# Protection Circuit Design for Ultrasound Transducers
## 3 Design Topology and Analysis of the Developed Protection Circuits  
This section describes the topology and analysis of previously developed protection circuits for ultrasound transducers or system applications. In addition, the specific parameters and the achieved performance of the protection circuits are described. In the analysis, the blocks of the transmitter, receiver, and protection circuit with the transducer were shown to emphasize the design topology of the protection circuit. Different labels and designs in each paper are shown in the schematic design. Identical designations and symbols were used for the resistor, capacitor, diode, transistor, and supply voltage so that the schematic designs of the protective circuits can be easily compared. The resistor, capacitor, and diode are selected as R, C, and D, the transformer is selected as T, and the N-channel and P-channel transistors are selected as \"N\" and \"P\", respectively. In addition, all protection circuit designs have been redrawn to facilitate comparison. Therefore, academic researchers can help select the types of protection circuits that are beneficial for ultrasound transducer research. Figure 3 shows the protection circuit developed by [PERSON] et al. [30]. The expander typically consists of a single-series cross-coupled diode pair (D1 and D2), and the limiter consists of a transmission line with a parallel cross-coupled diode pair (D3 and D4). The transmission line is used to match the impedance of the receiver for certain frequency  
ranges such as 30-35 or 40-45 MHz. The impedance of the transmission line depends on its length [31]. Thus, a suitable length for specific operating frequencies must be selected for impedance matching [32,33]. [PERSON] et al. have implemented the transmission line with a 20 cm step from 20 cm to 0.2 m for a 45 MHz polyvinylidene difluoride (PVDF) ultrasound transducer [30]. When high-voltage bipolar pulses pass through a transmission line, the diodes (D\({}_{3}\) and D\({}_{4}\)) should have low impedance so such discharged pulses can reach the ground. However, when low-voltage echoes are passed through a transmission line, the p-n diodes (D\({}_{3}\) and D\({}_{4}\)) should have a high impedance so that the echoes reach the receiver with a small IL. Figure 4 shows a duplexer design with a variable capacitive diode for an ultrasound imaging system [34]. Variable capacitive diodes (D\({}_{3}\) and D\({}_{4}\)) with opposite directions were used to separate the pulses and echoes while minimizing the IL and noise contributions. Variable capacitive diodes operate in the reverse direction for positive and negative pulses. The pin diode (D\({}_{5}\)) has a conducting voltage of 0.7 V. The resistor (R\({}_{1}\)) was attached to a bipolar transistor in the receiver [34]. Figure 5 shows the protection circuit with diodes and a transformer (T\({}_{1}\)) [35]. The bridge diodes (D\({}_{3}\), D\({}_{4}\), D\({}_{5}\), and D\({}_{6}\)) operate on the basis of the direct current (DC) bias voltages (S\({}_{1}\) and S\({}_{2}\)). The transmitted bipolar pulses could be coupled to the ground via the parallel diode pair (D\({}_{7}\) and D\({}_{8}\)). A transformer is typically used to block a DC signal and pass an AC signal [36,37]. The primary and secondary coils are connected to the receiver and transducer, respectively; therefore, an unwanted DC source to the transducer is blocked. Figure 4: The protection circuit is based on variable capacitive diodes for ultrasound imaging systems [34]. Figure 3: Protection circuit for 45 MHz PVDF ultrasound transducer. Adapted with permission from [PERSON] et al. [30]. Copyright 1991, IEEE. Figure 6 shows an expander- and a transformer-based limiter for a high-frequency ultrasound imaging system developed by [PERSON] [38]. A transformer with a wide bandwidth was used to reduce the IL for high-frequency operations. In addition, a transformer with a 1-N ratio (T\({}_{1}\)) could be useful to reduce the large amplitude current to reduce the stress on the receiver.