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.


Introduction
Among medical systems, ultrasound instruments are widely used in ambulances [1][2][3][4][5]. Compared to bench-top ultrasound instruments, which have a 100-240 V alternating current (AC) cord, other ultrasound instruments have a limited power supply [6,7]. Current ultrasound instruments have structures similar to those of laptop computers [8,9]. For example, the cooling fan of laptop computers must fit into a smaller structure than that of desktop computers, and laptop computers are restricted in their central processing unit (CPU) and graphic processing unit (GPU) capability owing to their limited power supply. The cooling fan must be controlled to minimize performance degradation due to the increased temperature caused by CPU and GPU operation [10][11][12][13]. Compared to laptop computers, ultrasound instruments have an even greater restriction in terms of high-power capability because power amplifiers must produce high-voltage signals to trigger piezoelectric transducers. In ultrasound instruments, power amplifiers are among the most critical sources of power consumption that need cooling fans [14][15][16]. Therefore, proper power amplifier design is crucial to achieve reasonable performance in ultrasound instruments.
Several nonlinear power amplifiers could be used to achieve efficient power amplifier design. In this paper, previous research is reviewed for different types of piezoelectric transducer applications. A class-D power amplifier was developed to generate 2 kW of output power, resulting in better signal quality with a power piezoelectric load [17][18][19]. A class-D amplifier was developed to generate a maximum output voltage of 125 V rms for a dielectric elastomer transducer [20]. A 1010 kHz class-DE power amplifier with output power of 800 mW was designed to generate a piezoelectric ultrasound transducer that can be integrated with MRI machines [21,22]. The MRI coils could interact with external inductor components in the power amplifier such that the designed power amplifier is implemented without any inductor components [23,24]. A 41.27 kHz and 133.3 mW class-E ductor components in the power amplifier such that the designed power amplifier is im plemented without any inductor components [23,24]. A 41.27 kHz and 133.3 mW class-E inverter with an impedance converter was designed for a Langevin transducer [25]. A 28.11 kHz class-E resonant inverter with output voltage of 112 Vrms operating under th zero-voltage switching condition was designed [26].
One of the most important specifications of power amplifiers for ultrasound instru ments is the output voltage, which is related to the sensitivity of the piezoelectric trans ducer; another is power consumption, which indicates the performance of the battery [27][28][29][30]. Nonlinear power amplifiers generate lower DC power consumption than linear powe amplifiers [31][32][33]. Therefore, they might be more useful for wireless electronic system with a limited power supply. However, they carry certain drawbacks such as a highe input voltage requirement [34][35][36]. To generate a high-amplitude input greater than 1 Vp p for a nonlinear power amplifier, a digital-to-analog converter (DAC) must be used [37][38][39].
Such high-amplitude input voltage can affect bias circuit stabilization and amplifie performance even when the power amplifier uses a shunt resistor or inductor to block unwanted input voltages, because the main transistors in the power amplifier must hav a proper DC bias voltage and AC input signal simultaneously [40,41]. If an amplitud input voltage less than 1 Vp-p is the input of the power amplifier, more stage power ampli fiers must be added because of the output of the amplifier [42,43]. Unfortunately, thi could produce higher temperatures, thus possibly reducing the output performance dur ing long-term operation [44]. Therefore, a new bias circuit to suppress unwanted inpu harmonic signals is proposed for use with high-amplitude input voltage. As depicted in Figure 1, a harmonic-reduced bias circuit could suppress several harmonic component of high-amplitude input signals (2fc, 3fc, and 4fc) simultaneously at the supply voltag point such that DC bias voltages could be more stabilized and more high-amplitude inpu signals go to the primary transistor for more effective amplification. In most analog circuit design for power amplifiers, the analog passive filter in th output matching network is well known [45][46][47]. The passive filter in the output matching network affects some performance parameters of the power amplifier. However, the pro posed approach could provide the harmonic-reduction capability in the bias circuit so tha performance parameters such as bandwidth and linearity would be less affected [48]. With the proposed harmonic-reduced bias circuit, the voltage gain of the class-C power ampli fier will be enhanced instead.
If the passive filter approach in the output matching network is used, the specifi harmonic distortion components could be reduced [49,50]. However, the voltage gain o bandwidth could be reduced accordingly. In particular, the high voltage generated by th power amplifier for ultrasound transducers could be more affected by the typical passiv filters in the output matching network because the linear voltage amplification and wid bandwidth are essential due to the nonlinear ultrasound transducer load impedance [51][52][53]. In most analog circuit design for power amplifiers, the analog passive filter in the output matching network is well known [45][46][47]. The passive filter in the output matching network affects some performance parameters of the power amplifier. However, the proposed approach could provide the harmonic-reduction capability in the bias circuit so that performance parameters such as bandwidth and linearity would be less affected [48]. With the proposed harmonic-reduced bias circuit, the voltage gain of the class-C power amplifier will be enhanced instead.
