Driver circuitry

The present disclosure relates to circuitry for driving a piezoelectric transducer. The circuitry comprises amplifier circuitry configured to receive a drive signal and to output an output signal, based on the drive signal, to the piezoelectric transducer, a variable capacitor configured to be coupled in series with the piezoelectric transducer, and control circuitry. The control circuitry is configured to control a capacitance of the variable capacitor to compensate for hysteresis in the piezoelectric transducer and to control a gain of the amplifier circuitry to compensate for signal attenuation caused by the variable capacitor.

FIELD OF THE INVENTION

The present disclosure relates to driver circuitry, and in particular to driver circuitry for piezoelectric transducers.

BACKGROUND

Piezoelectric transducers are increasingly being seen as a viable alternative to transducers such as speakers and resonant actuators for providing audio and/or haptic outputs in devices such as mobile telephones, laptop and tablet computers and the like, due to their thin form factor, which may be beneficial in meeting the demand for increasing functionality in such devices without significantly increasing their size. Piezoelectric transducers are also increasingly finding application as transducers for ultrasonic sensing and range-finding systems.

Piezoelectric transducers can be voltage-driven. However, when driven by voltage piezoelectric transducers exhibit both hysteresis and creep, which means that when the displacement of a piezoelectric transducer depends on both the currently-applied voltage and on a previously-applied voltage. Thus, for any given driving voltage there are multiple possible displacements of the piezoelectric transducer. For audio applications this manifests as distortion.

SUMMARY

According to a first aspect, the invention provides circuitry for driving a piezoelectric transducer, the circuitry comprising:amplifier circuitry configured to receive a drive signal and to output an output signal, based on the drive signal, to the piezoelectric transducer;a variable capacitor configured to be coupled in series with the piezoelectric transducer; andcontrol circuitry, wherein the control circuitry is configured to control a capacitance of the variable capacitor to compensate for hysteresis in the piezoelectric transducer and to control a gain of the amplifier circuitry to compensate for signal attenuation caused by the variable capacitor.

The control circuitry may be configured to control the gain of the amplifier circuitry and the capacitance of the variable capacitor based at least in part on a parameter of the drive signal received by the amplifier circuitry.

The parameter may comprise one or more of:a volume of an audio signal represented by the drive signal;an envelope of the drive signal; andan instantaneous value of the drive signal.

The control circuitry may be configured to monitor a signal at a node coupled to the piezoelectric transducer and to control the gain of the amplifier circuitry and the capacitance of the variable capacitor based at least in part on the monitored signal.

The monitored signal may comprise a voltage across the piezoelectric transducer or a current through the piezoelectric transducer, for example.

The control circuitry may be configured to control the gain of the amplifier circuitry such that the output signal is at a predefined level.

For example, the control circuitry may be configured to control the gain of the amplifier circuitry such that the output signal is at a full-scale signal level or a reduced signal level.

The control circuitry may be configured to determine the gain of the amplifier circuitry based on a predefined maximum value of a parameter of the output signal.

The control circuitry may be configured to determine a scaling factor for controlling the capacitance of the variable capacitor.

The control circuitry may be configured to determine the gain of the amplifier circuitry based on a predefined maximum value of a parameter of the output signal, and the control circuitry may be configured to determine the scaling factor based on the determined gain.

The variable capacitor may comprise a switched capacitor network, for example.

Alternatively, the variable capacitor may comprise active circuitry to effect a capacitance multiplier.

According to a second aspect, the invention provides circuitry for driving a piezoelectric transducer, the circuitry comprising:a controllable-gain amplifier for supplying an amplified drive signal to the piezoelectric transducer;a variable capacitor for coupling in series with the piezoelectric transducer; andcontrol circuitry, wherein the control circuitry is configured to adjust a capacitance of the variable capacitor based on a gain of the controllable-gain amplifier.

The gain of the controllable-gain amplifier may be based on a parameter of an input signal to the controllable-gain amplifier.

The gain of the controllable-gain amplifier may be selected such that the amplified drive signal is at a predefined level.

For example, the gain of the controllable-gain amplifier may be selected such that the amplified drive signal is at a full-scale signal level or a reduced signal level.

