# Patent

## Title
Multi-level pulser and related apparatus and methods

## Abstract

Apparatus and methods are provided directed to a device, including at least one ultrasonic transducer, a multi-level pulser coupled to the at least one ultrasonic transducer; the multi-level pulser including a plurality of input terminals configured to receive respective input voltages, an output terminal configured to provide an output voltage, and a signal path between a first input terminal and the output terminal including a first transistor having a first conductivity type coupled to a first diode and, in parallel, a second transistor having a second conductivity type coupled to a second diode.

## Background
BACKGROUND
Field
The present application relates to ultrasound devices having a multi-level pulser and/or a level shifter.
Related Art
Ultrasound devices may be used to perform diagnostic imaging and/or treatment. Ultrasound imaging may be used to see internal soft tissue body structures. Ultrasound imaging may be used to find a source of a disease or to exclude any pathology. Ultrasound devices use sound waves with frequencies which are higher than those audible to humans. Ultrasonic images are made by sending pulses of ultrasound into tissue using a probe. The sound waves are reflected off the tissue, with different tissues reflecting varying degrees of sound. These reflected sound waves may be recorded and displayed as an image to the operator. The strength (amplitude) of the sound signal and the time it takes for the wave to travel through the body provide information used to produce an image.
Many different types of images can be formed using ultrasound devices. The images can be real-time images. For example, images can be generated that show two-dimensional cross-sections of tissue, blood flow, motion of tissue over time, the location of blood, the presence of specific molecules, the stiffness of tissue, or the anatomy of a three-dimensional region.
SUMMARY
According to aspects of the present application, there are provided apparatus and methods directed to an apparatus, including at least one ultrasonic transducer, a multi-level pulser coupled to the at least one ultrasonic transducer; the multi-level pulser including a plurality of input terminals configured to receive respective input voltages, an output terminal configured to provide an output voltage, and a signal path between a first input terminal and the output terminal including a first transistor having a first conductivity type coupled to a first diode and, in parallel, a second transistor having a second conductivity type coupled to a second diode.
According to aspects of the present application, there are provided apparatus and methods directed to a multi-level pulser, including a plurality of input terminals configured to receive respective input voltages, an output terminal configured to provide an output voltage, and a signal path between a first input terminal and the output terminal including a transistor having a first conductivity type coupled to a first diode and, in parallel, a transistor having a second conductivity type coupled to a second diode.



BRIEF DESCRIPTION OF THE DRAWINGS
Various aspects and embodiments of the application will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same reference number in all the figures in which they appear.
FIG. 1 is a block diagram of an ultrasound device including a multi-level pulser and/or a level shifter, according to a non-limiting embodiment of the present application.
FIG. 2 illustrates a non-limiting circuit diagram of a multi-level pulser, according to a non-limiting embodiment of the present application.
FIG. 3A illustrates a circuit diagram of a first embodiment of a level shifter, according to a non-limiting embodiment of the present application.
FIG. 3B illustrates a circuit diagram of a second embodiment of a level shifter, according to a non-limiting embodiment of the present application.
FIG. 4A illustrates a non-limiting equivalent circuit of the circuit of FIG. 2, during a first phase of a multi-level pulse formation, according to a non-limiting embodiment of the present application.
FIG. 4B illustrates a non-limiting equivalent circuit of the circuit of FIG. 2, during a second phase of a multi-level pulse formation, according to a non-limiting embodiment of the present application.
FIG. 4C illustrates a non-limiting equivalent circuit of the circuit of FIG. 2, during a third phase of a multi-level pulse formation, according to a non-limiting embodiment of the present application.
FIG. 4D illustrates a non-limiting equivalent circuit of the circuit of FIG. 2, during a fourth phase of a multi-level pulse formation, according to a non-limiting embodiment of the present application.
FIG. 4E illustrates a non-limiting equivalent circuit of the circuit of FIG. 2, during a fifth phase of a multi-level pulse formation, according to a non-limiting embodiment of the present application.
FIG. 4F illustrates a non-limiting equivalent circuit of the circuit of FIG. 2, during a sixth phase of a multi-level pulse formation, according to a non-limiting embodiment of the present application.
FIG. 5 is a graph illustrating a non-limiting example of a time-dependent multi-level pulse and the control signals, according to a non-limiting embodiment of the present application.



