Amplifier interface and amplification methods for ultrasound devices

Amplifier architecture that allows low-cost class-D audio amplifiers to be compatible with ultrasonic signals, as well as loads presented by thin-film ultrasonic transducers. The amplifier architecture replaces the traditional capacitor used as an output filter in the class-D amplifier with the natural capacitance of the ultrasonic transducer load, and employs relative impedance magnitudes to create an under-damped low-pass filter that boosts voltage in the ultrasonic frequency band of interest. The amplifier architecture includes a secondary feedback loop to ensure that correct output voltage levels are provided.

BACKGROUND

Amplifying ultrasonic signals has traditionally been both challenging and costly. The frequency range required for such amplification is beyond most audio amplifier capabilities, and the highly capacitive load presented by an ultrasonic transducer is significantly different from the weakly inductive and mostly resistive load presented by a loudspeaker. Further, for many ultrasonic transducers, a much higher drive voltage (e.g., about 200-300 Vpp) is required compared with the drive voltage required for loudspeakers. For at least these reasons, ultrasonic amplifiers, whether designed to be linear or switching, are generally custom made to suit a specific application and/or device.

Class-D amplifiers were once relegated to subwoofer use, but are now capable of reproducing signals in the audio frequency band (i.e., 20-20,000 Hz) with good fidelity and at low cost, due in no small part to advancements made in the field of semiconductor technology. Recent class-D amplifier designs compatible with 192 kHz sample rate audio signals are now theoretically capable of driving up to one-half of this sample rate, or 96 kHz, which is well into a useful ultrasound frequency range. Such class-D amplifier designs are desirable because they are currently being manufactured in high quantities, and are generally available at low cost in a convenient “module” amplifier package, which typically includes signal processing, pulse width modulation, switch driving, power semiconductors, as well as amplifier protection and feedback correction. However, while such class-D amplifier designs theoretically allow high-frequency ultrasound reproduction, they generally do not permit practical use with ultrasonic transducers or parametric loudspeaker systems.

SUMMARY

In accordance with the present application, amplifier architecture is disclosed that allows low-cost class-D audio amplifiers to be compatible with ultrasonic signals, as well as loads presented by thin-film ultrasonic transducers. The disclosed amplifier architecture replaces the traditional capacitor used as an output filter in the class-D amplifier with the natural capacitance of the ultrasonic transducer load, and employs relative impedance magnitudes to create an under-damped low-pass filter that boosts voltage in the ultrasonic frequency band of interest. The disclosed amplifier architecture includes a secondary feedback loop to ensure that correct output voltage levels are provided.

In certain embodiments, an amplifier interface for driving an ultrasonic transducer is provided that includes a switch mode type amplifier configured to receive an ultrasonic signal, and an inductor coupled between the switch mode type amplifier and the ultrasonic transducer. The inductor has an associated inductance value, and the ultrasonic transducer has an associated capacitance value. The inductance value of the inductor and the capacitance value of the ultrasonic transducer form a low-pass filter for removing high frequency artifacts produced by the switch mode type amplifier, which can be configured as a class-D amplifier. The low-pass filter has an associated frequency response, and the inductance value and the capacitance value are selected to produce a voltage boost in the frequency response at about a center of an ultrasonic frequency band of interest.

In certain further embodiments, the amplifier interface can include a digital signal processor (DSP) coupled to an input of the switch mode type amplifier, and a feedback path disposed between the ultrasonic transducer and the DSP. The ultrasonic transducer is configured to produce an output signal, and the feedback path is configured to provide a signal representative of a level of the output signal to the DSP. The DSP is configured to correct the level of the output signal in response to variations in one or more of the inductance value of the inductor and the capacitance value of the ultrasonic transducer. The amplifier interface can also include a transformer disposed between the switch mode type amplifier and the inductor.