According to a third aspect, the invention provides circuitry for driving a piezoelectric transducer, the circuitry comprising:a first signal path for receiving a drive signal for driving the piezoelectric transducer, the first signal path comprising a first circuit node for coupling to a first terminal of the piezoelectric transducer;a second circuit node for coupling to a second terminal of the piezoelectric transducer;a capacitor for compensating for hysteresis in the piezoelectric transducer, the capacitor having a first terminal and a second terminal, wherein the first terminal is coupled to the second circuit node; anddriver circuitry coupled to the second terminal of the capacitor,wherein in operation of the circuitry the driver circuitry is operative to control a voltage at the second circuit node to compensate for signal attenuation caused by the capacitor.

The capacitor may be a variable capacitor, and the driver circuitry may be operative to maintain the second circuit node at 0 volts to compensate for signal attenuation caused by the variable capacitor.

The circuitry may further comprise control circuitry configured to control a capacitance of the variable capacitor based on a parameter of the drive signal.

The capacitor may be a fixed capacitor, the circuitry may further comprise controllable-gain amplifier circuitry, and the circuitry may be configured to control the voltage at the second circuit node based on a gain of the controllable-gain amplifier circuitry.

The circuitry may further comprise control circuitry configured to control the gain of the controllable-gain amplifier circuitry based on a parameter of the drive signal.

According to a fourth aspect, the invention provides a system comprising a piezoelectric transducer and the circuitry of the first, second or third aspect.

According to a fifth aspect, the invention provides an integrated circuit comprising the circuitry of the first, second or third aspect.

According to a sixth aspect, the invention provides a device comprising the circuitry of the first, second or third aspect.

The device may comprise, for example, a mobile telephone, a tablet or laptop computer, a gaming device, an accessory device, a headset, headphones, earphones, a smart speaker.

DETAILED DESCRIPTION

FIG.1ais a schematic representation of a model for hysteresis in a piezoelectric transducer. The hysteresis can be modelled as a charge Qhys (represented inFIG.1aas a current source120) that is added to the capacitance of the piezoelectric transducer110as a function of a previous state of the piezoelectric transducer.

The hysteresis can equivalently be modelled as shown inFIG.1b, as a voltage source130providing a voltage Vhys coupled in series with the piezoelectric transducer110.

The displacement of the piezoelectric transducer110is proportional to the charge on it. When the voltage Vhys changes and the piezoelectric transducer100is being driven by a constant drive voltage Vdrv, the charge stored on the piezoelectric transducer110changes, which cause unwanted displacement of the piezoelectric transducer110and creep.

The charge Qp on the piezoelectric transducer110when it is being driven by a constant drive voltage Vdrv can be expressed as:
Qp=Cp(Vdrv−Vhys)  (1),
where Cp is the capacitance of the piezoelectric transducer110.

The change in the charge Qp on the piezoelectric transducer110in response to a change in the hysteresis (i.e. a change in the voltage Vhys in the model ofFIG.1b) can be expressed as:

FIG.2is a schematic diagram illustrating an approach to mitigating the effect of hysteresis in a piezoelectric transducer. As can be seen, a capacitor210is introduced, in series with the piezoelectric transducer110(and thus also the modelled hysteresis voltage source130), such that a drive voltage Vdrv is supplied across the series combination of the piezoelectric transducer110and the capacitor210.

The charge Qp on the piezoelectric transducer110when it is being driven by a constant voltage source can be expressed as:
Qp=Ct(Vdrv−Vhys)  (3),
where Ct is the total capacitance of the series combination of the piezoelectric transducer110and the capacitor210.

Assuming that the capacitance has a capacitance C that is equal to αCp (where Cp is the capacitance of the piezoelectric transducer110), then the total capacitance of the series combination of the piezoelectric transducer110and the capacitor210can be expressed as:

Thus, the charge Qp on the piezoelectric transducer110when it is being driven by a constant drive voltage Vdrv can be expressed as:

The change in the charge Qp on the piezoelectric transducer110in response to a change in the hysteresis (i.e. a change in the voltage Vhys) can be expressed as:

Thus, the series capacitor210reduces the sensitivity of the charge on the piezoelectric transducer110to hysteresis

(by⁢⁢a⁢⁢factor⁢⁢of⁢⁢α1+α)
in comparison to the model ofFIG.1b.

In order to attenuate the hysteresis, a should be less than 1 (i.e. α<1).