DETAILED DESCRIPTION
The inventors have recognized and appreciated that the power necessary to transmit high-intensity pulses may be greatly decreased by forming electric pulses having multiple levels.
Aspects of the present application relate to high-intensity focused ultrasound (HIFU) procedures that may be used to focus high-intensity ultrasound energy on targets to treat diseases or damaged tissues by selectively increasing the temperature of the target or the region surrounding the target. HIFU procedures may be used for therapeutic or ablative purposes. Pulsed signals may be used to generate HIFUs. According to aspects of the present application, the generation of such high-intensity pulses may require driving voltages of several tens to several hundreds of volts.
The power consumption associated with the generation of typical 2-level pulses having a “low” voltage and a “high” voltage is proportional to the square of the high voltage. For example, the generation of a 2-level pulse having a “low” voltage equal to 0 requires a power equal to:

P(2)=C*V2*f 

where P(2) is the power needed to generate the 2-level pulse, C is the capacitance of the load receiving the pulse, V is the “high” voltage and f is the repetition frequency of the 2-level pulse.
According to aspects of the present application, the power consumption associated with the generation of pulses for HIFU procedures may exceed several tens to thousands of watts, thus causing the circuit to generate significant amounts of heat.
Aspects of the present application relate to multi-level pulsers designed to decrease power consumption and heat dissipation.
Furthermore, aspects of the present application relate to a level shifter circuit configured to drive the multi-level pulser. The level shifter disclosed herein may dissipate considerably less power compared to typical level shifters. Accordingly, power may be dissipated only when a level is switched, while static power consumption may be negligible.
The aspects and embodiments described above, as well as additional aspects and embodiments, are described further below. These aspects and/or embodiments may be used individually, all together, or in any combination of two or more, as the application is not limited in this respect.
FIG. 1 illustrates a circuit for processing received ultrasound signals, according to a non-limiting embodiment of the present application. The circuit 100 includes N ultrasonic transducers 102a . . . 102n, wherein N is an integer. The ultrasonic transducers are sensors in some embodiments, producing electrical signals representing received ultrasound signals. The ultrasonic transducers may also transmit ultrasound signals in some embodiments. The ultrasonic transducers may be capacitive micromachined ultrasonic transducers (CMUTs) in some embodiments. The ultrasonic transducers may be piezoelectric micromachined ultrasonic transducers (PMUTs) in some embodiments. Further alternative types of ultrasonic transducers may be used in other embodiments.
The circuit 100 further comprises N circuitry channels 104a . . . 104n. The circuitry channels may correspond to a respective ultrasonic transducer 102a . . . 102n. For example, there may be eight ultrasonic transducers 102a . . . 102n and eight corresponding circuitry channels 104a . . . 104n. In some embodiments, the number of ultrasonic transducers 102a . . . 102n may be greater than the number of circuitry channels.
According to aspects of the present application, the circuitry channels 104a . . . 104n may include transmit circuitry. The transmit circuitry may include level shifters 106a . . . 106n coupled to respective multi-level pulsers 108a . . . 108n. The multi-level pulsers 108a . . . 108n may control the respective ultrasonic transducers 102a . . . 102n to emit ultrasound signals.