If the passive filter approach in the output matching network is used, the specific harmonic distortion components could be reduced [49,50]. However, the voltage gain or bandwidth could be reduced accordingly. In particular, the high voltage generated by the power amplifier for ultrasound transducers could be more affected by the typical passive filters in the output matching network because the linear voltage amplification and wide bandwidth are essential due to the nonlinear ultrasound transducer load impedance [51][52][53].
Additionally, the variable resistors with low power levels in the passive filter could be hard to use because high-voltage output signals from power amplifiers for ultrasound applications are generated [54]. Compared to a typical passive filter in the output port of the power amplifier, the proposed harmonic-reduced bias circuit could be helpful to reduce the specific several harmonic distortion components in the echo signals generated by the ultrasound transducer. Especially in the ultrasound applications such as harmonic imaging, acoustic stimulation, and ultrasound therapy, harmonic signal reduction is more important than ultrasound imaging applications [55,56]. Unfortunately, simulation model libraries of high-voltage or high-power transistors used for power amplifier design have only sub-decibel-level distortion accuracy, so simulation data using high-voltage or high-power transistor models are inaccurate at predicting the voltage gain, power efficiency, and power consumption of power amplifiers [57][58][59]. A theoretical approach and experimental results are presented in this paper. In addition, the theoretical analysis of the harmonic-reduced bias circuit in the equivalent circuit model will be presented to predict how to design such a power amplifier. Section 2 presents detailed architecture, operating mechanisms, and equivalent circuit model analysis for a class-C power amplifier with a typical voltage divider and harmonic-reduced bias circuit. Section 3 shows the measurement results to characterize and evaluate the capability of harmonic signal compression with bias circuits. The voltage gain and power consumption performance of the class-C power amplifier with bias circuits including pulse-echo mode measurement were sufficient for ultrasound applications. Section 4 concludes the paper. Figure 2 shows a full schematic diagram and fabricated printed circuit board of the class-C power amplifier with harmonic-reduced and voltage divider bias circuits. The gate and drain sides in primary transistor (H 1 , ST Microelectronics, Inc., Geneva, Switzerland) were used by RF (radio frequency) choke inductors (L c and L d , Bourns, Inc., Riverside, CA, USA) to reduce the DC voltage reduction of the class-C power amplifier [60]. The electrolytic capacitor (220 µF, Panasonic North America, Newark, NJ, USA) and additional ceramic capacitors (0.1 µF, 1 nF, and 0.1 nF, Vishay Siliconix, Santa Clara, CA, USA) were used to reduce possible high-frequency noises generated by the DC power supply [61,62]. The specification of the primary transistor (H 1 ) is 1 GHz operating frequency, ±20 V gate-source voltage, and 65 V gate-drain voltage, which can be suitable for class-C power amplifier design. Two separate capacitors were used in the input port (C 1 and C 2 , Bourns, Inc.) and output port (C 3 and C 4 , Bourns, Inc.), respectively, because there is more freedom to select capacitance values when using two discrete capacitor components. Two resistors (R 3 and R 8 ) were used to smooth out the input and output waveforms.

The Class-C Power Amplifier Fabrication
The harmonic-reduced bias circuit is a kind of filter structure composed of inductors (L B1 , L B2 , and L B3 , Coilcraft, Inc., Cary, IL, USA), capacitors (C B1 , C B2 , and C B3 , Vishay Siliconix), transistors (T B1 , T B2 , and T B3 , Diodes, Inc., Plano, TX, USA), and variable resistors (R TB1 , R TB2 , and R TB3 , 10 kΩ, Bourns, Inc.) including one fixed resistor and one variable resistor (R VD1 and R VD2 , 2 kΩ and 0.5 kΩ, Bourns, Inc.). The harmonic-reduced bias circuit is designed to block unwanted high-amplitude input signals with specific frequency components from the gate (V gate ) to the supply voltage (V supply ) and minimize the DC voltage reduction simultaneously because one RF choke inductor (L c ) cannot completely block high-amplitude input signals higher than 1 V p-p . A voltage divider bias circuit that also has identical value resistors (R VD1 and R VD2 , Bourns, Inc.) in a harmonic-reduced bias circuit was used to compare the bias circuit performances. Figure 2a,b give the schematic diagram and printed circuit board of the power amplifier integrated with harmonic-reduced and voltage divider bias circuit. The harmonic-reduced bias circuit is a kind of filter structure composed of indu (LB1, LB2, and LB3, Coilcraft, Inc., Cary, IL, USA), capacitors (CB1, CB2, and CB3, Vishay conix), transistors (TB1, TB2, and TB3, Diodes, Inc., Plano, TX, USA), and variable res (RTB1, RTB2, and RTB3, 10 kΩ, Bourns, Inc.) including one fixed resistor and one variab sistor (RVD1 and RVD2, 2 kΩ and 0.5 kΩ, Bourns, Inc.). The harmonic-reduced bias circ designed to block unwanted high-amplitude input signals with specific frequency ponents from the gate (Vgate) to the supply voltage (Vsupply) and minimize the DC vo reduction simultaneously because one RF choke inductor (Lc) cannot completely high-amplitude input signals higher than 1 Vp-p. A voltage divider bias circuit tha has identical value resistors (RVD1 and RVD2, Bourns, Inc.) in a harmonic-reduced bia cuit was used to compare the bias circuit performances. Figure 2a,b give the sche diagram and printed circuit board of the power amplifier integrated with harmon duced and voltage divider bias circuit.
