# Protection Circuit Design for Ultrasound Transducers
## 3 Design Topology and Analysis of the Developed Protection Circuits
When the positive transmitted pulses reach the bridge diode, the diode (D\({}_{5}\)) is turned on, and the positive pulses are suppressed by the diode (D\({}_{6}\)). The suppressed positive discharged pulse is directed to the ground via the diode (D\({}_{8}\)). When the negative transmitted pulses reach the bridge diode, the diode (D\({}_{3}\)) is turned on and the positive pulses are suppressed by the diode (D\({}_{4}\)). The suppressed negative discharged pulse is conducted to the ground through the diode (D\({}_{7}\)). Like the schematic diagram in Figure 13, the commercial protection circuit like MD0100 also has the same diode bridge configuration [48]. This configuration requires the control to have the fast time of the turn on and off so that we could reduce the ring down of the discharged pulses which should not be combined with the echoes. The selected resistance values of the resistors (R\({}_{1}\) and R\({}_{2}\)) are 1 k\(\Omega\) and the value of the DC supply voltage (V\({}_{\text{dd}}\)) is 3 V. For the performance of the protection circuit, the measured amplitude of the discharged pulse was approximately 8 V\({}_{\text{P-P}}\) when the voltage applied by the transmitter was 400 V\({}_{\text{P-P}}\). The measured bandwidth and pulse recovery time were approximately 27 MHz and 25 ns, respectively. Figure 14 shows the protection circuit using a bipolar transistor. In comparison to the parallel diode pair in the diode limiter, the developed protection circuit has NPN (N\({}_{1}\) and N\({}_{2}\)) and PNP (P\({}_{1}\)) bipolar transistors [49]. The positive discharged pulse is routed to the ground through N\({}_{1}\) and N\({}_{2}\) whereas the negative discharged pulse is directed to the ground through P\({}_{1}\). The developed protection circuit ensures a lower IL and THD. However, the configuration is more complex and is not a cost-effective solution for ultrasound imaging systems that require a multi-array transducer. When a negative pulse of 20 MHz and 50 V\({}_{\text{P-P}}\) was applied, the measured values for IL, THD, NF, bandwidth, and recovery time of the protection circuit were \(-\)6.3 dB, \(-\)77.3 dB, 9.6 dB, 135 MHz, and 43 ns, respectively. Figure 3:0 Schematic design of the protection circuit based on a silicon carbide diode bridge configuration. Figure 15 shows a protection circuit for very high-frequency (\(\geq\)100 MHz) ultrasound system applications such as micro-particle stimulation and cell stimulation. To efficiently suppress the noise, two series diode pairs (D\({}_{1}\), D\({}_{2}\), D\({}_{3}\), and D\({}_{4}\)) were used as an expander circuit. To reduce the high insertion loss, a limiter circuit consisting of seven gate-drain connected MOSFET with a parallel diode pair was used [50]. The limiter functions as a high-pass filter and is therefore suitable for operating transducers at very high-frequencies owing to its low IL. The measured IL and THD values at 120 MHz were \(-\)1.0 dB and \(-\)69.89 dB, respectively. Figure 16 shows the protection circuit based on n-channel MOSFET devices. The transmitted pulses were suppressed using two resistors (R\({}_{1}\) and R\({}_{2}\)), complementary MOSFET devices (N\({}_{3}\)-N\({}_{6}\)), and gate source-connected MOSFET (N\({}_{1}\) and N\({}_{2}\)) devices without external power sources [51]. Therefore, this configuration could be useful for impedance adjusting, even when the design is complex and the chip area is large. The echoes from the transducer are passed through a gate-source connected to the MOSFET (N\({}_{1}\) and N\({}_{2}\)). The IL and bandwidth of the developed protection circuit are 0.5 dB and 100 MHz, respectively. Figure 3:1 Schematic design of protection circuit using a bipolar transistor. Adapted with permission from [PERSON] et al. [49]. Copyright 2014, Elsevier. Figure 3:2 Schematic design of the protection circuit used for the very high-frequency ultrasound system. Adapted with permission from [PERSON] et al. [50]. Copyright 2014, Springer Nature.