Circuitry channels 104a . . . 104n may also include receive circuitry. The receive circuitry of the circuitry channels 104a . . . 104n may receive the electrical signals output from respective ultrasonic transducers 102a . . . 102n. In the illustrated example, each circuitry channel 104a . . . 104n includes a respective receive switch 110a . . . 110n and an amplifier 112a . . . 112n. The receive switches 110a . . . 110n may be controlled to activate/deactivate readout of an electrical signal from a given ultrasonic transducer 102a . . . 102n. More generally, the receive switches 110a . . . 110n may be receive circuits, since alternatives to a switch may be employed to perform the same function. The amplifiers 112a . . . 112n may be trans-impedance amplifiers (TIAs).
The circuit 100 further comprises an averaging circuit 114, which is also referred to herein as a summer or a summing amplifier. In some embodiments, the averaging circuit 114 is a buffer or an amplifier. The averaging circuit 114 may receive output signals from one or more of the amplifiers 112a . . . 112n and may provide an averaged output signal. The averaged output signal may be formed in part by adding or subtracting the signals from the various amplifiers 112a . . . 112n. The averaging circuit 114 may include a variable feedback resistance. The value of the variable feedback resistance may be adjusted dynamically based upon the number of amplifiers 112a . . . 112n from which the averaging circuit receives signals. The averaging circuit 114 is coupled to an auto-zero block 116.
The auto-zero block 116 is coupled to a time gain compensation circuit 118 which includes an attenuator 120 and a fixed gain amplifier 122. Time gain compensation circuit 118 is coupled to an analog-to-digital converter (ADC) 126 via ADC drivers 124. In the illustrated example, the ADC drivers 124 include a first ADC driver 125a and a second ADC driver 125b. The ADC 126 digitizes the signal(s) from the averaging circuit 114.
While FIG. 1 illustrates a number of components as part of a circuit of an ultrasound device, it should be appreciated that the various aspects described herein are not limited to the exact components or configuration of components illustrated. For example, aspects of the present application relate to the multi-level pulsers 108a . . . 108n and the level shifters 106a . . . 106n. 
The components of FIG. 1 may be located on a single substrate or on different substrates. For example, as illustrated, the ultrasonic transducers 102a . . . 102n may be on a first substrate 128a and the remaining illustrated components may be on a second substrate 128b. The first and/or second substrates may be semiconductor substrates, such as silicon substrates. In an alternative embodiment, the components of FIG. 1 may be on a single substrate. For example, the ultrasonic transducers 102a . . . 102n and the illustrated circuitry may be monolithically integrated on the same semiconductor die. Such integration may be facilitated by using CMUTs as the ultrasonic transducers.
According to an embodiment, the components of FIG. 1 form part of an ultrasound probe. The ultrasound probe may be handheld. In some embodiments, the components of FIG. 1 form part of an ultrasound patch configured to be worn by a patient.
FIG. 2 illustrates the circuit diagram of a multi-level pulser, according to aspects to the present application. In some embodiments, multi-level pulser 200 may be configured to transmit a pulse to capacitor C. Capacitor C may represent the capacitance associated with an ultrasound transducer. For example, capacitor C may represent a capacitive micromachined ultrasonic transducer (CMUT). However, multi-level pulser 200 may be configured to transmit a pulse to a resistor, a resistive network or a network exhibiting any suitable combination of resistive and reactive elements.
In the non-limiting embodiment illustrated in FIG. 2, multi-level pulser 200 is configured to provide an N-level pulse, where N may assume any value greater than 2. The power consumption P(N) associated with the transmission of a N-level pulser to capacitor C is equal to:

P(N)=C*V2*f/(N−1)

