An ultrasound catheter is a type of rotational ultrasound transducer used in intravascular ultrasound (IVUS) systems [40; 41]. A dual-diode bridge was used to protect the input and output of the preamplifier used in the receiver. The DC bias voltages from the power supply (+V\({}_{\rm dd}\)) turned on the bridge diodes. A long coaxial cable must be used for the IVUS system; impedance matching is important. The values of capacitance (C\({}_{1}\) and C\({}_{2}\)) and resistance (R\({}_{1}\) and R\({}_{2}\)) can influence the impedance, bandwidth, and IL of the bridge diodes. Therefore, the proper selection of these values in the bridge diode configuration is essential for the transducer performance. Figure 8: Schematic representation of the protection circuit based on double bridge diode configuration. Adapted with permission from [PERSON] et al. [42]. Copyright 2003, IEEE. Figure 7: Schematic representation of the protection circuit used for the high-frequency ultrasound skin imaging system. Adapted with permission from [PERSON] et al. [39]. Copyright 2003, IEEE. As shown in Figure 9, [PERSON] et al. proposed a 64-channel on-chip Metal-oxide-semiconductor field-effect transistor (MOSFET) shunt device for protection circuits. Instead of high-voltage switches, the authors used a standard low-voltage 0.35-\(\upmu\)m complementary metal-oxide-semiconductor (CMOS) process to reduce the cost of portable high-channel ultrasound imaging system applications.