**4. Conclusions**

In point-of-care ultrasound systems, excessive overheating critically reduces the performance of piezoelectric transducers because class-A power amplifiers in ultrasound transmitters generate unwanted heat during the entire operation. To reduce excessive overheating, non-linear power amplifiers such as class-C power amplifiers can be used given that these amplifiers have on and o ff transition periods during its entire operation. Because of pulse signals a ffecting bias circuits, class-C power amplifiers could be a ffected by bias circuits, resulting in the generation of sensitive outputs for ultrasonic transducers. Therefore, a new diode expander architecture dedicated to improving input signal conditions for class-C power amplifiers was proposed. The diode expander architecture could reduce the e ffects of unwanted input pulse signals toward bias circuits, thus reducing the attenuation of the input pulse signals for class-C power amplifiers. As a result, higher input signals could be transferred to the class-C power amplifiers. We have shown that the gain of a class-C power amplifier with a diode expander (14.96 dB) was higher than that with a resistor diode expander (12.04 dB) for a 5 Vp–p input. However, the current consumption of a class-C power amplifier with a diode expander architecture (1.02 W) was a little bit higher than that with a resistor divider architecture (0.75 W).

To confirm the proposed idea, typical one-way pulse-echo response measurements were taken. The echo signal amplitude and its −6 dB bandwidth when using a class-C power amplifier with diode expander architecture (2.98 Vp–p and 18.25%) was higher and wider than those of a class-C power amplifier with resistor divider architecture (2.51 Vp–p and 17.51%). The limitation of the developed class-C power amplifier with a resistor divider architecture made it di fficult to block the unwanted pulse input signals. The limitation of the developed class-C power amplifier with a diode expander architecture is that the maximum voltage of the LDMOSFET in the diode expander needs to be much higher than the power supply with unwanted pulse signals. However, a class-C power amplifier with diode expander architecture can be a useful way to improve the output voltage amplitude. In the future, the developed architecture combined with a multiplexer/de-multiplexer will be applied to the array transducers because the sensitivity in point-of-care ultrasound systems is one of the critical performance issues.

**Funding:** This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2017R1C1B1003606).

**Conflicts of Interest:** The author declares no conflict of interest.