The reason to use the voltage divider in the harmonic-reduced bias circuit is to pare the performance of the voltage divider bias circuit with that of the harmonic-red bias circuit. In the proposed harmonic-reduced bias circuit, it is possible to reduce ce harmonic distortion components such as second, third, and fourth harmonic disto  The reason to use the voltage divider in the harmonic-reduced bias circuit is to compare the performance of the voltage divider bias circuit with that of the harmonic-reduced bias circuit. In the proposed harmonic-reduced bias circuit, it is possible to reduce certain harmonic distortion components such as second, third, and fourth harmonic distortion components simultaneously. The ultrasound transducer is a kind of capacitive load with resistance, capacitance, and inductance [63][64][65]. Therefore, the operating frequency in the second, third, and fourth harmonic distortion components of the power amplifier is not exactly two, three, and four times the operating frequency of the ultrasound transducer, respectively [66][67][68]. The specific harmonic distortion component technique can be controlled with the help of the proposed harmonic-reduced bias circuit. Therefore, the proposed approach could be helpful to improve the performance of certain ultrasound applications such as harmonic imaging, acoustic stimulation, and ultrasound therapy. reduced and voltage divider bias circuits. In the equivalent circuit model of the transistor (Figure 3a), the drain-source resistance was removed because its value in the transistor was very small. Therefore, the equivalent circuit model of the transistor had parasitic gate-source, gate-drain, and drain-source capacitances (C gs , C gd , and C ds ) [69][70][71][72].

Equivalent Circuit Analysis of Bias Circuits
respectively [66][67][68]. The specific harmonic distortion component technique can be controlled with the help of the proposed harmonic-reduced bias circuit. Therefore, the proposed approach could be helpful to improve the performance of certain ultrasound applications such as harmonic imaging, acoustic stimulation, and ultrasound therapy. Figure 3 shows the equivalent circuit models of the bias circuits for AC analysis. The equivalent circuit models could describe the operating mechanisms of the harmonic-reduced and voltage divider bias circuits. In the equivalent circuit model of the transistor (Figure 3a), the drain-source resistance was removed because its value in the transistor was very small. Therefore, the equivalent circuit model of the transistor had parasitic gatesource, gate-drain, and drain-source capacitances (Cgs, Cgd, and Cds) [69][70][71][72].  Figure 3a shows the equivalent circuit model of only one part of the harmonic-reduced bias circuit except for the resistors (RRD1 and RRD2). We expected that unwanted high-amplitude input signals would come from the gate of the transistor, so the input and output voltages (Vinput and Voutput) were labeled in opposite directions.

Equivalent Circuit Analysis of Bias Circuits
As shown in Figure 3a, one part of the equivalent circuit in the harmonic-reduced bias circuit can be constructed. Using the equivalent circuit model as shown in Figure 3a, the voltage (Vx) at each node could be calculated via Equations (1) and (2) [73][74][75]: where Rs is the source resistance, Cgs is the gate-source capacitance, Cgd is the gate-drain capacitance, fc is the center frequency, Cdb is the drain capacitance, gm is the transconductance, Rds is the drain-source resistance, CB is the shunt capacitance, and LB is the series inductance.  Figure 3a shows the equivalent circuit model of only one part of the harmonic-reduced bias circuit except for the resistors (R RD1 and R RD2 ). We expected that unwanted highamplitude input signals would come from the gate of the transistor, so the input and output voltages (V input and V output ) were labeled in opposite directions.
As shown in Figure 3a, one part of the equivalent circuit in the harmonic-reduced bias circuit can be constructed. Using the equivalent circuit model as shown in Figure 3a, the voltage (V x ) at each node could be calculated via Equations (1) and (2) [73][74][75]: where R s is the source resistance, C gs is the gate-source capacitance, C gd is the gate-drain capacitance, f c is the center frequency, C db is the drain capacitance, g m is the transconductance, R ds is the drain-source resistance, C B is the shunt capacitance, and L B is the series inductance.
