# Protection Circuit Design for Ultrasound Transducers
## 2 Fundamental Concepts and Parameters of the Protection Circuits  
The protection circuit consists of expander and limiter circuits. Figure 2 shows the operating mechanisms of the expander and limiter. In the ultrasound system, the transmitter is usually a pulser or a power amplifier used to generate the transmitted pulse, and the receiver is a preamplifier with a variable-gain amplifier or a time-gain compensation amplifier [16]. As shown in Figure 2a, the expander circuit is used to suppress the ring-down of the transmitted signals generated while minimizing the signal loss from the transmitter [17]. The reason for this is that the ring-down signal can aggravate the signal distortion caused by the voltage applied to the transducer. Therefore, unwanted harmonic distortion may be amplified in the received echo. As shown in Figure 2b, a limiter circuit, which is the main component of the protection circuit, has been used to suppress the discharged pulses and protect the receiver. Meanwhile, the received echo must be passed through with good signal integrity, with minimum loss or distortion. The core parameters of the protection circuit are the insertion loss (IL), harmonic distortion or total harmonic distortion (THD), noise figure (NF), input-referred noise, bandwidth, and recovery time. The IL is obtained by dividing the original signal from the protection circuit by the passed signal without the protection circuit, as shown in Equation (1) [18; 19; 20]. \[\mathrm{IL(dB)}=20\cdot\mathrm{Log}_{10}(\frac{output\;with\;protection\; circuit}{output\;without\;protection\;circuit}) \tag{1}\]  
The IL parameter is an important advantage because the protection circuit is located in the first-stage receiver electronics. Therefore, a low IL value in the circuit can affect the noise performance of the receiver. Equation (2) describes the general NF of an analog receiver circuit [21; 22]. If a passive device has only one loss, the gain is inversely proportional to the loss [23]. Therefore, Equation (2) changes to Equation (3). \[\mathrm{NF}_{\mathrm{total}}=\mathrm{NF}_{1}+\frac{\mathrm{NF}_{2}-1}{\mathrm{ G}_{1}}, \tag{2}\]  
Figure 2: Operating mechanism of the (**a**) expander and (**b**) limiter in the protection circuits. where NFtotal, NF1, and NF2 are the total NF of the protection circuit and receiver, NF of the protection circuit, and NF of the receiver, respectively, and G denotes the gain of the protection circuit. \[\text{NF}_{\text{total}}=\text{NF}_{1}+\text{L}_{1}(\text{NF}_{2}-1), \tag{3}\]  
where L1 is the loss of the protection circuit. Equation (3) shows that the NFtotal can be dependent on the NF1 and L1:  
The high signal distortion of the protection circuit can increase the ring-down of the discharged pulses and the harmonic distortion of the obtained echo signals. The THD for the protection circuit can be calculated using Equation (4) [24; 25; 26]. \[ \begin{array}{c}\text{THD}(\text{dB})=20\text{-}\text{Log}_{10}\big{(}\frac{ \text{V}_{2}{}^{2}+\text{V}_{2}{}^{2}+\text{V}_{2}{}^{2}+\dots+\text{V}_{n}{} ^{2}}{\text{V}_{1}{}^{2}}\big{)}\\ \text{THD}(\%)=\Big{(}\frac{\text{V}_{2}{}^{2}+\text{V}_{2}{}^{2}+\dots+\text{V }_{n}{}^{2}}{\text{V}_{1}{}^{2}}\dots+\text{V}_{n}{}^{2}}{\text{V}_{1}{}^{2}} \Big{)}\text{\cdot}100\end{array}  \tag{4}\]  
where V1, V2, V3, V4, and Vn are the amplitudes of the fundamental, the 2 nd harmonic, the 3 rd harmonic, the 4 th harmonic, and the nth harmonic signals of the protection circuit, respectively. In the limiter circuit, the discharged pulse signal must be suppressed as much as possible to prevent the echo signal from affecting the performance of the receiver. A relatively short recovery time for the discharged pulse is desirable when an echo signal with a long duration is obtained. The recovery time of the protection circuit is defined by Equation (5) [27; 28; 29].