However, as is apparent from equation (5) above, the series capacitor210also reduces the sensitivity of the piezoelectric transducer110to the drive voltage Vdrv, such that the displacement of the piezoelectric transducer110for a given drive voltage Vdrv is reduced

(by⁢⁢a⁢⁢factor⁢⁢of⁢⁢α1+α)
when a series capacitor210is employed.

To achieve the same displacement of the piezoelectric transducer110for a given drive voltage Vdrv when the series capacitor210is provided as when there is no series capacitor210, the drive voltage Vdrv should be increased to compensate for the effect of the charge capacitor210. This increase may be provided by way of a compensating gain β applied to the drive voltage Vdrv, where:

FIG.3is a schematic representation of driver circuitry for driving a piezoelectric transducer according to the present disclosure, in which the effects of hysteresis can be mitigated.

The circuitry, shown generally at300inFIG.3, includes variable gain amplifier circuitry310configured to apply a gain β to a drive signal Vdrv received by the amplifier circuitry310from upstream circuitry (not shown) for driving a piezoelectric transducer110. The drive signal may be, for example, an audio signal.

The circuitry300further includes a variable capacitor320coupled in series between the piezoelectric transducer110and a ground (or other reference voltage) supply terminal or rail.

The circuitry300further includes control circuitry330, which is operative to control the gain β that is applied to the drive signal Vdrv, and to control the capacitance value C of the variable capacitor320. Thus the control circuitry330is configured to receive the drive signal Vdrv and to output appropriate control signals to the amplifier circuitry310and the variable capacitor320to control the gain β and the capacitance value C of the variable capacitor320based (at least in part) on the received drive signal Vdrv.

More specifically, the control circuitry330is configured to monitor one or more parameters of the drive signal Vdrv, and to control the gain β and the capacitance value C of the variable capacitor320based (at least in part) on one or more of the monitored parameter(s). The monitored parameter(s) of the drive signal Vdrv may comprise, for example, a volume of an audio signal represented by the drive signal Vdrv, an envelope of the drive signal Vdrv, or an instantaneous value (e.g. an instantaneous magnitude) of the drive signal Vdrv.

In some examples the control circuitry330may also monitor a signal at a node322between the piezoelectric transducer110and the variable capacitor320, and control the capacitance value C of the variable capacitor320and/or the gain β based (at least in part) on the monitored signal at the node322. The monitored signal may be, or may be representative of, a voltage across the piezoelectric transducer110or a current through the piezoelectric transducer110, for example. Thus the capacitance value C of the variable capacitor320and/or the gain β may be controlled based on a parameter of the received drive signal and/or based on the monitored signal (e.g. voltage or current) at the node322.

The control circuitry330is configured to control the amplifier circuitry310such that the signal βVdrv output by the amplifier circuitry310is at a predefined level. For example, the control circuitry330may control the amplifier circuitry310such that the signal βVdrv output by the amplifier circuitry310is always full-scale (i.e. the signal βVdrv output by the amplifier circuitry310always covers the full range of output signal amplitudes that can be output without distortion by the amplifier circuitry310, rather than being scaled (reduced) in amplitude). Alternatively, the control circuitry330may control the amplifier circuitry310such that the signal βVdrv output by the amplifier circuitry310is at a reduced level, e.g. −6 dB (relative to a reference level such as a full-scale signal level).

To this end the control circuitry330may be provided with (e.g. programmed with) or may receive (e.g. from a memory of a host device incorporating the circuitry300) a predefined value, e.g. a predefined maximum value Vmax of a parameter (e.g. a maximum amplitude) of a signal that can be output without distortion by the amplifier circuitry310. The control circuitry330is configured to determine the gain β to be applied to the drive signal Vdrv by the amplifier circuitry310based on this predefined value and the monitored parameter of the drive signal Vdrv. For example, where the control circuitry330is configured to control the amplifier circuitry310such that the signal βVdrv output by the amplifier circuitry310is always full-scale, the control circuitry330may determine the gain β to be applied to the drive signal Vdrv by the amplifier circuitry310using the equation:

The control circuitry330is also configured to determine a value of a scaling factor α to be applied by the control circuitry330to adjust the capacitance value C of the variable capacitor320. The scaling factor α is determined by the control circuitry330, e.g. using the equation:

The control circuitry330controls the gain of the amplifier circuitry310according to the determined gain value β and controls the capacitance value C of the variable capacitor320according to the determined scaling factor α.

