U.S. patent application number 12/261209 was filed with the patent office on 2010-05-06 for ultrasound transmitter.
This patent application is currently assigned to TEXAS INSTRUMENTS INCORPORATED. Invention is credited to Myron J. KOEN, Ismail H. OGUZMAN.
Application Number | 20100113925 12/261209 |
Document ID | / |
Family ID | 42112466 |
Filed Date | 2010-05-06 |
United States Patent
Application |
20100113925 |
Kind Code |
A1 |
KOEN; Myron J. ; et
al. |
May 6, 2010 |
ULTRASOUND TRANSMITTER
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 plurality of drive
transistors. A bias network is coupled to at least one transistor
of the first plurality of drive transistors. A first switch is
coupled to the bias network. The first switch selectively connects
a first voltage to the bias network. The first switch is closed
when generating an ultrasonic drive signal. The first switch is
open when the transmitter is not generating an ultrasonic drive
signal.
Inventors: |
KOEN; Myron J.; (Tucson,
AZ) ; OGUZMAN; Ismail H.; (Plano, TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
US
|
Assignee: |
TEXAS INSTRUMENTS
INCORPORATED
Dallas
TX
|
Family ID: |
42112466 |
Appl. No.: |
12/261209 |
Filed: |
October 30, 2008 |
Current U.S.
Class: |
600/437 ;
310/311 |
Current CPC
Class: |
B06B 1/0215
20130101 |
Class at
Publication: |
600/437 ;
310/311 |
International
Class: |
A61B 8/02 20060101
A61B008/02 |
Claims
1. An ultrasound transmitter, comprising: a first plurality of
drive transistors; a bias network coupled to at least one
transistor of the first plurality of drive transistors; and a first
switch coupled to the bias network, the first switch selectively
connects a first voltage to the bias network; wherein the first
switch is closed when generating an ultrasonic drive signal.
2. The ultrasound transmitter of claim 1, further comprising: a
second plurality of drive transistors; and a second switch coupled
to the bias network, the second switch selectively connects a
second voltage to the bias network; wherein at least one transistor
of the second plurality of drive transistors is coupled to the bias
network and the second switch is closed when generating the
ultrasonic drive signal.
3. The ultrasound transmitter of claim 2, further comprising a
first driver that couples the bias network to the at least one
transistor of the second plurality of drive transistors.
4. The ultrasound transmitter of claim 1, wherein the first switch
is open when the transmitter is not generating an ultrasonic drive
signal.
5. The ultrasound transmitter of claim 1, further comprising a
second driver that couples the bias network to the at least one
transistor of the first plurality of drive transistors.
6. The ultrasound transmitter of claim 1, further comprising a
clamping circuit that shunts a transmitter output node to ground
when the first switch is open.
7. A method, comprising: closing a first switch that connects a
first power supply voltage to an ultrasound driver bias network;
generating a bias voltage in the bias network that substantially
equalizes the voltage drop across a plurality of drive transistors;
and generating an ultrasonic drive signal.
8. The method of claim 7, further comprising closing a second
switch that connects a second power supply to the bias network.
9. The method of claim 7, further comprising driving at least one
transistor of the plurality of drive transistors with a driver
controlled by a first voltage produced by the bias network.
10. The method of claim 7, further comprising driving at least two
transistors of the plurality of drive transistors each with a
different driver controlled by a different voltage produced by the
bias network.
11. The method of claim 7, further comprising clamping an
ultrasonic transmitter output node to ground when the first switch
is open.
12. The method of claim 7, further comprising opening the first
switch when ultrasonic drive signal generation is disabled.
13. An ultrasound transmitter, comprising: a plurality of drive
transistors; a bias network that substantially equalizes the
voltages dropped across each of the plurality of drive transistors;
and a first driver that drives at least a first transistor of the
plurality of drive transistors; wherein the first driver provides a
buffered version of a first voltage generated by the bias network
to at least the first transistor of the plurality of drive
transistors.
14. The ultrasound transmitter of claim 13, further comprising a
second driver that drives at least a second transistor of the
plurality of drive transistors, wherein the second driver provides
a buffered version of a second voltage generated by the bias
network to at least the second transistor of the plurality of drive
transistors.
15. The ultrasound transmitter of claim 13, further comprising a
first switch that connects a first power supply voltage to the bias
network.
16. The ultrasound transmitter of claim 15, further comprising a
second switch that connects a second power supply voltage to the
bias network.
