Abstract:
A system and method for providing a high voltage ultrasonic drive signal from an ultrasound transmitter are disclosed herein. An ultrasound transmitter includes a first driver and a bias network. The first driver includes a first plurality of drive transistors that when activated drive an ultrasound transmitter output to a first voltage. The first bias network is coupled to the first plurality of drive transistors, and, at least in part, controls distribution, across the drive transistors, of voltage at the ultrasound transmitter output. Control inputs of the first driver are decoupled from the ultrasound transmitter output.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application contains subject matter that may be related to U.S. patent application Ser. No. ______, entitled “Low Power Continuous Wave Ultrasound Transmitter” [attorney docket TI-66536 (1962-54400)], U.S. patent application Ser. No. ______, entitled “Ultrasound Transmitter” [attorney docket TI-66537 (1962-54500)], and U.S. patent application Ser. No. ______, entitled “Ultrasound Transmitter” [attorney docket TI-66538 (1962-54600)]. 
     
    
     BACKGROUND 
       [0002]    Ultrasonic imaging has become a widely used tool in medical applications. Ultrasound techniques introduce high-frequency acoustic waves into a subject&#39;s body. The received echoes of those waves provide information allowing a trained observer to view the subject&#39;s internal organs. Ultrasound imaging equipment uses transducers that convert electrical energy into acoustic energy. Piezo-electric crystals are one commonly used type of electrical to acoustical transducer. To obtain a clear image, a high signal to noise ratio is desirable to overcome random noise associated with the imaging process. One way to increase the signal-to-noise ratio is to increase the amplitude of the signal driving the transducer. Generally, the transducer drive signal may require voltages in the range of ±75 volts to ±100 volts. 
         [0003]    There are two broad categories of ultrasound transmitters, digital and analog. The analog type takes a signal generated digitally and after being converted to analog form, by a digital-to-analog converter, the signal is amplified to the required higher voltage by a power amplifier. This type of transmitter is capable of generating complex waveforms by using a high-resolution digital-to-analog converter with a resolution of, for example, 12 bits. This technique is expensive and finds application in high-end ultrasound imaging systems. 
         [0004]    Digital transmitters are simpler and less expensive than analog transmitters. Unfortunately, the semiconductor process technologies used to fabricate digital circuits, which are often less expensive and provide better performance than high voltage processes, do not typically accommodate the high voltages required to produce an acceptable signal-to-noise ratio in an ultrasound imager. Furthermore, users of ultrasound imaging systems demand both power efficiency and portability in modern ultrasound equipment. 
       SUMMARY 
       [0005]    Various systems and methods for implementing a high-voltage ultrasound transmitter are disclosed herein. In accordance with at least some embodiments, an ultrasound transmitter includes a first driver and a first bias network. The first driver includes a first plurality of drive transistors that when activated drive an ultrasound transmitter output to a first voltage. The first bias network is coupled to the first plurality of drive transistors. The first bias network, controls, at least in part, distribution across the drive transistors of voltage at the ultrasound transmitter output. Control inputs of the first driver are decoupled from the ultrasound transmitter output. 
         [0006]    In accordance with at least some other embodiments, an ultrasound imaging system includes a receiver and a transmitter. The receiver receives detected ultrasonic signals. The transmitter drives a transducer that converts electrical signals into acoustic signals. The transmitter includes a high-side driver, a low-side driver, and a bias network. The high-side driver drives a transmitter output to a positive voltage. The low-side driver drives the transmitter output to a negative voltage. The bias network substantially equalizes the voltage across drive transistors of each of the high-side driver and the low-side driver. A node of the bias network is connected to ground. 