Combining Equations (1) and (2), the voltage (V x ) could be obtained as per Equation (3): (3) After the voltage (V x ) in Equation (3) is applied to Equation (1), Equation (4) could be obtained: The transfer function of one equivalent circuit of the harmonic-reduced bias circuit is represented in Equation (5): An unwanted high-amplitude input signal was passed through one part of the harmonic-reduced bias circuit from the "input" (V input ) to "output" (V output ) ports. This worked as a kind of filter [76][77][78]. Figure 3b shows the equivalent circuit model of the harmonic-reduced bias circuit with the resistance of the power supply (R supply ). To minimize several higher-harmonic components of the input signal, three succeeding circuits were adopted in the harmonic-reduced bias circuit as shown in Figure 3b. The transfer function of three filter circuits of the harmonic-reduced bias circuit with the power supply resistance can be obtained via Equation (6): where V input1 , V input2 , and V input3 are the first, second, and third inputs of the harmonicreduced bias circuit, respectively. V output1 , V output2 , and V output3 are the first, second, and third outputs of the harmonic-reduced bias circuit, respectively. Figure 4 shows the equivalent circuit model of the harmonic-reduced bias circuit for DC analysis.
After the voltage (Vx) in Equation (3) is applied to Equation (1), Equation (4) cou obtained: The transfer function of one equivalent circuit of the harmonic-reduced bias circ represented in Equation (5): An unwanted high-amplitude input signal was passed through one part of the monic-reduced bias circuit from the "input" (Vinput) to "output" (Voutput) ports. This wo as a kind of filter [76][77][78]. Figure 3b shows the equivalent circuit model of the harm reduced bias circuit with the resistance of the power supply (Rsupply). To minimize se higher-harmonic components of the input signal, three succeeding circuits were ado in the harmonic-reduced bias circuit as shown in Figure 3b. The transfer function of filter circuits of the harmonic-reduced bias circuit with the power supply resistanc be obtained via Equation (6): where Vinput1, Vinput2, and Vinput3 are the first, second, and third inputs of the harmon duced bias circuit, respectively. Voutput1, Voutput2, and Voutput3 are the first, second, and outputs of the harmonic-reduced bias circuit, respectively. Figure 4 shows the equivalent circuit model of the harmonic-reduced bias circu DC analysis. According to the voltage divider theory, the DC voltage of the gate (Vgate) ca simply represented by Equation (7)   According to the voltage divider theory, the DC voltage of the gate (V gate ) can be simply represented by Equation (7) [79]. The values of three shunt resistances (R TB1 , R TB2, and R TB3 ) are much larger than the values of the parallel impedance of the transconductances (g mtb1 , g mtb2 , and g mtb3 ) and drain-source resistances (r dstb1 , r dsb2 , and r dsb3 ) of the transistors, so we can ignore those values. If three shunt resistances (R TB1 , R TB2 , and R TB3 ) are much larger than one shunt resistance (R VD2 ), the combinational resistances (R VD2 , R TB1 , R TB2 , and R TB3 ) could be simplified into one shunt resistance (R VD2 ), as shown in Equation (8). Thus, the DC voltage of the gate in the harmonic-reduced and voltage divider bias circuits (V gate ) could be similar. As shown in Equation (8), three shunt resistances were not affected accordingly. Consequently, the harmonic-reduced bias circuit could suppress the unwanted high-amplitude input signal while sustaining the same DC bias voltages between the voltage divider and harmonic-reduced bias circuits if the shunt resistors (R TB1 , R TB2 , and R TB3 ) are much larger than one shunt resistor (R VD2 ).
The relationship between the i output and V gate of the class-C power amplifier with temperature effect can be represented by Equation (9) [80,81]: [θ conduction − sin(θ conduction )], (9) where q is the charge, N A is the constant doping density of the p-type substrate, ε is the permittivity of the oxide, Φ f is the Femi level, C ox is the oxide capacitance per unit, Φ f is the work function difference between the oxide and silicon interface, Q ss is the positive charge density in the oxide at the silicon interface, and θ conduction is the conduction angle. The gain and power consumption of the class-C power amplifier with harmonicreduced and voltage divider bias circuits are apparently the same, except for the conduction angle (θ conduction ) and output current (i output ) values, owing to different DC bias voltages. The voltage gain (G C ) of the class-C power amplifier with bias circuits can be expressed by Equation (10) [82,83]: where I drain , θ conduction , and V drian are the drain current, conductance angle, and drain voltage of the transistor. R Load is the load impedance of the oscilloscope, and i output and V input are the output current and input voltage of the class-C power amplifier with bias circuits, respectively The static and dynamic power consumption (P C ) of the class-C power amplifier with bias circuits is represented in Equation (11) [84][85][86]. In a class-A power amplifier, power consumption can be obtained by multiplying the supply voltage by the output current. However, the class-C power amplifier depends on the conduction angle (θ conduction ) and dynamic power consumption [87].
where C p is the dynamic power-dissipation capacitance. The next section shows the measurement results of the designed circuits such as spectrum data in the input port to show the effects of the input signal waveform. The gain and power consumption were measured to estimate the performance of the power amplifier. In addition, the pulse-echo responses using the ultrasound transducer and designed circuits were shown. The transistor model used for the power amplifier has only sub-decibel level distortion accuracy, so that the simulation data are inaccurate at predicting the voltage gain and power consumption [80]. In addition, there are no simulation libraries for variable resistors, transistors, electrostatic capacitors, RF inductors, and choke inductors.