Thus the control circuitry330controls the capacitance of the variable capacitance to compensate for (e.g. attenuate) hysteresis in the piezoelectric transducer, and controls the gain of the amplifier circuitry310to compensate for signal attenuation (i.e. attenuation of the signal output by the amplifier circuitry310) caused by the variable capacitor320, so as to ensure that the signal βVdrv that is output by the amplifier circuitry310has a predefined signal level. The control circuitry330therefore controls the capacitance value C of the variable capacitor320both to compensate for the gain β that is applied to the drive signal Vdrv, and to mitigate the effects of hysteresis.

As those of ordinary skill in the art will appreciate, the variable capacitor320may be implemented in a number of different ways. For example, the variable capacitor320may be implemented using active circuitry to effect a capacitance multiplier, or using a switched capacitor network of the kind illustrated generally at400inFIG.4.

The switched capacitor network400in this example comprises first to fourth banks410-440of switched capacitances.

The first bank410comprises a first capacitance412of value C coupled in series with a first switch414between a first rail450that is coupled to the piezoelectric transducer110and a second rail460that is coupled to the ground (or other reference supply) terminal of the circuitry300. Although for clarity the first capacitance412is shown inFIG.4as a single capacitor, it will be appreciated by those skilled in the art that the first capacitance412could be made up of a number of separate capacitances coupled in parallel or in series in order to achieve the capacitance value C.

The second bank420comprises a second capacitance422of value 2C coupled in series with a second switch424between the first rail450and the second rail460. Again, for clarity the second capacitance422is shown inFIG.4as a single capacitor, but it will be appreciated by those skilled in the art that the second capacitance422could be made up of a number of separate capacitances coupled in parallel or in series in order to achieve the capacitance value 2C.

The third bank430comprises a third capacitance432of value 4C coupled in series with a third switch434between the first rail450and the second rail460. As before, for clarity the third capacitance432is shown inFIG.4as a single capacitor, but it will be appreciated by those skilled in the art that the third capacitance432could be made up of a number of separate capacitances coupled in parallel or in series in order to achieve the capacitance value 4R.

The fourth bank440comprises a fourth capacitance442of value 8C coupled in series with a fourth switch444between the first rail450and the second rail460. Again, for clarity the fourth capacitance442is shown inFIG.4as a single capacitor, but it will be appreciated by those skilled in the art that the fourth resistance442could be made up of a number of separate capacitances coupled in series or parallel in order to achieve the resistance value 8C.

The switched capacitor network400further includes a fifth switch470, coupled in series between the first rail450and the second rail460, which can be actuated to bypass the first to fourth banks410-440such that the variable capacitor320provides no capacitance.

The capacitance value of the variable capacitor320can be adjusted by selectively opening and closing the switches414-444in accordance with, in this example, a four-bit input digital word or code.

Thus for an input digital word of value 0001, the first switch414would be closed and the second, third and fourth switches424-444would be open. The capacitance value of the variable capacitor320would thus be equal to C.

For an input digital word of value 0010, the second switch424would be closed and the first, third and fourth switches414,434,444would be open. The capacitance value of the variable capacitor320would thus be equal to 2C.

For an input digital word of value 0011, the first and second switches414,424would be closed and the third and fourth switches434,444would be open. The capacitance value of the variable capacitor320would thus be equal to the parallel combination of C and 2C, i.e. 3C.

It will be appreciated thatFIG.4illustrates the principle of using a switched capacitor network as a variable capacitance. The specific number of banks of switched capacitances, and the values of the capacitances within the banks, will be determined or selected according to the particular application for which the switched capacitor network400is used.

FIG.5is a schematic representation of alternative driver circuitry for driving a piezoelectric transducer according to the present disclosure, in which the effects of hysteresis can be mitigated.

The driver circuitry, shown generally at500inFIG.5, includes a first signal path510for receiving a drive signal Vdrv for driving a piezoelectric transducer110. The first signal path510terminates in a first circuit node512, to which a first terminal of the piezoelectric transducer110can be coupled.