where f is the repetition frequency of the pulsed waveform. Accordingly, power consumption is reduced by a factor N−1 compared to typical 2-level pulsers.
In some embodiments, N-level pulser 200 may comprise 2N−2 transistors and 2N−4 diodes. However, any suitable number of transistors may be used. Among the 2N−2 transistors, N−1 may exhibit one type of conductivity and N−1 may exhibit the opposite type of conductivity. However any other suitable combination of types of conductivity may be used. For example, N−1 transistors may be nMOS and N−1 transistors may be pMOS. However any other suitable type of transistor may be used.
N-level pulser 200 may comprise N circuit blocks 2011, 2012 . . . 201N. The N circuit blocks may be connected to node 202. One terminal of capacitor C may also be connected to node 202. The second terminal of capacitor C may be connected to ground. Circuit block 2011 may comprise pMOS transistor T1, having the source connected to a reference voltage VDD and the drain connected to node 202. Reference voltage VDD may be a voltage supply. The gate of transistor T1 may be driven by signal VG1.
Circuit block 201N may comprise nMOS transistor T2N−2, having the source connected to a reference voltage VSS and the drain connected to node 202. In some embodiments, reference voltage VSS may be less than reference voltage VDD. However, pulser 200 is not limited in this respect. Furthermore, reference voltage VSS may positive, negative or equal to zero. The gate of transistor T2N−2 may be driven by signal VG2N−2.
In some embodiments, circuit blocks 2012 may comprise two transistors T2 and T3 and two diodes D2 and D3. Transistor T2 and diode D2 may be connected in series and transistor T3 and diode D3 may also be connected in series. The two series may be connected in parallel. In some embodiments, T2 may be a pMOS transistor, having the source connected to the reference voltage VMID2 and the drain connected to the anode of D2 and T3 may be an nMOS transistor, having the source connected to VMID2 and the drain connected to the cathode of D3. In some embodiments, VMID2 may be greater than VSS and less than VDD. The cathode of D2 and the anode of D3 may be connected to node 202. Furthermore, the gate of T2 may be driven by signal VG2 and the gate of T3 may be driven by signal VG3.
In some embodiments, circuit blocks 201i where i may assume any value between 3 and N−1, may comprise two transistors T2i−2 and T2i−1 and two diodes D2i−2 and D2i−1. Transistor T2i−2 and diode D2i−2 may be connected in series and transistor T2i−1 and diode D2i−1 may also be connected in series. The two series may be connected in parallel. In some embodiments, T2i−2 may be a pMOS transistor, having the source connected to the reference voltage VMIDi and the drain connected to the anode of D2i−2 and T2i−1 may be an nMOS transistor, having the source connected to VMIDi and the drain connected to the cathode of D2i−1. In some embodiments, VMIDi may be greater than VSS and less than VMID2. The cathode of D2i−2 and the anode of D2i−1 may be connected to node 202. Furthermore, the gate of T2i−2 may be driven by signal VG2i−2 and the gate of T2i−1 may be driven by signal VG2i−1.
VDD, VSS and VMIDi, for any value of i, may have values between approximately −300V and 300V, between approximately −200V and 200V, or any suitable value or range of values. Other values are also possible.
FIG. 3A and FIG. 3B illustrate two non-limiting embodiments of a level shifter circuit, according to aspects of the present application. In some embodiments, level shifter 301, shown in FIG. 3A, may be integrated on the same chip as pulser 200. In some embodiments, level shifter 301 may be used to drive any of the pMOS transistors of pulser 200. For example, level shifter 301 may be used to output signal VG2i−2 to drive the gate of transistor T2i−2. The input voltage VIN2i−2 to level shifter 301 may be a control signal having two possible voltage levels: VSS and VSS+δV, where δV may assume any suitable value or range of values. In some embodiments, control signal VIN2i−2 may be generated by a circuit integrated on the same chip as level shifter 301. However, control signal VIN2i−2 may also be generated by a circuit integrated on a separate chip. In some embodiments, level shifter 301 may comprise an inverter IM1, followed by capacitor CM. The power supply pins of inverter IM1 may be connected to voltages VSS and VSS+δV. Capacitor CM may be followed by the series of a number of inverters. In some embodiments, capacitor CM is followed by three inverters IM2, I3 and IM4. The “−” and “+” power supply pins of inverter IM2, IM3 and IM4 may be connected to voltages VMIDi−ΔV and VMIDi respectively. In some non-limiting embodiments, level shifter 301 may comprise diode DM. The cathode or diode DM may be connected to the output of capacitor CM, while the anode may be connected to the VMIDi−ΔV rail. While level shifter 301 comprises four inverters in the non-limiting embodiment of FIG. 3A, any suitable number of inverters may otherwise be used. Output voltage VG2i−2 may assume two possible voltages: VMIDi−ΔV and VMIDi.
