An Inverse Class-E Power Amplifier for Ultrasound Transducer

A Dual Load-Modulated Doherty Power Amplifier Design Method for Improving Power Back-Off Efficiency

In this paper, the load modulation process of a Doherty power amplifier (DPA) is analyzed to address the issue of why designed DPAs have a very low efficiency in the back-off state in some cases. A general formula of the real load modulation process is also given for analyzing the load modulation of a peak PA matching network. This provides a new perspective for improving the back-off efficiency of a DPA. To improve the power back-off efficiency of a DPA, a dual load-modulated DPA (D-DPA) design method is proposed. The core principle of the proposed design method is to control the load modulation process from the carrier PA to the peaking PA based on the design method of the traditional two-way DPA. The efficiency of the peaking PA in the back-off region is enhanced, thereby improving the efficiency in the entire back-off region of the DPA. Based on the proposed design method, a D-DPA operating at 2 GHz is designed and fabricated. The test results show that the saturated output power and gain are 43.7 dBm and 9.7 dB, respectively, while the efficiency at 6 dB output power back-off is 59.2%. The designed D-DPA eliminates the efficiency pit of the traditional two-way DPA in the output power back-off region.

Power Amplifier Design for Ultrasound Applications

A design analysis of the power amplifiers developed for ultrasound applications was conducted because ultrasound applications require different types of power amplifiers, which are one of the most critical electronic components in ultrasound systems. To generate acoustic signals using transducers, which are among the most important mechanical devices in ultrasound systems, an appropriate output voltage, current, or power signal must be produced by a power amplifier. Therefore, an appropriate design analysis of the power amplifier must be conducted to obtain the optimal performance from a transducer. In addition, because of new ultrasound research trends, such as ultrasound systems with other imaging modalities and wireless ultrasound systems, the selection of an appropriate power amplifier could improve the performance of an ultrasound system with other imaging and therapy modalities. This paper describes the design parameters of a power amplifier, including the gain, bandwidth, harmonic distortion, and efficiency. Each power amplifier has specific applications and limitations. Therefore, this review will assist design engineers and ultrasound researchers who need to develop or use power amplifiers in ultrasound applications.

Harmonic-Reduced Bias Circuit for Ultrasound Transducers

The gain of class-C power amplifiers is generally lower than that of class-A power amplifiers. Thus, higher-amplitude input voltage signals for class-C power amplifiers are required. However, high-amplitude input signals generate unwanted harmonic signals. Therefore, a novel bias circuit was proposed to suppress the harmonic signals generated by class-C power amplifiers, which improves the output voltage amplitudes. To verify the proposed idea, the input harmonic signals when using a harmonic-reduced bias circuit (-61.31 dB, -89.092 dB, -90.53 dB, and -90.32 dB) were measured and were found to be much lower than those when using the voltage divider bias circuit (-57.19 dB, -73.49 dB, -70.97 dB, and -73.61 dB) at 25 MHz, 50 MHz, 75 MHz, and 100 MHz, respectively. To further validate the proposed idea, the pulse-echo measurements were compared using the bias circuits. The peak-to-peak echo amplitude and bandwidth of the piezoelectric transducer, measured when using a harmonic-reduced bias circuit (27.07 mV and 37.19%), were higher than those achieved with a voltage divider circuit (18.55 mV and 22.71%). Therefore, the proposed scheme may be useful for ultrasound instruments with low sensitivity.