         [0007]    In accordance with yet other embodiments, an ultrasound transmitter includes a plurality of drive transistors that drive a transmitter output. The transmitter further includes means for decoupling the control inputs of the plurality of drive transistors from the transmitter output. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    For a detailed description of exemplary embodiments of the invention, reference will now be made to the accompanying drawings in which: 
           [0009]      FIG. 1  shows a block diagram of an exemplary ultrasound imaging system in accordance with various embodiments; 
           [0010]      FIG. 2  shows an exemplary ultrasound transmitter circuit that employs a symmetrical output configuration and provides fast output signal zeroing without a clamp circuit in accordance with various embodiments; and 
           [0011]      FIG. 3  shows a flow diagram for a method for generating a high voltage ultrasonic drive signal in accordance with various embodiments. 
       
    
    
     NOTATION AND NOMENCLATURE 
       [0012]    Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” or “couples” is intended to mean either an indirect or direct electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
       DETAILED DESCRIPTION 
       [0013]    The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment. 
         [0014]    The performance and cost efficiency of low-voltage semiconductor processes make it desirable to use those processes to implement high-voltage circuits. High-voltage circuits can be so implemented by connecting transistors (e.g., field effect transistors (“FETs”)) in series (i.e., stacked), and in such a way as to ensure that the voltage across the transistors is distributed in a predictable manner. If transistors are stacked without considering voltage distribution, it may be possible for the voltage across an individual transistor to exceed the process specification. Moreover, a bias network that achieves predictable voltage distribution can result in undesirable power dissipation and/or poor switching performance. Drivers employing stacked transistors can also suffer from an undesirable lack of output symmetry due to process and/or temperature variations. Further, to reduce power dissipation, a low on resistance is needed, requiring the stacked transistors to be large. 
         [0015]    Moreover, in order to produce a compact ultrasound imaging system and a low cost, it is desirable to reduce the area consumed by the ultrasonic transmitters. Embodiments of the present disclosure employ symmetrically configured high-side and low-side drivers wherein each driver includes serially connected complementary transistors (i.e., a N-type transistor in series with a P-type transistor in each of the high and low side drivers). Additionally, embodiments of the present disclosure are configured to quickly return the transmitter output to zero when the transmitter is disabled. Embodiments achieve quick return to zero while avoiding a dedicated clamping circuit. Such features advantageously reduce circuit area, provide improved performance across process and temperature, and result in reduced transmitter quiescent power dissipation. 
         [0016]      FIG. 1  shows a block diagram of an exemplary ultrasound imaging system  100  in accordance with various embodiments. The terms “ultrasound” or “ultrasonic” generally refer to acoustic waves at frequencies beyond the range of human hearing (e.g., frequencies above 20 KHz). The system  100  comprises a transducer  102 , a transmitter  104 , a receiver  106 , a signal processor  108 , and a display  110 . The transducer  102  converts the electrical drive signals generated by the transmitter  104  into sound waves (i.e., pressure waves) that are introduced into the subject to be imaged, for example, a human body when considering medical ultrasound. The transducer  102  can comprise a piezoelectric crystal, electromagnetic transducer, micro-electro-mechanical system (“MEMS”) transducer or other device that converts an electrical signal into sound waves. Moreover, the transducer  102  can comprise one or more transducer elements. The transducer  102  also detects ultrasonic waves reflected by internal structures of the subject and converts the detected waves into electrical signals. In some embodiments, the same transducer elements are used to generate ultrasonic waves and to detect ultrasonic waves. In other embodiments, separate transducer elements are used for wave generation and detection. 
         [0017]    The transmitter  104  is coupled to the transducer  102 . The transmitter  104  produces an oscillating electrical signal at a frequency and amplitude suitable for imaging desired structures internal to the subject. For example, transmitter output signals for use in imaging the internal organs of a human body may range in frequency from 1 to 20 megahertz with lower frequencies providing lower resolution and greater imaging depth. Other applications may use different frequencies. The transmitter  104 , while not limited to any particular signal amplitudes, may provide, for example, a drive signal amplitude in the range of ±75 volts. The transmitter  104  employed in embodiments of the present disclosure advantageously uses transmitter circuitry that allows for efficient implementation of a high voltage ultrasonic driver on a low voltage semiconductor process, while reducing circuit area and power dissipation, and improving performance across temperature and process variation. Embodiments employ a variety of novel means to reduce circuit area, for example, embodiments of the present disclosure are configured to eliminate the need for a dedicated transmitter output clamping circuit. 