In some embodiments, level shifter 302, shown in FIG. 3B, may be integrated on the same chip as pulser 200. In some embodiments, level shifter 302 may be used to drive any of the nMOS transistors of pulser 200. For example, level shifter 302 may be used to output signal VG2i−1 to drive the gate of transistor T2i−1. The input voltage VIN2i−1 to level shifter 302 may be a control signal having two possible voltage levels: VSS and VSS+δV. In some embodiments, control signal VIN2i−1 may be generated by a circuit integrated on the same chip as level shifter 302. However, control signal VIN2i−1 may also be generated by a circuit integrated on a separate chip. In some embodiments, level shifter 302 may comprise an inverter IP1, followed by capacitor CP. The power supply pins of inverter IP1 may be connected to voltages VSS and VSS+δV. Capacitor CP may be followed by the series of a number of inverters. In some embodiments, capacitor CP is followed by two inverters IP2 and IP3 The power supply pins of inverter IM2 and IM3 may be connected to voltages VMIDi and VMIDi+ΔV. In some non-limiting embodiments, level shifter 302 may comprise diode DP. The cathode or diode DP may be connected to the output of capacitor CP, while the anode may be connected to the VMIDi rail. While level shifter 302 comprises three inverters in the non-limiting embodiment of FIG. 3B, any suitable number of inverters may otherwise be used. Output voltage VG2i−i may assume two possible voltages: VMIDi and VMIDi+ΔV.
According to aspects of the present application, level shifters 301 and 302 may dissipate power only when a level is switched, while static power may be negligible. Capacitors CM and CP may be used to shift the voltage level by storing a constant voltage drop across them. For example, the static power consumption may be less than 100 mW, less than 1 mW, less than 1 μW or less than any suitable value.
FIG. 4A, FIG. 4B, FIG. 4C, FIG. 4D, FIG. 4E, and FIG. 4F illustrate six snapshots of pulser 200 corresponding to the six phases associated with the formation of a 4-level pulse, according to aspects on the present application. In the figures, only the active blocks are shown. While in the non-limiting example N is equal to 4, any other suitable value of N, such that N is greater than 2, may otherwise be used. In the example, VSS is set to 0.
FIG. 5 illustrates a non-limiting example of multi-level pulse 500 generated according to aspects of the present application. In the non-limiting example, pulse 500 exhibits 4 levels: 0, VMID3, VMID2, and VDD. In addition, FIG. 5 illustrates the 6 control signals VG1, VG2, VG3, VG4, VG5, and VG6 used to respectively drive the gates of transistors T1, T2, T3, T4, T5, and T6. The process associated with the pulse generation can be divided in 6 phases. Between t1 and t2, pulse 500 may be increased from 0 to VMID3 by providing a negative pulse 504 to transistor T4 through VG4 as shown in FIG. 5. FIG. 4A illustrates pulser 201 between t1 and t2. During this period, the gate of transistor T4 may be driven by a voltage equal to VMID3−ΔV. ΔV may be chosen so as to create a conductive channel and cause transistor T4 to drive a current between the source and the drain passing through diode D4. Such current may charge capacitor C, such that an output voltage of VMID3 is obtained, neglecting any voltage drop on T4 and D4. Pulse 504 may be obtained through level shifter 301.
Between t2 and t3, pulse 500 may be increased from VMID3 to VMID2 by providing a negative pulse 502 to transistor T2 through VG2 as shown in FIG. 5. FIG. 4B illustrates pulser 201 between t2 and t3. During this period, the gate of transistor T2 may be driven by a voltage equal to VMID2−ΔV. ΔV may be chosen so as to create a conductive channel and cause transistor T2 to drive a current between the source and the drain passing through diode D2. Such current may charge capacitor C, such that an output voltage of VMID2 is obtained, neglecting any voltage drop on T2 and D2. Pulse 502 may be obtained through level shifter 301.