In certain additional embodiments, the ultrasonic transducer is configured as a thin-film ultrasonic transducer. The amplifier interface further includes a DC bias circuit configured to provide a DC bias voltage to the thin-film ultrasonic transducer. The DC bias circuit includes a DC bias generator, an isolation capacitor disposed between the inductor and the ultrasonic transducer, and an isolation resistor disposed between the ultrasonic transducer and the DC bias generator.

Other features, functions, and aspects of the invention will be evident from the Detailed Description that follows.

DETAILED DESCRIPTION

The disclosure of U.S. Provisional Patent Application No. 62/441,468 filed Jan. 2, 2017 entitled AMPLIFIER INTERFACE AND METHODS FOR ULTRASOUND is hereby incorporated herein by reference in its entirety.

Amplifier architecture is disclosed that allows low-cost class-D audio amplifiers to be compatible with ultrasonic signals, as well as loads presented by thin-film ultrasonic transducers. The disclosed amplifier architecture replaces the traditional capacitor used as an output filter in the class-D amplifier with the natural capacitance of the ultrasonic transducer load, and employs relative impedance magnitudes to create an under-damped low-pass filter that boosts voltage in the ultrasonic frequency band of interest. The disclosed amplifier architecture includes a secondary feedback loop to ensure that correct output voltage levels are provided.

FIG. 1adepicts an illustrative embodiment of a conventional amplifier interface100for driving a loudspeaker110. As shown inFIG. 1a, the amplifier interface100includes a class-D (switch mode type) amplifier104, and a pair of low-pass (L-C) filters105.1,105.2. The low-pass filter105.1includes an inductor106.1and a capacitor108.1. Likewise, the low-pass filter105.2includes an inductor106.2and a capacitor108.2. In the illustrative embodiment ofFIG. 1a, the class-D amplifier104can be configured to include a full-bridge (or H-bridge) output stage, which provides a pair of outputs (Output1, Output2) for driving the loudspeaker110. It is noted, however, that the class-D amplifier104may alternatively be configured to include a half-bridge output stage or any other suitable output stage configuration.

In an exemplary mode of operation, an audio source102produces an audio signal in the audio frequency band (i.e., 20-20,000 Hz), and provides the audio signal to an input of the class-D amplifier104, which includes suitable components for synthesizing and amplifying pulse width modulated (PWM) signals based on the audio input signal, as known in the art. The PWM signals include the audio signal to be reproduced at the loudspeaker110, as well as high frequency switching artifacts outside the audible frequency band. The class-D amplifier104provides a PWM signal at each of its Outputs1,2, which, in turn, provide the respective PWM signals to the low-pass filters105.1,105.2. The low-pass filter105.1is configured to filter the PWM signal at the Output1of the class-D amplifier104to remove the high frequency switching artifacts from the respective PWM signal. Likewise, the low-pass filter105.2is configured to filter the PWM signal at the Output2of the class-D amplifier104to remove the high frequency switching artifacts from the respective PWM signal. Having removed the high frequency switching artifacts from the PWM signals, the low-pass filters105.1,105.2provide the audio signal in the audio frequency band to the loudspeaker110for subsequent reproduction.

FIG. 1bdepicts an exemplary frequency response112of the low-pass filters105.1,105.2included in the amplifier interface100ofFIG. 1a. As shown inFIG. 1b, the cutoff frequency, Fc, of the low-pass filters105.1,105.2can be expressed, as follows:

Fc=12⁢⁢π⁢LC,(1)
in which “L” corresponds to the inductance value of the inductors106.1,106.2, and “C” corresponds to the capacitance value of the capacitors108.1,108.2. It is noted that, for a full-band audio device, the cutoff frequency Fc is typically about 20 kHz.