         [0018]    The receiver  106  is coupled to the transducer  102 . As explained above, the transducer  102  detects ultrasonic waves reflected by subject internal structures. The transducer  102  converts the detected waves into electrical signals. The electrical signals are provided to the receiver  106 . The receiver  106  performs initial processing of the received signals. Processing performed by the receiver  106  can comprise, for example, amplifying, filtering, digitizing, etc. 
         [0019]    The signal processor  108  is coupled to the receiver  106 . The signal processor  108  may, for example, provide post-digitization filtering of received signals, detect signal reflections, and prepare output signals for display on the display  110 . The signal processor  108  may comprise, for example, a digital signal processor or other microprocessor or microcomputer and associated software programming along with attendant memory and interface devices, or dedicated hardware circuitry adapted to perform the processing functions. The display  110  may be a liquid crystal display, a cathode ray display, or any other suitable display device. 
         [0020]      FIG. 2  shows an exemplary ultrasound transmitter circuit  200  that employs a symmetrical output configuration and provides fast output signal zeroing without an output clamping circuit in accordance with various embodiments. The transmitter  200  is configured to provide symmetry between the high side and the low side of the transmitter  200  output circuitry. Accordingly, the high side driver and the low side driver each include both a P-type transistor and an N-type transistor. The high-side driver comprises N-type transistor MN_HIGH  202  and P-type transistor MP_HIGH  204  connected in series, and the low-side driver comprises N-type transistor MN_LOW  206  and P-type transistor MP_LOW  208  connected in series. When enabled, stacked drive transistors MN_HIGH  202  and MP_HIGH  204  provide high voltage, +HV, to the transmitter output  238 . Similarly, stacked drive transistors MN_LOW  206  and MP_LOW  208  provide high voltage, −HV, to the transmitter output  238  when enabled. 
         [0021]    The stacked N-type and P-type transistors employed by embodiments of the present disclosure provide a number of advantages. Using an N-type transistor in series with a P-type transistor on both the high and low sides compensates for the lower mobility of the P-type transistor, and results in a smaller circuit area (for example, 15-20% less area) than would be required by an embodiment employing only P-type transistors on one side. Because the characteristics of one transistor type compensate for the characteristics of the other, high/low side symmetry also results in significantly improved performance in less than nominal conditions, for example, at temperature extremes or at process limits. Symmetry can provide a substantial improvement in the harmonic distortion present in the output signal. For example, at the process limits, symmetry can result in as much as a 25% reduction in the second harmonic content of the output with respect to the fundamental when compared to an asymmetrical embodiment. 
         [0022]    As explained above, voltage should be predictably distributed across each transistor of a set of stacked transistors. The bias network comprising resistors R 0   210 , R 1   212 , R 2   214 , and R 3   216  ensures that voltage is approximately equally distributed across each transistor of transistor pair MN_HIGH  202  and MP_HIGH  204 , and each transistor of transistor pair MN_LOW  206  and MP_LOW  208  to assure that the breakdown voltage of the transistors is not exceeded. In some embodiments, for example, the voltage drop across a selected drive transistor may be within 10% of the voltage drop across the other drive transistor of the transistor pair. 
