# A Wideband High-Voltage Power Amplifier Post-Linearizer for Medical Ultrasound Transducers
## 1 Introduction  
Ultrasound systems have been used in various applications, such as nondestructive testing, medical imaging, acoustic tweezers, cellular stimulation and surface acoustic wave filters [1; 2; 3; 4; 5]. In ultrasound systems, the transmitters are one of the key components affecting the ultrasound transducer performance [6; 7; 8]. The high-voltage power amplifier (HVPA) is a critical ultrasound transmitter component that influences the generation of ultrasound waves by the transducer [9; 10]. More specifically, adjacent channels in the miniaturized array transducer result in cross-intermodulation and signal harmonic distortions [11; 12]. Therefore, the HVPA in medical ultrasound transmitters should yield low-harmonic distortions [13; 14]. Harmonic imaging technique (HIT) in ultrasound systems is commonly used to enhance image contrast by using a relatively higher bandwidth of the received echo signals [15; 16; 17]. When applying the HIT, the transmitting signal produced from the HVPA should have low-harmonic components because the image is formed solely from the several harmonic componentsgenerated by the target and these harmonic components should not interfere with the transmitting harmonic components of the HVPA [7]. The coded excitation method is generally utilized to increase the signal-to-noise ratio of echo signals. However, this method also requires low-harmonic signal distortion to prevent spectral overlap [18]. In wireless communication systems, power amplifier linearizing techniques have been widely used to eliminate the narrow-band channel selection of the operating data signal carrier frequency, and to reduce leakage signal from the transmitter [19; 20]. A linear HVPA is also desirable in ultrasound systems because the multiple-order (second, third, and fourth) harmonic components of the echo waveform generated by the medical ultrasound transducer deteriorate the echo signal quality of ultrasound systems [7]. The harmonic distortion components in ultrasound applications are critical issues to address, because higher harmonic distortion components of the echo signals lower image resolution [21]. The non-linear ultrasound wave (ultrasound wave with high harmonic distortions) causes ultrasound wave distortion. As a result of the distorted ultrasound wave, image resolution of ultrasound instrumentation is degraded, and even the penetration depth into the medium is reduced [10]. In order to reduce the high-order harmonic distortion components, several methods using contrast agents or signal-processing techniques have been reported. The first method involves using a contrast agent in which the pulse-echo response capability is improved using insonification, thus reducing unwanted backscattered echo signals [22]. However, dynamic quantification of contrast agents is not possible with this method. The second method is to suppress harmonic signals by performing a series of signal-processing operations without using contrast agents [17], described as follows: (1) pulse-inversion is first applied to the ultrasound system followed by harmonic post-processing [23]. However, this method critically reduces the frame rate of the ultrasound system. (2) Source pre-biasing can be used to shift the phase between the fundamental pulse and leakage pulse of the transmission signal [24]. However, this method is not efficient because it is difficult to generate the appropriate leakage signal amplitude. (3) Nonlinear schemes have been reported to filter out the second harmonic signal before feeding to the transducer [25]. The disadvantage of this scheme is that pre-definition of the inverted and mixed signals before obtaining the echo signal in the system level is necessary. There is very little research that aims to improve performance of the high-voltage power amplifiers at the hardware level. A pre-linearizer scheme using look-up tables was developed to reduce harmonic distortion of the ultrasound echo signal [26].