Between t3 and t4, pulse 500 may be increased from VMID2 to VDD by providing a negative pulse 501 to transistor T1 through VG1 as shown in FIG. 5. FIG. 4C illustrates pulser 201 between t3 and t4. During this period, the gate of transistor T1 may be driven by a voltage equal to VDD−ΔV. ΔV may be chosen so as to create a conductive channel and cause transistor T1 to drive a current between the source and the drain. Such current may charge capacitor C, such that an output voltage of VDD is obtained, neglecting any voltage drop on T1. Pulse 501 may be obtained through level shifter 301.
Between t4 and t5, pulse 500 may be decreased from VDD to VMID2 by providing a positive pulse 503 to transistor T3 through VG3 as shown in FIG. 5. FIG. 4D illustrates pulser 201 between t4 and t5. During this period, the gate of transistor T3 may be driven by a voltage equal to VMID2+ΔV. ΔV may be chosen so as to create a conductive channel and cause transistor T3 to drive a current between the drain and the source. Such current may discharge capacitor C, such that an output voltage of VMID2 is obtained, neglecting any voltage drop on T3 and D3. Pulse 503 may be obtained through level shifter 302.
Between t5 and t6, pulse 500 may be decreased from VMID2 to VMID3 by providing a positive pulse 505 to transistor T5 through VG5 as shown in FIG. 5. FIG. 4E illustrates pulser 201 between t5 and t6. During this period, the gate of transistor T5 may be driven by a voltage equal to VMID3+ΔV. ΔV may be chosen so as to create a conductive channel and cause transistor T5 to drive a current between the drain and the source. Such current may discharge capacitor C, such that an output voltage of VMID3 is obtained, neglecting any voltage drop on T5 and D5. Pulse 505 may be obtained through level shifter 302.
After t6, pulse 500 may be decreased from VMID3 to 0 by providing a positive pulse 506 to transistor T6 through VG6 as shown in FIG. 5. FIG. 4F illustrates pulser 201 after t6. During this period, the gate of transistor T6 may be driven by a voltage equal to ΔV. ΔV may be chosen so as to create a conductive channel and cause transistor T6 to drive a current between the drain and the source. Such current may discharge capacitor C, such that an output voltage of 0 is obtained, neglecting any voltage drop on T6. Pulse 506 may be obtained through level shifter 302.
In the non-limiting example in connection to FIG. 5, pulse 500 is unipolar. However, multi-level pulser 200 in not limited in this respect. Multi-level pulser 200 may alternatively be configured to transmit bipolar pulses exhibiting levels having positive and negative voltages. In accordance with another aspect of the present application, the multi-level pulser 200 may be considered a multi-level charge recycling waveform generator in that charge recycling occurs on the decrementing step as charge is transferred from the output capacitance back into the power supply. In accordance with another aspect of the present application, although the multi-level pulser has been described as being used to drive a capacitive output, it may also be used to drive a resistive output.
The amount of power saving when using a level shifter of the types described herein may be significant. In some embodiments, utilizing a level shifter of the types described herein may provide substantial power saving by setting the static power consumption to approximately zero. Accordingly, power may be dissipated only during switching states.
Having thus described several aspects and embodiments of the technology of this application, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those of ordinary skill in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology described in the application. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described.
As described, some aspects may be embodied as one or more methods. The acts performed as part of the method(s) may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.
All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and/or ordinary meanings of the defined terms.
The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases.
As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements.
As used herein, the term “between” used in a numerical context is to be inclusive unless indicated otherwise. For example, “between A and B” includes A and B unless indicated otherwise.
In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively.