Performance Evaluation of the Bias Circuits
The measured performance of the harmonic-reduced bias circuit is provided in Figure 5. Figure 5a,b show the measurement setup to validate the capability of the harmonic-reduced bias circuit. The unwanted high-amplitude input voltage blocking performance was compared between the implemented bias circuits. A 25 MHz, 5-cycle, and 5 V p-p sinusoidal waveform from a function generator (DG5071, Rigol Technologies, Inc., Beijing, China) was the input, and the output voltage (V supply ) was recorded as spectrum data with an oscilloscope (MSO2024B, Tecktronics, Inc., Beaverton, OR, USA). The transistor model used for the power amplifier has only sub-decibel level distortion accuracy, so that the simulation data are inaccurate at predicting the voltage gain and power consumption [80]. In addition, there are no simulation libraries for variable resistors, transistors, electrostatic capacitors, RF inductors, and choke inductors.

Performance Evaluation of the Bias Circuits
The measured performance of the harmonic-reduced bias circuit is provided in Figure 5. Figure 5a,b show the measurement setup to validate the capability of the harmonicreduced bias circuit. The unwanted high-amplitude input voltage blocking performance was compared between the implemented bias circuits. A 25 MHz, 5-cycle, and 5 Vp-p sinusoidal waveform from a function generator (DG5071, Rigol Technologies, Inc., Beijing, China) was the input, and the output voltage (Vsupply) was recorded as spectrum data with an oscilloscope (MSO2024B, Tecktronics, Inc., Beaverton, OR, USA).  The unwanted amplitudes at the center frequency, second, third, and fourth harmonic components could be resonated out to be filtered. This measured performance would prove this theory.
The harmonic-reduced bias circuit showed better harmonic signal suppression than the voltage divider bias circuit at all harmonic signal components. Therefore, this capability can help improve class-C power amplifier performance because more input signal am-    Figure 5c, the spectrum data measured when using the harmonicreduced bias circuit at the center frequency (25 MHz), second harmonic (50 MHz), third harmonic (75 MHz), and fourth harmonic (100 MHz) components were −61.31 dB, −89.02 dB, −90.53 dB, and −90.32 dB, respectively. In Figure 5d, the spectrum data measured when using the voltage divider bias circuit at the center frequency (25 MHz), second harmonic (50 MHz), third harmonic (75 MHz), and fourth harmonic (100 MHz) components were −57.19 dB, −73.49 dB, −70.97 dB, and −73.61 dB, respectively. The unwanted amplitudes at the center frequency, second, third, and fourth harmonic components could be resonated out to be filtered. This measured performance would prove this theory.
The harmonic-reduced bias circuit showed better harmonic signal suppression than the voltage divider bias circuit at all harmonic signal components. Therefore, this capability can help improve class-C power amplifier performance because more input signal amplitudes may go to the primary transistor. In the next section, the gain and power consumption performance were measured to further validate our proposed idea. Table 1 summarizes the measured results of the harmonic-reduced and voltage divider bias circuits.

Power Amplifier Performance Evaluation
The gain of the power amplifier is related to the echo signal sensitivity, and the power consumption is related to the instrument battery [88,89]. Therefore, the parameters of the power amplifiers could be the output voltage and power consumption. The gain and power consumption of the class-C power amplifier with bias circuits are presented accordingly. Figure 6a,b show the schematic diagram and photo of the measurement setup for the gain and power consumption of the class-C power amplifiers with bias circuits. A 25 MHz and 5-cycle input waveform generated from a function generator (DG5071) was used as the input of the class-C power amplifier with bias circuits. The output voltage was obtained in the oscilloscope (MSO2024B), and the gain was calculated by dividing the output voltage by the input voltage.