17. The ultrasound transmitter of claim 15, wherein the first
switch is open when the transmitter is not generating an ultrasonic
drive signal.
18. The ultrasound transmitter of claim 13, wherein the first
driver isolates the bias network from the input capacitance of at
least the first transistor of the plurality of drive
transistors.
19. The ultrasound transmitter of claim 13, wherein opening the
first switch reduces the power dissipated in the bias network.
20. The ultrasound transmitter of claim 13, further comprising a
clamping circuit that shunts a transmitter output node to ground
when the first switch is open.
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-66538 (1962-54600)],
and U.S. patent application Ser. No. ______, entitled "Ultrasound
Transmitter" [attorney docket TI-66539 (1962-54700)].
BACKGROUND
[0002] Ultrasonic imaging has become a widely used tool in medical
applications. Ultrasound techniques introduce high-frequency
acoustic waves into a subject's body. The received echoes of those
waves provide information allowing a trained observer to view the
subject'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 do not typically
accommodate the high voltages required to produce an acceptable
signal-to-noise ratio in an ultrasound imager. Moreover, lower
voltage processes are often faster and less expensive. Thus, an
ultrasound transmitter compatible with low-voltage semiconductor
processes is desirable.
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
plurality of drive transistors. A bias network is coupled to at
least one transistor of the first plurality of drive transistors. A
first switch is coupled to the bias network. The first switch
selectively connects a first voltage to the bias network. The first
switch is closed when generating an ultrasonic drive signal.
[0006] In accordance with at least some other embodiments, a method
includes closing a first switch that connects a first power supply
voltage to an ultrasound driver bias network. The bias network
generates a bias voltage that substantially equalizes the voltage
drop across a plurality of drive transistors.
[0007] In accordance with yet other embodiments, an ultrasound
transmitter includes a plurality of drive transistors. A bias
network substantially equalizes the voltages dropped across each of
the plurality of drive transistors. A first driver drives at least
a first transistor of the plurality of drive transistors. The first
driver provides a buffered version of a first voltage generated by
the bias network to at least the first transistor of the plurality
of drive transistors.
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 provides a high voltage output with reduced power dissipation
in accordance with various embodiments;
[0011] FIG. 3 shows a diagram of various signals produced when
generating high voltage ultrasonic drive signals in accordance with
various embodiments; and
[0012] FIG. 4 shows a flow diagram for a method for generating high
voltage ultrasonic drive signals in accordance with various
embodiments.
NOTATION AND NOMENCLATURE
[0013] 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
[0014] 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.
[0015] 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. Embodiments of the present
disclosure switch power to the bias network of a stacked transistor
driver to reduce dissipation and employ drivers to buffer the bias
voltages provided to the drive transistors, thus improving
transmitter performance and advantageously reducing bias network
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 generating acoustic waves
(i.e., an ultrasonic drive signal) useful 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 from 1 to 20 megahertz with lower frequencies providing lower
resolution and greater imaging depth. 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 power dissipation and
improving switching performance.
[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 further 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 exemplary ultrasound transmitter circuitry 200
that provides high-voltage ultrasonic drive signals while reducing
power dissipation and enhanced switching performance. The
transmitter 200 comprises drive transistors Q1 220, Q2 218, Q3 216,
and Q4 214. When enabled, stacked drive transistors Q1 220 and Q2
218 conduct high voltage, +HV, onto the transmitter output 238.
Similarly, stacked drive transistors Q3 216 and Q4 214 conduct high
voltage, -HV, onto the transmitter output 238 when enabled. As
explained above, voltage should be predictably distributed across
each transistor of a set of stacked transistors. The bias network
comprising resistors R1 230, R2 232, R3 234, and R4 236 ensures
that voltage is approximately equally distributed across each
transistor of transistor pair Q1 220 and Q2 218, and each
transistor of transistor pair Q3 216, and Q4 214 to assure that the
breakdown voltage of the transistors is not exceeded. In some
embodiments, R1 230, R2 232, R3 234, and R4 236 are of
approximately equal value. 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.
[0021] 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
214, 216, 218, 220 may conduct a relatively large amount of
current, the large amount of current is required for only a short
period of time.
[0022] Transmitter 200 preferably comprises transistor switches Q10
202 and Q5 204 coupled in series with the resistors R1-R4 230-236
to connect voltages +HV and -HV to the bias resistor network. When
the transmitter 200 is inactive (i.e., no ultrasonic drive signal
is being generated), the switches Q10 202 and Q5 204 are open.