         [0023]    As shown in  FIG. 2 , in embodiments of the present disclosure, the center node  226  of the bias network is preferably grounded. By grounding node  226 , embodiments of the present disclosure advantageously decouple the gate drive of transistors MP_HIGH  204  and MN_LOW  206  from the transmitter output  238 . Thus, the gates of drive transistors MP_HIGH  204  and MN_LOW  206  are not affected by voltage present on the output  238 , and consequently are able to respond more quickly than possible in an embodiment that connects node  226  to transmitter output  238 , or couples the control inputs of drive transistors MP_HIGH  204  and MN_LOW  206  in some fashion. The time required for the drive transistors MP_HIGH  204  and MN_LOW  206  to discharge voltage on the nodes  228  and  230  respectively, is one factor determining how quickly the transmitter output  238  returns to zero when transmitter  200  drive is disabled. If MP_HIGH  204  and MN_LOW  206  are slow to discharge nodes  228  and  230 , then the voltage on the transmitter output  238  may require a considerable period of time to decay to zero. While connecting node  226  to the transmitter output  238  may help equalize voltage across the bias network, it also serves to degrade gate drive to transistors MP_HIGH  204  and MN_LOW  206  by causing the gates to follow the output  238 . By connecting bias network node  226  to ground, embodiments of the present disclosure provide fast discharge of nodes  228  and  230  by the drive transistors MP_HIGH  204  and MN_LOW  206  through transducer  102 . Thus, embodiments of the present disclosure can exclude a dedicated transmitter output  238  clamping circuit as required by other embodiments. Consequently, embodiments of the present disclosure advantageously reduce transmitter circuit area and associated cost. 
         [0024]    In ultrasound applications, the duty cycle of the transmitter  200  can be low (i.e., the transmitter on time is short relative to the transmitter off time). For example, the transmitter  200  duty cycle may be in the range of 1% (i.e., on 1% of the time and off 99% of the time), so that even though the drive transistors  202 ,  204 ,  206 ,  208  may conduct a relatively large amount of current, the large amount of current is required for only a short period of time. 
         [0025]    The high-side driver, comprising MN_HIGH  202  and MP_HIGH  204 , is enabled to provide voltage HV to output  238  by asserting signal T 1   218  (i.e., bringing the T 1   218  signal voltage near HV). Similarly, the low side driver, comprising MN_LOW  206  and MP_LOW  208 , is enabled to provide voltage −HV to output  238  by asserting signal T 2   220  (i.e., bringing the T 2   220  signal voltage near −HV). Either of the high-side or the low-side drivers can be disabled by bringing the corresponding control signal (T 1   218  or T 2   220 ) near to ground. Thus, when both the high and low side drivers are disabled, the voltages present on T 1   218  and T 2   220  are preferably approximately at ground. Consequently, the voltage drop across the bias network comprising R 0   210 , R 1   212 , R 2   214 , and R 3   216  can be zero or very small when the transmitter  200  is disabled. By way of contrast, the current flowing in the bias network of a disabled asymmetrical output driver can be on the order of 10 milli-amperes. Such a reduction in quiescent current is significant when the 1% duty cycle of the ultrasound transmitter  200  is considered. 
         [0026]    The drive transistors, for example MP_HIGH  204  and MN_LOW  206 , can be very large to achieve a low on resistance. Correspondingly, the gate capacitance of large field effect transistors (“FETs”) can also be very large. Transmitter  200  preferably comprises buffer drivers  222 ,  224  to drive the gates of drive transistors MP_HIGH  204  and MN_LOW  206  respectively. The buffer drivers  222 ,  224  provide current suitable to enable fast switching of the drive transistors MP_HIGH  204  and MN_LOW  206 . In some embodiments, the buffers  222 ,  224  are source followers. Ultrasound transmitter embodiments not incorporating buffer drivers  222 ,  224  suffer from slower switching of the drive transistors MP_HIGH  204  and MN_LOW  206  and consequently may not provide ultrasonic drive signals at frequencies as high as those produced by embodiments of the present disclosure. 
         [0027]    The input capacitance of the buffers  222 ,  224  is preferably substantially lower than the gate capacitance of the drive transistors MP_HIGH  204  and MN_LOW  206 , for example, in some embodiments by approximately a factor of 20 or more. Consequently, in embodiments of the present disclosure, the values of resistors R 0 -R 3   210 - 216  can preferably be 20 times larger than in an embodiment without the drivers  222 ,  224 . Thus, the current flowing through bias resistors R 0 -R 3   210 - 216  when the transmitter  200  is active can be 20 times lower than in an embodiment omitting the buffer drivers  222 ,  224 . 