Sensors 2023, 23, 4438 9 of plitudes may go to the primary transistor. In the next section, the gain and power co sumption performance were measured to further validate our proposed idea. Table 1 su marizes the measured results of the harmonic-reduced and voltage divider bias circuit

Power Amplifier Performance Evaluation
The gain of the power amplifier is related to the echo signal sensitivity, and the pow consumption is related to the instrument battery [88,89]. Therefore, the parameters of t power amplifiers could be the output voltage and power consumption. The gain a power consumption of the class-C power amplifier with bias circuits are presented a cordingly. Figure 6a,b show the schematic diagram and photo of the measurement set for the gain and power consumption of the class-C power amplifiers with bias circuits. 25 MHz and 5-cycle input waveform generated from a function generator (DG5071) w used as the input of the class-C power amplifier with bias circuits. The output voltage w obtained in the oscilloscope (MSO2024B), and the gain was calculated by dividing the ou put voltage by the input voltage.   Figure 6c, the maximum gain when usi the class-C power amplifier with the harmonic-reduced bias circuit (18.06 dB) was high than that when using the class-C power amplifier with the voltage divider bias circ (15.11 dB) at a 5 Vp-p input voltage. In Figure 6d, the minimum gain when using the cla C power amplifier with the harmonic-reduced bias circuit (0.82 dB) was lower than th when using the class-C power amplifier with the voltage divider bias circuit (5.10 dB) a 5 MHz input frequency. However, the maximum gain when using the class-C power a plifier with the harmonic-reduced bias circuit (15.56 dB) was higher than that when usi the class-C power amplifier with the voltage divider bias circuit (11.69 dB) at a 50 MH input frequency. Figure 6e,f show the measured power consumption of the class-C power amplifi with the bias circuits versus the input voltage and input frequency. The power consum tion performance is an important parameter for ultrasound instruments because of t battery issue. In Figure 6e, the measured power consumption of the class-C power amp fier with the harmonic-reduced bias circuit (21.25 W) was less than that of the class power amplifier with the voltage divider circuit (23.25 W) versus the input voltage at 5 In Figure 6f, the measured power consumption of the class-C power amplifier with t harmonic-reduced bias circuit (17.75 W) was less than that of the class-C power amplifi with the voltage divider circuit (18.75 W) versus the input frequency at 50 MHz. Figu 6g shows the measured output voltage versus the input voltage of the class-C power a plifier with the bias circuits. In Figure 6g, the output voltage of the class-C power amplifi with the harmonic-reduced bias circuit (40.0 V) was higher than that of the class-C pow amplifier with the voltage divider circuit (28.5 V) versus the input voltage at a 5 V inp voltage. Figure 6h shows the measured output voltage versus the input frequency of t class-C power amplifier with the bias circuits. In Figure 6h, the output voltage of the cla   Figure 6c, the maximum gain when using the class-C power amplifier with the harmonic-reduced bias circuit (18.06 dB) was higher than that when using the class-C power amplifier with the voltage divider bias circuit (15.11 dB) at a 5 V p-p input voltage. In Figure 6d, the minimum gain when using the class-C power amplifier with the harmonic-reduced bias circuit (0.82 dB) was lower than that when using the class-C power amplifier with the voltage divider bias circuit (5.10 dB) at a 5 MHz input frequency. However, the maximum gain when using the class-C power amplifier with the harmonic-reduced bias circuit (15.56 dB) was higher than that when using the class-C power amplifier with the voltage divider bias circuit (11.69 dB) at a 50 MHz input frequency. Figure 6e,f show the measured power consumption of the class-C power amplifier with the bias circuits versus the input voltage and input frequency. The power consumption performance is an important parameter for ultrasound instruments because of the battery issue. In Figure 6e, the measured power consumption of the class-C power amplifier with the harmonic-reduced bias circuit (21.25 W) was less than that of the class-C power amplifier with the voltage divider circuit (23.25 W) versus the input voltage at 5 V. In Figure 6f, the measured power consumption of the class-C power amplifier with the harmonic-reduced bias circuit (17.75 W) was less than that of the class-C power amplifier with the voltage divider circuit (18.75 W) versus the input frequency at 50 MHz. Figure 6g shows the measured output voltage versus the input voltage of the class-C power amplifier with the bias circuits. In Figure 6g, the output voltage of the class-C power amplifier with the harmonic-reduced bias circuit (40.0 V) was higher than that of the class-C power amplifier with the voltage divider circuit (28.5 V) versus the input voltage at a 5 V input voltage. Figure 6h shows the measured output voltage versus the input frequency of the class-C power amplifier with the bias circuits. In Figure 6h, the output voltage of the class-C power amplifier with the harmonic-reduced bias circuit (30.0 V) was higher than that of the class-C power amplifier with the voltage divider circuit (19.0 V) versus the input voltage at a 50 MHz input frequency. Table 2 shows the measured gain and power consumption versus input voltage and input frequency of the class-C power amplifiers with harmonic-reduced and voltage divider bias circuits, respectively. Table 2. Measured results of the gain and power consumption versus input voltage or input frequency of the designed power amplifiers with harmonic-reduced and voltage divider bias circuits, respectively.