Thus, if the transmitter 200 has a 1% duty cycle, then by opening
switches Q10 202 and Q5 204 when no drive signal is required (e.g.,
99% of the time) no current flows through the bias resistors R1-R4
230-236 resulting in a substantial reduction in transmitter 200
quiescent current.
[0023] The drive transistors, for example Q1 220 and Q2 218, 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 comprises buffer drivers 240,
242 to drive the gates of drive transistors Q2 218 and Q3 216
respectively. As shown in the illustrative embodiment of FIG. 2,
buffer driver 240 can comprise complementary transistors Q8 208 and
Q9 206, and buffer driver 242 can comprise complementary
transistors Q6 212 and Q7 210. The buffer drivers 240, 242 provide
current suitable to enable fast switching of the drive transistors
Q2 218 and Q3 216. Ultrasound transmitter embodiments not
incorporating drivers 240, 242 suffer from slower switching of the
drive transistors Q2 218 and Q3 216 and consequently may not
provide ultrasonic drive signals at frequencies as high as those
produced by embodiments of the present disclosure.
[0024] The input capacitance of the buffers 240, 242 preferably is
substantially lower than the gate capacitance of the drive
transistors Q2 218 and Q3 216, for example, in some embodiments by
approximately a factor of 20 or more. Consequently, in embodiments
of the present disclosure, the values of resistors R1-R4 230-236
can be, for example, 20 times larger than in an embodiment without
the drivers 240, 242. Furthermore, embodiments of the present
disclosure allow for a reduction in the size of the switches Q5
204, Q10 202 because the switches Q5 204, Q10 202 need not source
as much current to the bias network.
[0025] Transistors Q11 224, Q12 222, and diodes 226, 228 are part
of a clamping circuit that, when enabled, shunts the transmitter
output 238 to ground. In some embodiments, the clamping circuit is
enabled when the transmitter 200 is not generating ultrasonic drive
signals.
[0026] An ultrasonic drive signal is generated by the illustrative
transmitter 200 as follows. The output clamp of the illustrative
embodiment is disabled by turning off transistors Q11 224 and Q12
222. Switches Q5 204 and Q10 202 are closed to connect the +HV and
-HV voltages to the bias network comprising resistors R1-R4
230-236, thus preferably biasing the stacked drive transistor pairs
Q1 220, Q2 218 and Q3 216, Q4 214 to switch +HV and -HV. Q1 220 is
turned on and Q4 214 is turned off to drive the output 238 to +HV.
Q1 220 is turned off and Q4 214 is turned on to drive the output
238 to -HV. Thus, embodiments alternately turn Q1 220 and Q4 214 on
and off at the desired frequency to generate an ultrasonic drive
signal on output 238. Some embodiments activate the output clamp
(transistors Q11 224 and Q12 222) between deactivation of Q1 220
and activation of Q4, and vice versa, to clamp the output 238 to
ground between half-cycles. Some embodiments generate pulses of one
polarity by repetitively enabling and disabling only one of Q1 220
and Q4 214 with clamping (at least one of Q11 224 and Q12 222
turned on) during the disabled intervals. During intervals when no
ultrasonic drive signal is being generated, embodiments shunt the
output 238 to ground by turning on transistors Q11 224 and Q12 222,
and transmitter quiescent current is preferably reduced by opening
bias network switches Q5 204 and Q10 202 in accordance with at
least some embodiments.
[0027] FIG. 3 shows a diagram of various signals produced when
generating high voltage ultrasonic drive signals in accordance with
various embodiments. The diagram begins, in period 302, with the
transmitter driver 200 in shunt mode where embodiments clamp the
output 238 to ground through diodes 226, 228 and transistors Q11
224 and Q12 222. Signals T5 and T6 are asserted to enable
transistors Q12 222 and Q11 224 respectively. The signals T3 and T4
are preferably negated to maintain switches Q10 202 and Q5 204 in
an open state to reduce transmitter 200 quiescent current. Because
no drive signals are being generated, the signals T1 and T2 are
negated, disabling drive transistors Q1 220 and Q4 214. Note that
the states of signals T3 and T4 may be the same as those of signals
T6 and T5 respectively, but the amplitudes of T3 and T4 may differ
from the amplitudes of T6 and T5 because the voltage levels
required to drive the switches Q10 202 and Q5 204 can differ from
the voltages required to drive the clamp transistors Q11 224 and
Q12 222.