         [0028]    An ultrasonic drive signal is generated by transmitter  200  as follows. MN_HIGH  202  and MP_HIGH  204  are turned on and MN_LOW  206  and MP_LOW  208  are turned off to drive the output  238  to +HV. MN_HIGH  202  and MP_HIGH  204  are turned off and MN_LOW  206  and MP_LOW  208  are turned on to drive the output  238  to −HV. Thus, the high and low side drivers are alternately turned on and off at the desired frequency to generate an ultrasonic drive signal on the transmitter output  238 . During intervals when no ultrasonic drive signal is being generated, the high and low side drivers are disabled, and the transmitter output  238  returned to zero by bringing T 1   218  and T 2   220  to ground. In at least some embodiments, the output  238  is returned to zero for an interval between when one driver polarity is disabled and the other is enabled by disabling both drivers for the interval. In some embodiments, a single polarity output is generated by repetitively enabling and disabling a single polarity of driver. For example, by repetitively enabling and disabling MN_HIGH  202  and MP_HIGH  204 , and holding MN_LOW  206  and MP_LOW  208  disabled, only positive voltage pulses are generated. 
         [0029]      FIG. 3  shows a flow diagram for a method for generating a high voltage ultrasonic drive signal in accordance with various embodiments. Though depicted sequentially as a matter of convenience, at least some of the actions shown can be performed in a different order and/or performed in parallel. Additionally, some embodiments may perform only some of the actions shown. In block  302 , the transmitter  200  is producing no ultrasonic drive signal. Consequently, the high-voltage drive transistors MN_HIGH  202 , MP_HIGH  204 , MN_LOW  206 , and MP_LOW  208  are turned off. 
         [0030]    If transducer drive is requested, in block  304 , then generation of the drive signal proceeds in block  306  where the positive portion of the high-voltage ultrasonic drive signal is generated. +HV drive is enabled by turning on high-side drive transistors MN_HIGH  202  and MP_HIGH  204 , and turning off low-side drive transistors MN_LOW  206 , and MP_LOW  208 . 
         [0031]    The negative portion of the high-voltage ultrasonic drive signal is generated in block  308 , where −HV drive is enabled by turning on drive transistors MN_LOW  206  and MP_LOW  208 , and turning off high-side drive transistors MN_HIGH  202  and MP_HIGH  204 . 
         [0032]    Embodiments may repetitively perform the operations of blocks  306  and  308  to generate any number of cycles of a high-voltage ultrasonic drive signal. Some embodiments generate a drive signal of a single polarity by omitting the operations of one of blocks  306  and  308  to produce an output signal alternating between ground and either −HV or +HV. In some embodiments, the operation of block  310  is performed between blocks  306  and  308  to provide a zero output between positive and negative drive signals. 
         [0033]    In block  310 , the desired number of high-voltage cycles have been generated and ultrasonic drive is not required for at least a predetermined time period. The drive transistors MN_HIGH  202 , MP_HIGH  204 , MN_LOW  206 , and MP_LOW  208  are turned off to disable high-voltage drive onto the transmitter output  238 , and to advantageously reduce transmitter  200  quiescent power consumption. As explained above the duty cycle of the high voltage transmitter may be approximately 1% in some embodiments, thus reducing current flow in the bias resistors R 0 -R 3   210 - 216  when the transmitter  200  is disabled can result in substantial power reduction. Moreover, in embodiments of the present disclosure, disabling the transmitter  200  causes the transmitter output  238  to quickly return to zero without aid of a dedicated clamping circuit. 
         [0034]    If, in block  312 , transducer drive is to be continued, that is, another ultrasonic signal burst is required, then after a predetermined time delay, in block  314 , signal generation continues in block  306  as described above. 
         [0035]    The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.