Voltage Divider
Bias Circuit  Table 3 shows comparison data of the maximum gain and power consumption versus input voltage and input frequency of the class-C power amplifiers with harmonic-reduced and voltage divider bias circuits, respectively. The maximum gain of the harmonic-reduced bias circuit was higher than that of the voltage divider circuit. In addition, the maximum power consumption of the harmonic-reduced bias circuit was lower than that of the voltage divider circuit. These results confirm that the proposed harmonic-reduced bias circuits could reduce the effects of the high-voltage input signals on the bias points of the main transistor in the class-C power amplifiers. Table 3. Comparison data of the measured results of the gain and power consumption of the designed power amplifiers with harmonic-reduced and voltage divider bias circuits.

Pulse-Echo Mode Measurement
Pulse-echo measurement is a basic operating method for evaluating piezoelectric transducers and electronics [90][91][92]. Figure 7a,b show the schematic diagram and photo of the pulse-echo measurement setup to evaluate the performance of the class-C power amplifier with bias circuits. A 25 MHz, 5-cycle, and 5 V p-p sinusoidal waveform from a function generator was used as the input. The input was fed into the developed class-C power amplifier with bias circuits. The generated pulses were transmitted into a piezoelectric transducer to produce the acoustic pulses to be delivered to the target, and the discharged pulses were blocked by a limiter comprising a resistor shunt with a single cross-coupled diode [93]. The echoes were detected by a piezoelectric transducer with the element size of 0.25 inch and operating frequency of 25 MHz converted into the echo waveform [9]. The weak echo waveforms were amplified by a preamplifier (AU-1114, MITEQ, Inc., Hauppauge, NJ, USA), and the waveform and its spectrum were displayed on an oscilloscope (MSO2024B). The peak-to-peak voltage (V p-p ), center frequency (f c ), −6 dB BW (bandwidth), and total harmonic distortion (THD) of the measured echo signal were calculated using Equations (12)- (15) [94][95][96][97]: where V + and V − were the measured positive maximum and negative minimum voltages, respectively; f c1 and f c2 were measured frequency range located at the left and right points −6 dB below the frequency at maximum spectrum data; V 1 is the amplitude of the fundamental signal; and V 2 , V 3 , and V 4 are the amplitudes of the second, third, and fourth harmonic signals, respectively. Figure 8a,b show the echo signal amplitude comparison measured when using different bias circuits. The measured echo signal amplitude when using the harmonic-reduced bias circuit (27.07 mV p-p ) was higher than that when using the voltage divider bias circuit (18.55 mV p-p ) because of the higher gain measured when using the harmonic-reduced bias circuit. Figure 8c,d show the comparison of the echo signal spectrum measured when using different bias circuits. The center frequency measured when using the harmonic-reduced bias circuit (22.56 MHz) was higher than that when using the voltage divider bias circuit (20.87 MHz). The −6 dB BW measured when using the harmonic-reduced bias circuit (37.19%) was also larger than that when using the voltage divider bias circuit (22.71%).
Figure 8e,f show enlarged normalized spectrum data of Figure 8c,d to compare the harmonic distortion signals (HD2, HD3, and HD4). The harmonic distortion signals (HD2 = −37.68 dB, HD3 = −37.83 dB, and HD4 = −43.80 dB) when using the harmonicreduced bias circuit were lower than those (HD2 = −24.45 dB, HD3 = −30.60 dB, and HD4 = −29.09 dB) when using the voltage divider circuit. The calculated THD value (−34.82 dB) when using the harmonic-reduced bias circuit was also lower than that when using the voltage divider circuit. These harmonic distortion data confirm that the proposed harmonic-reduced bias circuit could affect the signal distortions of the echo signals generated by the ultrasound transducer.  Figure 8a,b show the echo signal amplitude comparison measured when u ferent bias circuits. The measured echo signal amplitude when using the harm duced bias circuit (27.07 mVp-p) was higher than that when using the voltage div circuit (18.55 mVp-p) because of the higher gain measured when using the harm duced bias circuit. Figure 8c,d show the comparison of the echo signal spectrum m when using different bias circuits. The center frequency measured when using monic-reduced bias circuit (22.56 MHz) was higher than that when using the vo vider bias circuit (20.87 MHz). The −6 dB BW measured when using the harmonic bias circuit (37.19%) was also larger than that when using the voltage divider bi (22.71%).   Figure 8a,b show the echo signal amplitude comparison measured when using different bias circuits. The measured echo signal amplitude when using the harmonic-reduced bias circuit (27.07 mVp-p) was higher than that when using the voltage divider bias circuit (18.55 mVp-p) because of the higher gain measured when using the harmonic-reduced bias circuit. Figure 8c,d show the comparison of the echo signal spectrum measured when using different bias circuits. The center frequency measured when using the harmonic-reduced bias circuit (22.56 MHz) was higher than that when using the voltage divider bias circuit (20.87 MHz). The −6 dB BW measured when using the harmonic-reduced bias circuit (37.19%) was also larger than that when using the voltage divider bias circuit (22.71%).  