[0028] Generation of a high voltage ultrasonic drive signal is
shown in period 304. To produce the high voltage signal on the
output 238, embodiments turn off the shunt transistors Q11 224 and
Q12 222 by negating T6 and T5 as illustrated. Further, the bias
network switches Q5 204 and Q10 202 are closed to provide voltage
(e.g., .+-.HV) to the bias network by asserting T4 and T3 as shown.
Thereafter, embodiments toggle signals T1 and T2 as shown to
alternately turn on and off high side drive transistor Q1 220 and
low side drive transistor Q4 214 so that the output 238 is
alternately driven near to .+-.HV (some voltage is dropped across
the driving transistors). As illustrated, embodiments generate a
first half cycle of the ultrasonic drive signal by asserting T1 to
turn on transistor Q1 220 while negating T2 to turn off transistor
Q4 214. Embodiments generate a second half cycle of the ultrasonic
drive signal by asserting T2 to turn on transistor Q4 214 while
negating T1 to turn off transistor Q1 220. As many cycles of the
signal as may be desired can be generated in this manner. In some
embodiments, the output 238 is pulled to ground between the +HV
half-cycle and the -HV half-cycle by asserting T5 and T6 to enable
shunt transistors Q11 224 and Q12 222.
[0029] In period 306, which may occur between high voltage
ultrasonic bursts or when generation of ultrasonic drive is
terminated, the transmitter 200 preferably returns to shunt mode as
described above. By negating signals T3 and T4, the bias network
switches Q10 202 and Q5 204 are preferably opened during this
period to reduce transmitter 200 power dissipation.
[0030] FIG. 4 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
402, the transmitter 200 is producing no ultrasonic drive signal,
and consequently the shunt mode is enabled. The clamp transistors,
Q11 224 and Q12 222 are turned on to preferably clamp the output
238 to ground. The high voltage drive transistors Q1 220, Q2 218,
Q3 216 and Q4 214 are turned off. The bias network switches Q10 202
and Q5 204 are open.
[0031] If transducer drive is requested, in block 404, then the
clamp transistors Q11 224 and Q12 222 holding the output 238 to
ground are turned off, and the switches Q10 202 and Q5 204 are
closed to connect voltage (e.g., .+-.HV) to the bias resistors
R1-R4 230-236 in block 406. The bias network R1-R4 230-236
preferably substantially equalizes the voltage drop across the each
pair of drive transistors Q1 220 and Q2 218, and Q3 216 and Q4
214.
[0032] In block 408, the first portion of the high voltage
ultrasonic drive signal is generated. +HV drive is enabled by
turning on drive transistor Q1 220 and -HV drive is disabled by
turning off drive transistor Q4 214. The second portion of the high
voltage ultrasonic drive signal is generated in block 410 where +HV
drive is disabled by turning off drive transistor Q1 220 and -HV
drive is enabled by turning on drive transistor Q4 214. Embodiments
may repetitively perform the operations of blocks 408 and 410 to
generate any number of cycles of the high voltage ultrasonic drive
signal. In some embodiments, at least some of the operations of
block 412 (e.g., disabling drive transistors and enabling output
clamping) and block 406 (e.g., disabling output clamping) can be
performed between block 408 and block 410 to produce a zero output
between the +HV and -HV output drive. Furthermore, some embodiments
can perform the operations of only one of blocks 408 and 410 in
conjunction with blocks 406 and 412 to produce a drive signal
oscillating between ground and either of +HV or -HV.
[0033] In block 412, the required number of high voltage cycles
have been generated and ultrasonic drive is not required for at
least a predetermined time period. The drive transistors Q1 220 and
Q4 214 are preferably turned off to disable high voltage drive onto
output 238. The bias switches Q5 204 and Q10 202 are opened to
remove voltage across the bias resistors R1-R4 230-236 and
preferably 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 opening switches Q5
204 and Q10 202 can result in substantial power reduction. To
discharge the output 238 (i.e., to clamp the output to ground), the
clamp transistors Q11 224 and Q12 222 are turned on in some
embodiments.
[0034] If, in block 414, transducer drive is to be continued, that
is another ultrasonic signal burst is required, then after a
predetermined time delay, in block 416, signal generation continues
in block 406 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.
* * * * *