The circuitry500further includes a variable capacitor520having a first terminal which is coupled to a second circuit node522, to which a second terminal of the piezoelectric transducer110can be coupled. The variable capacitor320may be implemented, for example, using active circuitry to effect a capacitance multiplier, or using a switched capacitor network of the kind shown inFIG.4.

The circuitry500further includes a subtractor530, having a first input which is coupled to a ground or 0 volts reference source, and a second input which is coupled to the second circuit node522.

An output of the subtractor530is coupled to an input of drive circuitry540, which in this example implements a buffer amplifier. An output of the drive circuitry540is coupled to a second terminal of the variable capacitor520.

The circuitry500may further include control circuitry550, configured to receive the drive signal Vdrv and to control the capacitance of the variable capacitor520based on a parameter such as a volume of an audio signal represented by the drive signal Vdrv, an envelope of the drive signal Vdrv or an instantaneous value (e.g. an instantaneous magnitude) of the drive signal Vdrv.

In operation of the circuitry500, the series combination of the capacitance Cpiezo of the piezoelectric transducer110and the variable capacitor520forms a capacitive voltage divider, and a voltage Vpiezo develops at the second circuit node522. As will be understood by those of ordinary skill in the art,

The subtractor530subtracts the voltage Vpiezo received at its second input from the 0 volts or ground reference voltage received at its first input and outputs a voltage −Vpiezo to the driver circuitry540. Thus the voltage at the second terminal of the variable capacitor520is equal to −Vpiezo.

As a result, the second circuit node522, to which the second terminal of the piezoelectric transducer110is coupled, is effectively at 0 volts, such that the full-scale drive signal Vdrv appears across the piezoelectric transducer110.

Thus, in contrast to the circuitry300ofFIG.3, in which the variable capacitor320compensates for hysteresis in the piezoelectric transducer110and the gain β applied to the drive signal Vdrv by the amplifier circuitry310compensates for the attenuation of the drive signal that would otherwise occur due to the variable capacitor320, in the circuitry500ofFIG.5the variable capacitance520compensates for hysteresis in the piezoelectric transducer110, and the drive signal attenuation caused by the variable capacitor520is compensated by driving the second terminal of the variable capacitor520such that the second circuit node522is effectively at 0 volts. Thus in the circuitry500no amplification of the drive signal Vdrv is required to compensate for the attenuation caused by the variable capacitor520.

FIG.6is a schematic representation of further alternative driver circuitry for driving a piezoelectric transducer according to the present disclosure, in which the effects of hysteresis can be mitigated.

The driver circuitry, shown generally at600inFIG.6, shares many elements in common with the circuitry500ofFIG.5. Such common elements are denoted by common reference numerals inFIGS.5and6and will not be described in detail here.

The circuitry600differs from the circuitry500in that it includes inverting differential amplifier circuitry640in place of the subtractor530and driver circuitry540.

The differential amplifier circuitry640has a first, inverting, input coupled to the second circuit node522and a second, non-inverting, input coupled to a 0 volts or ground reference source.

In operation of the circuitry600, a voltage Vpiezo develops at the second circuit node522as a result of the drive signal Vdrv, and is received at the first, inverting, input of the amplifier circuitry640. As the voltage Vpiezo is greater than the voltage (0 volts) at the second, non-inverting, input of the amplifier circuitry640, the amplifier circuitry640outputs a voltage −Vpiezo to the second terminal of the variable capacitor520.

As in the circuitry500, the second circuit node522, to which the second terminal of the piezoelectric transducer110is coupled, is thus effectively at 0 volts, such that the full-scale drive signal Vdrv appears across the piezoelectric transducer110.

Thus in the circuitry600ofFIG.6the variable capacitance520compensates for hysteresis in the piezoelectric transducer110, and the drive signal attenuation caused by the variable capacitor520is compensated by driving the second terminal of the variable capacitor520such that the second circuit node522is effectively at 0 volts. Thus no amplification of the drive signal Vdrv is required to compensate for the attenuation caused by the variable capacitor520.

Where the circuitry500,600is used for audio applications (i.e. where the piezoelectric transducer is used as an audio output transducer) a change in the capacitance of the variable capacitor520may give rise to audible artefacts such as click or pop sounds in the signal output by the piezoelectric transducer110. Thus it may be desirable to synchronise changes in the capacitance of the variable capacitor520to points at which the input signal Vdrv crosses 0v. Alternatively, if the voltage across the variable capacitor520can be copied to one or more reserve capacitors in advance of a change in the capacitance of the variable capacitor520, the capacitance may be changed at any time. However, both of these solutions require additional circuitry and give rise to increased complexity in controlling the circuitry500,600.