For enhanced accuracy and lower distortion, the class-D amplifier104can employ real-time feedback (generally depicted by arrows111.1,111.2; seeFIG. 1a) derived from its output stage. The class-D amplifier104can process (e.g., filter) the real-time feedback, compare the processed feedback to the audio input signal, and employ a resulting error function to suppress distortion and enhance reproduction accuracy. Because the output filter is flat in the audio frequency band of interest, it is generally not necessary to obtain such feedback from the load presented by the loudspeaker110, as the output from the class-D amplifier is representative and accurate, at least within the audio frequency band of interest.

FIG. 2adepicts an illustrative embodiment of an amplifier interface200for driving an ultrasonic transducer214, in accordance with the present application. As shown inFIG. 2a, the amplifier interface200includes a class-D (switch mode type) amplifier204, a transformer206, an inductor208, a DC bias circuit211, and a peak detector210. The DC bias circuit211includes an isolation capacitor212, an isolation resistor216, and a DC bias generator218. The transformer206is configured to provide isolation and a ground reference, and to allow the impedance seen by the class-D amplifier204to be adjusted to a suitable range, as well as allow the output voltage to be adjusted to a suitable level. In the illustrative embodiment ofFIG. 2a, the class-D amplifier204can be configured to include a full-bridge (or H-bridge) output stage, which provides a pair of outputs (Output1, Output2) coupled to an input winding of the transformer206. It is noted, however, that the class-D amplifier204may alternatively be configured to include a half-bridge output stage or any other suitable output stage configuration. Further, for enhanced accuracy and lower distortion, the class-D amplifier204can employ real-time feedback (generally depicted by one or more primary feedback paths219.1,219.2; seeFIG. 2a) derived from its output stage. A compensation filter can also be used to flatten the net ultrasound response of the ultrasonic transducer214.

The amplifier interface200for driving the ultrasonic transducer214(seeFIG. 2a) can be contrasted with the conventional amplifier interface100for driving the loudspeaker110(seeFIG. 1a), at least as follows. First, the capacitance provided by the capacitors108.1,108.2included in the low-pass filters105.1,105.2is replaced by the natural capacitance of the ultrasonic transducer214. This eliminates the need for the capacitors108.1,108.2, and, by reducing the total capacitance, reduces the amount of current that the class-D amplifier204is required to supply to drive the ultrasonic transducer214to a desired voltage level. The capacitance of the ultrasonic transducer214can be trimmed by adding a capacitor of a relatively small capacitance value, preferably in parallel with the ultrasonic transducer214, such that the cutoff frequency Fc (seeFIG. 2b) is located in a desired ultrasound frequency range. In addition, the inductance value of the inductor208is selected such that a resulting frequency response222(seeFIG. 2b) has a cutoff frequency, Fc, located approximately in the center of the ultrasonic frequency band of interest. The inductance of the inductor208can be trimmed, as desired and/or required, using tunable inductor devices or one or more additional inductors in series or parallel. Because the resistance of the load presented by the ultrasonic transducer214is low, the inductance of the inductor208and the natural capacitance of the ultrasonic transducer214create an under-damped (or “peaking”) low-pass filter, as illustrated by the frequency response222ofFIG. 2b. As shown inFIG. 2b, a strong resonance peak is created at approximately the cutoff frequency Fc, thereby naturally amplifying the output voltage in the desired ultrasonic frequency band. InFIG. 2b, a frequency response220(shown in phantom) similar to the frequency response112ofFIG. 1bis shown for comparison with the under-damped or peaking frequency response222.

It is noted that the strong resonance peak at the cutoff frequency Fc (seeFIG. 2b) allows the frequency response222to have characteristics of a band-pass filter. It is further noted that, near the cutoff frequency Fc (seeFIG. 2b), the series-resonance combination of the inductor208and the natural capacitance of the ultrasonic transducer214presents a load to the class-D amplifier204that appears to be mostly resistive, thereby making the ultrasonic transducer214compatible with standard class-D amplifier modules designed for use with resistive loads.