## Claims

1. An apparatus, comprising:
at least one ultrasonic transducer on a substrate; and
a multi-level pulser on the substrate coupled to the at least one ultrasonic transducer; the multi-level pulser including
a plurality of input terminals configured to receive respective input voltages;
an output terminal configured to provide an output voltage; and
a signal path between a first input terminal of the plurality of input terminals and the output terminal including a first transistor coupled to a first diode and, in parallel, a second transistor coupled to a second diode,
wherein the first transistor is either a pMOS transistor or an nMOS transistor and the second transistor is an NMOS transistor if the first transistor is a pMOS transistor or a pMOS transistor if the first transistor is an nMOS transistor.

2. The apparatus of claim 1, further comprising a controller configured to control a charge and discharge of an output capacitance associated with the ultrasonic transducer so as to provide charge recycling.

3. The apparatus of claim 1, wherein the multi-level pulser comprises a plurality of signal paths between respective input terminals of the plurality of input terminals and the output terminal, each signal path including a pMOS or nMOS transistor coupled to a first diode and, in parallel, a pMOS or nMOS transistor coupled to a second diode, wherein the pMOS or nMOS transistor coupled to the second diode is an nMOS transistor when the pMOS or nMOS transistor coupled to the first diode is a pMOS transistor, and wherein the pMOS or nMOS transistor coupled to the second diode is a pMOS transistor when the pMOS or nMOS transistor coupled to the first diode is an nMOS transistor.

4. The apparatus of claim 1, wherein the output voltage is equal to a predetermined input voltage.

5. The apparatus of claim 1, further comprising a capacitor coupled to the output terminal.

6. The apparatus of claim 1, further comprising a resistor coupled to the output terminal.

7. The apparatus of claim 1, wherein the first transistor is pMOS and the second transistor is nMOS.

8. The apparatus of claim 1, wherein the first diode has an anode connected to the first transistor and a cathode connected to the output terminal.

9. The apparatus of claim 1, wherein the second diode has a cathode connected to the second transistor and an anode connected to the output terminal.

10. A multi-level pulser configured to be coupled to an ultrasonic transducer, comprising:
a plurality of input terminals configured to receive respective input voltages;
an output terminal configured to provide an output voltage;
a signal path between a first input terminal of the plurality of input terminals and the output terminal including a first transistor coupled to a first diode and, in parallel, a second transistor coupled to a second diode, wherein the first transistor is either a pMOS transistor or an nMOS transistor and the second transistor is an NMOS transistor if the first transistor is a pMOS transistor or a pMOS transistor if the first transistor is an nMOS transistor; and
a capacitor coupled to the output terminal.

11. The multi-level pulser of claim 10, further comprising a controller configured to control a charge and discharge of the capacitor coupled to the output terminal so as to provide charge recycling.

12. The multi-level pulser of claim 10, comprising a plurality of signal paths between respective input terminals of the plurality of input terminals and the output terminal, each signal path including a pMOS or nMOS transistor coupled to a first diode and, in parallel, a pMOS or nMOS transistor coupled to a second diode, wherein the pMOS or nMOS transistor coupled to the second diode is an nMOS transistor when the pMOS or nMOS transistor coupled to the first diode is a pMOS transistor, and wherein the pMOS or nMOS transistor coupled to the second diode is a pMOS transistor when the pMOS or nMOS transistor coupled to the first diode is an nMOS transistor.

13. The multi-level pulser of claim 10, wherein the output voltage is equal to a predetermined input voltage.

14. The multi-level pulser of claim 10, further comprising a resistor coupled to the output terminal.

15. The multi-level pulser of claim 10, wherein the first transistor is pMOS and the second transistor is nMOS.

16. The multi-level pulser of claim 10, wherein the first diode has an anode connected to the first transistor and a cathode connected to the output terminal.

17. The multi-level pulser of claim 10, wherein the second diode has a cathode connected to the second transistor and an anode connected to the output terminal.