# Protection Circuit Design for Ultrasound Transducers
## 5 Conclusions  
High- and very high-frequency ultrasound transducers have a low sensitivity. Therefore, transmit pulses with high amplitude voltage or power signals must be used to trigger the ultrasound transducers. Therefore, a protection circuit may affect the performance of the ultrasound transducer. However, there are no review articles describing the proper selection of protection circuits for ultrasound transducers and system applications. Therefore, \begin{table}
\begin{tabular}{c c c c c} \hline
**Paper** & **Type** & **Performance Parameter** & **Characteristics** & **Application** \\ \hline \multirow{3}{*}{[45]} & \multirow{3}{*}{Dual gate-source connected MOSFET device} & Bandwidth, & & \\  & & input-referred noise, and & & \\  & & recovering time are & & \\  & & recovering time are & & \\  & & 41 MHz, 2 nV/VHz, and & & \\  & & \textless{1}\textbackslash{S}, respectively. & & \\ \hline \multirow{3}{*}{[46]} & \multirow{3}{*}{N-channel MOSFET switch} & & Useful for unipolar pulse & Capacitive \\  & & transmission and chip area & & micromachined \\  & & reduction & & ultrasonic transducer \\ \hline \multirow{3}{*}{[47]} & \multirow{3}{*}{Bridge diode based on SiC diode} & Bandwidth and & & \\  & & recovering time are & & \\  & & 27 MHz and 25 ns. & & \\ \hline \multirow{3}{*}{[49]} & \multirow{3}{*}{Bipolar-transistor} & IL, THD, bandwidth, & & \\  & & input-referred noise, and & & \\  & & recovering time are & & \\  & & 6.3 dB, \(-\)77.3 dB, & & \\  & & 135 MHz, 96 dB, and & & \\  & & 43 ns, respectively. & & \\ \hline \multirow{3}{*}{[50]} & Gate-drain connected MOSFET device & IL and THD are \(-\)1.0 dB and \(-\)69.89 dB at & Useful for improving IL for very high-frequency & High-frequency \\  & & 120 MHz, respectively. & & transducer & system \\ \hline \multirow{3}{*}{[51]} & Complementary and & IL and bandwidth are & Useful for impedance & \\  & gate-source connected MOSFET device & \(-\)0.5 dB and 100 MHz, & & \\  & & respectively. & & \\ \hline \multirow{3}{*}{[52]} & Gate-source connected MOSFET switch & Static power & Useful for lowering static & Ultrasound imaging \\  & & consumption is 12.1 μW. & power consumption & system \\ \hline \multirow{3}{*}{[54]} & \multirow{3}{*}{Diode limiter} & IL, THD, and recovering time are \(-\)3.87 dB, 0.29\%, & Improved sensitivity with conventional diode limiter & Multi-channel \\  & & and 6.1 μS. & design & \\ \hline \multirow{3}{*}{[55]} & \multirow{3}{*}{RTZ circuit and switch} & The recovery time is & Chip area reduction using a & Ultrasound imaging \\  & & 77.23 ns. & low-voltage switch circuit & system \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._ \begin{table}
\begin{tabular}{l l l l} \hline
**Type** & **Contribution** & **Limitation** \\ \hline Protection circuits based on a limiter diode & Useful for array imaging systems due to & High IL at high frequency \\ Protection circuits based on a bridge diode & Low IL at high frequency & Required to control fast turn-on and \\ Protection circuits based on a MOSFET & Low IL and low noise at high frequency & To reduce space, the chip process needs to be used \\ \hline \end{tabular}
\end{table} 
Table 2: Contribution and limitation of the currently developed protection circuits. the design topology and specifications of protection circuits for ultrasound transducers used in ultrasound transducer, imaging, and stimulation applications are reviewed. The protection circuit consists of expander and limiter circuits. The expander usually consists of a single series-crossed coupled pair and is used to suppress the ring-down of the transmitted high-voltage pulses, whereas the limiter is used to minimize IL while suppressing the high-voltage pulses. In particular, a protection circuit must be designed considering a low IL and wide bandwidth to avoid the suppression of the low-amplitude echo signal generated by high-frequency or very high-frequency ultrasound transducers. In this review, a specific design topology and suitable applications have been proposed.