# Protection Circuit Design for Ultrasound Transducers
## 4 Discussion
Therefore, high-voltage chip processes have been used to implement protection circuits using MOSFET or high-voltage discrete MOSFET devices. Although high-voltage MOSFET devices offer high-voltage tolerance, they have very large parasitic gate-source, gate-drain, and drain-source capacitances [63; 64; 65]. To reduce such unwanted parasitic capacitances, gate source-connected or gate drain-connected MOSFET devices have been used. Compared to the low-voltage chip process, high-voltage MOSFET devices require a very large size [66]. To solve this problem, a protection circuit that uses both low-voltage and high-voltage MOSFETs was used to save space and thus reduce the size of the multi-array ultrasound transducers. The measured IL and equivalent input noises were \(-5\) dB and \(2.5\) nV/\(\sqrt{\text{Hz}}\), respectively [44]. The measured bandwidth and input-referred noise were 41 MHz and \(2\) nV/\(\sqrt{\text{Hz}}\), respectively [45]. The measured IL and bandwidth were 0.5 dB and 100 MHz, respectively [51]. Therefore, the protection circuit based on a MOSFET would be beneficial for high-frequency operation due to low IL and noise performances. The main parameters of the protection circuit are IL, harmonic distortion, input-referred noise, bandwidth, and recovery time. The IL of the protection circuit was reduced when the operating frequency of the protection circuit was increased. Thus, the high-frequency operation of the ultrasound transducer may be affected by the reduced IL of the protection circuit. A lower IL of the protection circuit may degrade the overall noise performance of the receiver used in the ultrasound system, as the noise figure of the first-stage electronics, such as the protection circuit, mainly determines the overall noise figure of the receiver electronics. This can affect the resolution of the ultrasound systems. The high harmonic distortion of the protection circuit can burden the preamplifier and variable-gain amplifier; therefore, it is desirable for the protection circuit to have a low harmonic distortion. A lower bandwidth of the protection circuit can reduce the echo signal amplitude of the ultrasound transducer. This can be critical for high-frequency ultrasound transducer operations. Therefore, the protection bandwidth must be as wide as possible. Table 1 gives an overview of the protection circuits developed for ultrasound transducers and system applications. Table 1 lists the types of protection circuits and describes the performance parameters and desired applications. \begin{table}
\begin{tabular}{c c c c c} \hline
**Paper** & **Type** & **Performance Parameter** & **Characteristics** & **Application** \\ \hline \multirow{3}{*}{[30]} & \multirow{3}{*}{Transmission line with diode} & \multicolumn{2}{c}{Advantageous to be matched} \\  & & with the transmitter and & & PVDF ultrasound \\  & & & receiver when using a certain & & transducer \\  & & & length of the transmission line. & & system \\ \hline \multirow{3}{*}{[34]} & \multirow{3}{*}{Duplexer design with variable capacitive diode} & \multicolumn{2}{c}{Reduction in the IL and noise} & \multicolumn{2}{c}{Ultrasound imaging} \\  & & & system \\ \hline \multirow{3}{*}{[35]} & \multirow{3}{*}{Transformer with diode} & \multicolumn{2}{c}{Unwanted DC source blocking.} & \multicolumn{2}{c}{Ultrasound imaging} \\  & & & system \\ \hline \multirow{3}{*}{[38]} & \multirow{3}{*}{Wideband transformer-based limiter} & IL is \(-4\) to \(-8\) dB at 20 & Wideband transformers can & \\  & & MHz and \(-55\) to \(-66\) dB & improve IL at high-frequency & & Ultrasound imaging \\  & & at \(45\) MHz. & operation. & & system \\ \hline \multirow{3}{*}{[39]} & \multirow{3}{*}{Bridge diode} & \multicolumn{2}{c}{Using a transmission line} & High-frequency \\  & & & improves impedance & ultrasound skin \\  & & & matching. & imaging \\ \hline \multirow{3}{*}{[42]} & \multirow{3}{*}{Dual bridge diode} & \multicolumn{2}{c}{Advantageous for impedance} \\  & & matching and the input and & & ultrasound catheter \\  & & & output receiver.