Because the under-damped or peaking low-pass filter created by the inductance of the inductor208and the natural capacitance of the ultrasonic transducer214does not have a flat frequency response, and the inductance value of the inductor208and the capacitance/resistance value(s) of the ultrasonic transducer214may vary and/or drift, the amplifier interface200is configured to include a secondary feedback path209from a node between the inductor208and the capacitor212to an ultrasonic source/digital signal processor (DSP) coupled to an input of the class-D amplifier204. It is noted that the change in phase near the cutoff frequency Fc (seeFIG. 2b) can be rapid, and therefore a direct feedback comparison with the output of the amplifier interface200can be difficult to implement. For this reason, a slower feedback loop implemented by the secondary feedback path209with a signal representative of the output level is employed.

As described herein, the inductance, L, of the inductor208and the natural capacitance, C, of the ultrasonic transducer214create an under-damped or peaking low-pass filter, as illustrated by the frequency response222ofFIG. 2b. In one embodiment, the ultrasonic transducer214is configured as a thin-film ultrasonic transducer, in which the internal series resistance is relatively small compared to the impedance of L and C in the ultrasound frequency range of interest. Such a thin-film ultrasonic transducer generally requires a high voltage (and a DC bias) for proper operation. For this reason, the broadband, flat frequency response of the class-D amplifier204is effectively converted into a band-pass frequency response with voltage boost, which is useful for amplifying band-limited ultrasonic signals centered at the cutoff frequency, Fc, as expressed as in equation (1) above. The magnitude of the voltage boost near the cutoff frequency, Fc, is dependent upon the relative values of L, C, as well as the load resistance. It is noted that there can be a tradeoff between the useful bandwidth and the magnitude of the voltage boost. In an exemplary configuration of the amplifier interface200, a 24 Vdc power supply can be used to drive the ultrasonic transducer load to 200-500 Vpp near the cutoff frequency, Fc, allowing maximum drive limits of the ultrasonic transducer load to be reached over a reasonable bandwidth. It is further noted that a carrier frequency of a modulated ultrasonic signal produced by the ultrasonic transducer214can be selected to be near the cutoff frequency, Fc. For certain types of ultrasonic signal modulation, such as single side-band (SSB), the cutoff frequency, Fc, can be approximately at the center of the ultrasonic frequency band of interest.

FIG. 3depicts an illustrative phase response302and frequency response304of the under-damped or peaking low-pass filter created by the inductance, L, of the inductor208, and the natural capacitance, C, of the ultrasonic transducer214. Exemplary values of the inductance, L, of the inductor208, the natural capacitance, C, of the ultrasonic transducer214, as well as the internal series resistance of the ultrasonic transducer214, are 80 uH, 100 nF, and 2Ω, respectively. As shown inFIG. 3, the frequency response304provides a voltage boost of about 23 dB at a cutoff frequency (Fc) of about 56 kHz, thereby boosting the voltage level at the ultrasonic transducer load by a factor of over ten (10) and, for nearby frequencies, providing a voltage boost of about a factor of four (4) or more over a bandwidth of about 14 kHz. Because the frequency response304rolls off at higher frequencies, high frequency switching harmonics are effectively eliminated (i.e., filtered out) from the output of the class-D amplifier204. It is noted that proper selection of the inductance, L, of the inductor208in conjunction with the natural capacitance, C, of the ultrasonic transducer214ensures that the load seen by the class-D amplifier204is mostly resistive, at least in the ultrasonic frequency band of interest. The class-D amplifier204, which is typically designed for use with resistive loads, can therefore be used to drive the load of the ultrasonic transducer214at high voltage both efficiently and at low cost.

