# A Wideband High-Voltage Power Amplifier Post-Linearizer for Medical Ultrasound Transducers
## 3 Results and Discussion
### Performance of the Pulse-Echo Response of the Ultrasound Bio-Microscopy (UBM) System  
Linearizing the HVPA could reduce harmonic distortion of the HVPA itself [19]. The HVPA stimulates the generation of the ultrasound waves by the transducer. Thus, linearizing the HVPA can suppress the harmonic distortion components of the ultrasound transducer, thereby improving the echo signal quality of the ultrasound system.  
Figure 8: (a) Gain vs. frequency and (b) gain deviation vs. frequency of the HVPA without the post-linearizer (black-dashed line) and HVPA with post-linearizer when 17 \(\mathrm{dB_{m}}\) input power was applied.  
Figure 7: Output power vs. input power of the HVPA without the post-linearizer (value provided in each figure) and HVPA with post-linearizer (solid black lines) with 10 MHz, (a) 10-, (b) 15-, (c) 20-, and (d) 25-cycle pulse signals sent to the HVPA with and without the post-linearizer, respectively.  
Figure 9a presents the design of a proposed ultrasound biomicroscopy (UBM) system using an HVPA with and without the post-linearizer. The UBM system is useful for estimating ultrasound transducer and ultrasound system performance because newly-developed transducers or electronic components need to be tested before integration in ultrasound systems [33]. In this study, the input power signal was sent to the HVPA with and without the post-linearizer in order to activate the 10 MHz immersion ultrasound transducer (V31-SU, Olympus NDT. Inc., Waltham, MA, USA). The detected echo signal produced by the transducer passed through the protection circuit composed of a 50 \(\Omega\) power resistor (Caddock Electronics Inc.) shunt with a single crossed-diode pair (NXP Semiconductors) and was subsequently amplified by the 40 dB voltage gain preamplifier (AU-1114, MITEQ Inc., Hauppauge, NY, USA). The echo signal was displayed on the oscilloscope (MSOX4154A) in order to process the spectrum data on a personal computer.  
Figure 9b,c illustrates the measured normalized spectrum as a function of the echo signal frequency generated by a 10 MHz ultrasound transducer for the HVPA with and without the post-linearizer circuit, respectively. As shown in Figure 9b, a 10 MHz, 20-cycle, 17 dBm input power sinewave pulsed signal was applied to the HVPA, and 4 V DC voltage was applied to the linearizer. With 17 dBm input, the second-, third-, fourth- and fifth-harmonic distortion components of the echo signal of the HVPA with post-linearizer (\(-\)8.94 dB, \(-\)10.01 dB, \(-\)11.95 dB, and \(-\)19.34 dB) were lower than those of the echo signal of HVPA without the post-linearizer (\(-\)4.32 dB, \(-\)8.19 dB, \(-\)8.94 dB, and \(-\)15.31 dB),  
Figure 9: Block diagram of the UBM system and spectrum data of the echo signal generated by a 10 MHz ultrasound transducer with and without post-linearizer circuit: (**a**) Block diagram of the UBM system; (**b**) 10 MHz, 20-cycle and 17 dBm input power, and 4 V DC voltage applied to the HVPA with and without post-linearizer; (**c**) 10 MHz, 5-cycle and 26 dBm input power, and 4 V DC voltage applied to the HVPA with and without post-linearizer.  
respectively. As shown in Figure 9c, a 10 MHz, 5-cycle 26 dBm input power sinewave burst signal was applied to the HVPA and 4 V DC voltage was applied to the post-linearizer. With a 26 dB\({}_{\text{m}}\) input, the second-, third-, fourth- and fifth-harmonic distortion components of the echo signal of HVPA with post-linearizer (\(-13.11\) dB, \(-11.8\) dB, \(-16.33\) dB and \(-22.78\) dB) were lower than those of the echo signal of HVPA without the post-linearizer (\(-4.58\) dB, \(-8.87\) dB, \(-10.51\) dB and \(-15.31\) dB), respectively. As shown in Figure 9b,c, harmonic signal components of the HVPA with post-linearizer were suppressed more than those of HVPA without post-linearizer. Therefore, we conclude that the post-linearizer circuit can suppress various undesirable harmonic distortion components of the ultrasound transducer-generated echo signal.