Figure 8e,f show enlarged normalized spectrum data of Figure 8c,d to compare the harmonic distortion signals (HD2, HD3, and HD4). The harmonic distortion signals (HD2 = −37.68 dB, HD3 = −37.83 dB, and HD4 = −43.80 dB) when using the harmonic-reduced bias circuit were lower than those (HD2 = −24.45 dB, HD3 = −30.60 dB, and HD4 = −29.09 dB) when using the voltage divider circuit. The calculated THD value (−34.82 dB) when using the harmonic-reduced bias circuit was also lower than that when using the voltage divider circuit. These harmonic distortion data confirm that the proposed harmonic-reduced bias circuit could affect the signal distortions of the echo signals generated by the ultrasound transducer. Table 4 summarizes the measured comparison data of the echo signal amplitude and spectrum data when using the harmonic-reduced and voltage divider bias circuits. The class-C power amplifier with the harmonic-reduced bias circuit may be a better candidate for reducing signal distortions in electronic devices.   Table 4 summarizes the measured comparison data of the echo signal amplitude and spectrum data when using the harmonic-reduced and voltage divider bias circuits. The class-C power amplifier with the harmonic-reduced bias circuit may be a better candidate for reducing signal distortions in electronic devices.  Table 5 shows the comparison data of the proposed work with previous publications related to the nonlinear power amplifiers only for ultrasound applications because the designed scheme is useful for nonlinear power amplifiers that have large harmonic distortions. Therefore, the linear power amplifier schemes were excluded in Table 5 as below. Their target applications are piezoelectric transducer applications, even though their design circuit topology is different. Therefore, comparison data are provided here.  The target applications of the other power amplifier design are low-frequency (≤15 MHz) ultrasound transducers. Compared to other previous publications, the target application of the designed class-C power amplifier with bias circuits is a high-frequency (≥15 MHz) piezoelectric transducer. Because the designed bias circuit to suppress harmonic signals from high-amplitude input signals was proposed, the harmonic distortion performance needs to be properly produced with the help of the designed harmonic-reduced bias circuit. The HD3 (−37.83 dB) when using the proposed bias circuit of the class-C power amplifier was measured.

Conclusions
Ultrasound instruments are used in hospitals. Compared to bench-top ultrasound instruments, other ultrasound instruments have the advantage of being cordless for portability. However, their performance is severely restricted by power consumption owing to battery limitations. Because thermal problems could affect the performance of ultrasound instruments, the cooling fan must be utilized, resulting in unavoidable noise, which affects the image resolution of the ultrasound instruments. To solve this problem, nonlinear power amplifiers would be more desirable than linear power amplifiers owing to their low power consumption. However, nonlinear power amplifiers may produce lower output power owing to the low conduction angle of the main transistors. Consequently, a higher input amplitude is required to generate adequate echo signal outputs from the ultrasound transducers.
To utilize the class-C power amplifiers in the ultrasound instruments, high-amplitude (>1 V p-p ) input voltage signals must be applied to obtain proper output voltages, because the gain of the class-C power amplifiers is lower than that of the class-A power amplifier. Therefore, a new type of harmonic-reduced bias circuit was proposed to block unwanted high-amplitude input signals. To verify the capability of the bias circuit, the measured spectrum data at a DC supply voltage point were measured. The measured spectrum of the harmonic-reduced bias circuit was lower (−61.31 dB, −89.02 dB, −90.53 dB, and −90.32 dB) than that of 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. The measured gain of the class-C power amplifier with the harmonic-reduced bias circuit was higher (18.06 dB) than that of the class-C power amplifier with the voltage divider circuit (15.11 dB). The measured power consumption when using the class-C power amplifier with the harmonic-reduced bias circuit (21.25 W) was lower than that when using the class-C power amplifier with the voltage divider circuit (23.25 W).
To further verify the capabilities of the harmonic-reduced bias circuit, a pulse-echo measurement was also performed. The echo signal amplitude and bandwidth measured when using the class-C power amplifier with the harmonic-reduced bias circuit (27.07 mV and 37.19%) were higher than those of the class-C power amplifier with the voltage divider circuit (18.55 mV and 22.71%). The THD value (−34.82 dB) calculated when using the class-C power amplifier with the harmonic-reduced bias circuit was lower than that when using the class-C power amplifier with the voltage divider circuit. These data confirm that the proposed harmonic-reduced bias circuit could help reduce the signal distortions of the echo signals. Therefore, the designed class-C power amplifier with a harmonic-reduced bias circuit could be a candidate to reduce harmonic distortion in transducer applications.