As described herein, the transformer206can be included in the amplifier interface200ofFIG. 2ato provide isolation and a ground reference, and to allow the impedance seen by the class-D amplifier204to be adjusted to a suitable range. More specifically, in the case where the class-D amplifier204is configured to include a full-bridge (or H-bridge) output stage, the transformer206can effectively convert a differential driving capability of the full-bridge output stage to a single-ended, ground-referenced output signal, without sacrificing the output voltage amplitude. Moreover, the transformer206can be configured as a step-up or step-down transformer to tailor the maximum voltage swing at the output, as well as alter the impedance seen by the class-D amplifier204, thereby ensuring that the class-D amplifier204is operating within appropriate current limits and at the rated load impedance.

As further described herein, the class-D amplifier204can employ real-time feedback (generally depicted by the primary feedback path(s)219.1,219.2; seeFIG. 2a) from its output stage for enhanced accuracy and lower distortion. Such feedback, which is typically provided in off-the-shelf class-D amplifier modules, is useful in conventional architectures such as the amplifier interface100(seeFIG. 1a), because the cutoff frequency, Fc, of the output filter tends to be well outside the audio frequency band of interest, and the signal seen by the load presented by the loudspeaker110is about the same as that seen at the output of the class-D amplifier104, at least within the audio frequency band of interest. In the architecture of the amplifier interface200(seeFIG. 2a), however, an under-damped resonance is created between the class-D amplifier204and the load presented by the ultrasonic transducer214, so the amplitude of the signal seen by the ultrasonic transducer load is different from that seen by the loudspeaker load and is highly frequency dependent. Further, the phase response302changes rapidly in the ultrasonic frequency band of interest, and therefore a direct feedback of the signal at the output of the amplifier interface200is difficult to implement. For this reason, rather than using the entire output signal for feedback purposes, the amplifier interface200employs a signal representative of the output level in the secondary feedback path209. In the amplifier interface200ofFIG. 2a, such a signal is provided by the peak detector210to an ultrasonic source/digital signal processor (DSP)202to ensure that the signal seen by the ultrasonic transducer load has accurate amplitude, especially with variations in the values of inductance, L, and the capacitance, C. Because the secondary feedback path209can include a nonlinear element (e.g., the peak detector210) rather than a linear filter, such feedback provided by the secondary feedback path209can be viewed as being nonlinear. It should be noted, however, that a full-signal, linear filtered, or any other suitable secondary feedback path may be employed. In an alternative embodiment, the peak detector210can be replaced with a rectifier or any other suitable signal-conditioning configuration. For higher-speed DSP interfaces, the nonlinear element210can be omitted, and the requisite signal conditioning can be performed within the DSP (see reference numeral202) for the secondary feedback path209.

The DC bias circuit211can apply a suitable DC bias voltage (e.g., 250 Vdc) to the ultrasonic transducer214for increased sensitivity and maximum output. In the amplifier interface200ofFIG. 2a, the DC bias generator218feeds the DC bias voltage through the resistor216, which protects the DC bias generator218from a high voltage AC drive signal seen by the ultrasonic transducer load. Further, the capacitor212is configured to block the DC bias voltage from the class-D amplifier204, as well as the transformer206. The capacitor212is provided with a capacitance value suitably high enough to avoid substantially influencing the high voltage AC drive signal being fed to the ultrasonic transducer214. In an alternative embodiment, the capacitor212can be located between the transformer206and the inductor208, and the feedback provided by the secondary feedback path209can be obtained directly from the ultrasonic transducer load.

As still further described herein, a compensation filter can be used to flatten the net ultrasound response of the ultrasonic transducer214. More specifically, the compensation filter can be used to compensate for the non-flat frequency response222(seeFIG. 2b) in the vicinity of the cutoff frequency, Fc. Such a compensation filter can be implemented by the ultrasonic source/DSP202, which can be configured to measure the frequency response222using the output level provided via secondary feedback path209. It is noted that the compensation filter can be either fixed to provide a correction for expected values of inductance (L) and/or capacitance (C), or adjustable to account for possible drift in the values of inductance (L) and/or capacitance (C).

