# Protection Circuit Design for Ultrasound Transducers
## 1 Introduction  
In medical ultrasound imaging systems, the transmitter, receiver, and transducer are the three main electronic and mechanical components [1, 2]. In ultrasound systems, the transducer should be located between the transmitter and receiver [3]. High-voltage pulse signals generated by the transmitter trigger the transducer to generate acoustic signals from the desired sample target [4, 5]. The received acoustic signals are converted into electrical echo signals using a transducer [6]. Before the echo signals are received by the transducer, high-voltage pulse signals reach the receiver electronics because of the shared paths between the transmitter and receiver [7, 8].  
Figure 1 shows the signal path between the transmitter/receiver and the transducer to describe the fundamental mechanism of the transmitted pulse and the received echo. First, the transmitted pulse signals are passed through the expander to the transducer. Typically, an expander circuit is used to remove the ring-down of the transmitted high-voltage pulse signals while minimizing the reduction in the transmitted pulse amplitude [9]. An acoustic wave is transmitted from the transducer to the desired sample. The received acoustic waves are converted using a transducer. Secondly, the discharged pulse signal travels to the limiter before the electrically reflected echo reaches the limiter. Third, a limiter circuit was used tosuppress the discharged pulse and pass the reflected echo before it reaches the receiver [10]. Therefore, a limiter circuit is used to suppress the discharged pulse while minimizing the reduction in the reflected echo. In diagnostic ultrasound imaging applications, one or two pulses are usually used to obtain a wide bandwidth [11]. For Doppler imaging and cell or particle stimulation applications, multicycle pulses are generally used, which provide more stress to the receiver electronics to obtain a narrow bandwidth [12].  
Pulse signals must be used to trigger the ultrasound transducers. However, as the operating frequency of the ultrasound transducers increases, their size decreases, which reduces their sensitivity [13; 14]. In addition, the transmitted voltage applied to the ultrasound transducer must be elevated [15]. Therefore, proper selection of the protection circuit for operating high-frequency or very high-frequency transducers is important for optimal ultrasound system performance.  
First, several database search engines, such as Google Scholar, MedLine, PubMed, Embase, ProQuest, and EBSCOhost, were used to find suitable research articles on protection circuits for medical ultrasound systems. Second, duplicate articles were excluded from the analysis. The titles, introductions, methods, abstracts, and conclusions of each research article were reviewed to identify the specific design topologies, parameter specifications, and performances. Articles in which the specific design topology of the protection circuits was not presented were removed. The selected articles were examined in this review paper.  
The rest of this paper is organized as follows: Section 2 describes the general design topology and the specific design parameters, Section 3 presents detailed design concepts for different transducer applications and describes how to select such protection circuits for specific ultrasound applications, Section 4 discusses currently developed protection circuits used for ultrasound transducer applications; this allows academic researchers to determine the selection of protection circuits for ultrasound research, and Section 5 provides the conclusions and a summary of this review.  
Figure 1: Description of the signal path when the protection circuit is located with the transmitter, receiver, and transducer.