FIG.7is a schematic representation of further alternative driver circuitry for driving a piezoelectric transducer according to the present disclosure, in which the effects of hysteresis can be mitigated.

The driver circuitry, shown generally at700inFIG.7, shares many elements in common with the circuitry500ofFIG.5. Such common elements are denoted by common reference numerals inFIGS.5and6and will not be described in detail here.

The circuitry700differs from the circuitry500in that, instead of being coupled to a 0 volts or ground reference source, the first input of the subtractor530is coupled to an output of controllable-gain amplifier circuitry710that is provided in a feedforward path between the first signal path510and the first input of the subtractor530. Thus an input of the controllable-gain amplifier circuitry710is coupled to the first signal path510so as to receive the drive signal Vdrv.

The circuitry700may further include control circuitry720configured to receive the drive signal Vdrv and to control a gain β of the controllable-gain amplifier circuitry710based on a parameter such as a volume of an audio signal represented by the drive signal Vdrv, an envelope of the drive signal Vdrv or an instantaneous value (e.g. an instantaneous magnitude) of the drive signal Vdrv.

In operation of the circuitry700, the controllable gain amplifier710outputs a voltage βVdrv to the first input of the subtractor530. The voltage Vpiezo (which develops at the second circuit node522as a result of the drive signal Vdrv) is received at the second input of the subtractor530and an output signal (equal to βVdrv−Vpiezo) is output by the subtractor530to the driver circuitry540and thus appears at the second terminal of the variable capacitor520.

By adjusting the gain β of the controllable-gain amplifier circuitry710, a level of drive signal attenuation can be adjusted.

For example, where the drive signal Vdrv is a high amplitude signal (e.g. a high-volume audio signal), the control circuitry720may reduce the gain β of the controllable-gain amplifier circuitry710to zero. Thus βVdrv=0 and the circuitry700operates in the same way as the circuitry500described above, providing a voltage −Vpiezo at the second terminal of the variable capacitor520such that the second circuit node522is effectively at 0 volts and thus the full-scale drive signal Vdrv appears across the piezoelectric transducer110.

Where the drive signal is a lower amplitude signal (e.g. a lower-volume audio signal), the control circuitry may increase the gain β of the controllable-gain amplifier circuitry710, such that the level of attenuation of the hysteresis is reduced.

The resolution with which the gain β of the controllable-gain amplifier circuitry710can be adjusted may be sufficiently high as to permit smooth changes in the gain β at any time without giving rise to audible artefacts.

As will be appreciated by those of ordinary skill in the art, the subtractor530and driver circuitry540ofFIG.7could be replaced by differential amplifier circuitry of the kind shown at640inFIG.6, with the exception that the non-inverting input of the amplifier circuitry640would be coupled to the output of the controllable-gain amplifier circuitry710rather than to a 0 volt or ground reference supply.

The circuitry300,500,700may be provided as an integrated circuit (or as part of an integrated circuit). The present disclosure also extends to a system comprising the circuitry300,500,700(whether implemented as an integrated circuit or part of an integrated circuit or implemented in discrete circuitry) and a piezoelectric transducer110.

As will be apparent from the foregoing description, the circuitry300,500,700of the present disclosure is able to compensate for hysteresis in a piezoelectric transducer, and thus can reduce distortion in an audible output of the piezoelectric transducer.

Embodiments may be implemented as an integrated circuit which in some examples could be a codec or audio DSP or similar. Embodiments may be incorporated in an electronic device, which may for example be a portable device and/or a device operable with battery power. The device could be a communication device such as a mobile telephone or smartphone or similar. The device could be a computing device such as a notebook, laptop or tablet computing device, or a gaming device such as a games console. The device could be a wearable device such as a smartwatch, eyewear (e.g. smart glasses) or the like. The device could be a virtual reality (VR) or augmented reality (AR) device such as a VR or AR headset. The device could be a device with voice control or activation functionality such as a smart speaker. In some instances the device could be an accessory device such as a headset, headphones, earphones, earbuds or the like to be used with some other product.