In one mode of operation, the ultrasonic source/DSP202produces an ultrasonic signal in the ultrasonic frequency band (i.e., 50-70 kHz), and provides the ultrasonic signal to an input of the class-D amplifier204, which can include a full-bridge output stage. The transformer206converts the differential driving capability of the full-bridge output stage to a single-ended, ground-referenced output signal, and provides the output signal to the under-damped or peaking low-pass filter created by the inductance of the inductor208and the natural capacitance of the ultrasonic transducer214, which can be a thin-film ultrasonic transducer. The peak detector210provides a signal representative of the output level via the secondary feedback path209to the ultrasonic source/DSP202, which uses the output level to compensate for the non-flat frequency response of the under-damped or peaking low-pass filter near the cutoff frequency, Fc. The DC bias circuit211applies a suitable DC bias voltage to the thin-film ultrasonic transducer for increased sensitivity and maximum output.

Having described the above illustrative embodiments of the amplifier interface200(seeFIG. 2a) and associated amplification methods for ultrasonic devices, other variations and/or modifications can be made and/or practiced. For example, the amplifier interface and amplification methods described herein can be employed in a parametric loudspeaker system configured (i) to receive an audio input signal, (ii) to process the audio signal using a DSP, (iii) to modulate the audio signal to the ultrasonic frequency band using a carrier frequency near the cutoff frequency, Fc, and (iv) to deliver the modulated ultrasonic signal to the amplifier interface200for reproduction by the ultrasonic transducer214. An ultrasonic output produced by the ultrasonic transducer214is then demodulated by the nonlinear characteristics of the propagation medium (e.g., the air), converting the ultrasonic output into audible sound that can be heard by a human subject.

It was described herein that the amplifier interface200incorporating the class-D amplifier204can be used to drive the ultrasonic transducer214. In an alternative embodiment, the amplifier interface200can be used to drive any suitable reactive load, such as an antenna (capacitive) or a coil or motor (inductive—the positions of capacitance, C, and inductance, L, are reversed). In another alternative embodiment, the amplifier interface200can be configured to incorporate a class-A amplifier, a class-B amplifier, a class-T amplifier, or any other suitable amplifier normally used to amplify audio signals. In a further alternative embodiment, one or more transient-voltage-suppression (TVS) diodes can be used at the output of the amplifier interface200to protect the class-D amplifier204from damage due to current spikes that may cause voltage ripples to reflect back through the amplifier interface circuit, potentially reaching the class-D amplifier204.

As further described herein, the carrier frequency of the modulated ultrasonic signal produced by the ultrasonic transducer214can be selected to be near the cutoff frequency, Fc. In an alternative embodiment, such a carrier frequency can be adjusted to enhance the voltage boost of the frequency response of the amplifier interface200, thereby accounting for inherent variations in the inductance of the inductor208and/or the natural capacitance of the ultrasonic transducer214. In such an alternative embodiment, the carrier frequency can be adjusted at the ultrasonic source/DSP202either dynamically during operation or during programmed calibration sequences (such as upon startup), based on the signal representative of the output level provided via the secondary feedback path209. In a further alternative embodiment, the frequency response222of the amplifier interface200can be tuned by (i) inserting one or more additional resistors in the signal chain to reduce the peak of the frequency response222, and/or (ii) inserting one or more inductors and/or capacitors in the signal chain to move the position of the peak of the frequency response222, as well as the frequency region affected by the voltage boost provided by the frequency response222, etc.

It was further described herein that the amplifier interface200can include the series inductor208. In an alternative embodiment, the series inductor208can be replaced with a secondary inductance in the transformer206, or an inductor disposed in parallel with the ultrasonic transducer load, resulting in a flat voltage response and reduced current consumption at frequencies close to where the inductance of the inductor208and the natural capacitance of the ultrasonic transducer214are resonant. In such an alternative embodiment, another low-pass filter (e.g., another L-C filter) can be added before or after the transformer206, either separately or combined with the inductance of the transformer206.