# Protection Circuit Design for Ultrasound Transducers
## 4 Discussion  
High-frequency operation of ultrasound transducers is preferable for IVUS [56; 57]. During cell stimulation, a smaller-sized cell-trapping performance can be achieved by increasing the operating frequency of the ultrasound transducer [58]. In IVUS applications, a higher-frequency operation in the ultrasound transducer provides a wider bandwidth and higher lateral resolution [59]. The higher the operating frequency of the piezoelectric materials in an ultrasound transducer, the smaller the piezoelectric materials become [60]. High-amplitude pulsed signals with smaller materials must be used to obtain suitable echo signals [61]. Therefore, protection circuits can be vulnerable in applications operating at higher frequencies. Therefore, proper design of protection circuits is highly desirable. Figure 17: Schematic of the protection circuit with a dual diode limiter. Adapted from [PERSON] [54] with permission under the terms of the CCBY 4.0 License, Copyright 2022, IOS Press. Figure 18: Schematic design of the protection circuit with an RTZ circuit and switch. Adapted with permission from [PERSON] et al. [55]. Copyright 2023, Wiley. The protection circuits are typically categorized into three major types: the protection circuits based on a limiter diode, a bridge diode, and MOSFET. Some protection circuits combine these different types of circuits. A protection circuit based on a diode limiter is easy to implement, but increases the IL at a high frequency operation and thus reduces the bandwidth. The measured IL was 3.87 dB at 1 MHz [54]. Therefore, a protection circuit based on a limiter diode may not be suitable for high-frequency ultrasound transducer applications. However, protection circuits based on a limiter diode are still the most commonly used for ultrasound array imaging systems because of their simple architecture. Therefore, academic researchers still prefer to use protection circuits based on a limiter diode for lab equipment and low-intensity stimulation. However, the high-voltage suppression capability of such a protection circuit based on a limiter diode was no better than that of a protection circuit based on a bridge diode or a MOSFET. A protection circuit based on a transformer can reduce the IL during high-frequency operation but causes excessive ring-down of the echoes, which may not be suitable for Doppler ultrasound imaging applications that require a long pulse duration. However, it is useful for blocking unwanted DC voltages and adjusting the impedance. Therefore, this configuration is useful for some ultrasound transducers which need to have matching with small impedances. The measured IL value (\(-4\) to \(-8\) dB) at 20 MHz was relatively low [38]. Therefore, the protection circuit based on a transformer may be suitable for high-frequency operation. The protection circuit based on a bridge diode can provide a low IL at high-frequency operation, but without proper selection of the terminating resistor, it generates a long ring-down and oscillation. In addition, the resistance used to control the bridge diode can significantly affect the impedance, IL, and bandwidth. The measured bandwidth was around 27 MHz [47]. Therefore, a protection circuit based on a bridge diode may not be suitable for high-frequency transducer applications. The bridge diodes must be turned on and off quickly to reduce the ring-down of the discharged pulses. Therefore, the duration of the discharge pulses should not be mixed with the duration of the echoes. The commercial protection circuits also have the same bridge diode configuration to use a control circuit which requires fast turn on and off times. For wearable ultrasound electronics, triboelectric nanogenerators, and energy harvesting circuits--protection circuits based on a bridge diode like commercial protection circuits--have been widely used due to low IL at high-frequency operation [62]. A protection circuit using a MOSFET can provide a low IL and low noise in high-frequency operation.