TRANSDUCER DRIVER

Abstract: An apparatus and method for driving a capacitative transducer is described. A switching (Class D) power supply has a switching output stage (21) feeding through, and inductor (25) to the transducer (27). Instead of using voltage feedback, a sensor (23) measures the current through the inductor and feeds that back as negative feedback.

Description: TITLE: TRANSDUCER DRIVER DESCRIPTION The invention relates to transducers, e.g. loudspeaker transducers, and more particularly to means for driving transducers, particularly piezoelectric and other capacitative transducers. Advances in materials technology have allowed the use of more and more "voltage driven" transducers such as piezoelectric, in audio and related applications. These transducers tend to present a predominantly capacitative load to the driving amplifier, thus requiring precautions to be taken against instability or excessive high-frequency current flow. The use of highly capacitative loads with Class B and Class AB type amplifiers also results in very low efficiency due to the low power factor of such loads. The present invention provides a new approach to driving such devices allowing safe and very efficient power transfer. According to the invention there is provided a capacitative transducer drive apparatus including a signal input for accepting an input signal, power inputs for accepting electrical power, an output inductor connected to an output, a switching output stage having at least one switch connected between the power inputs and the output inductor, a modulator for controlling the switching output stage in accordance with the input signal, a current sensor for sensing the output current supplied through the output inductor, and a negative feedback loop for negatively feeding back the sensed output current through the modulator. The switching amplifier may efficiently drive a capacitative transducer. The current drive output may drastically reduce problems with resonance of the output inductor and the capacitative load. The current drive output may provide natural equalisation of the transducer characteristic. An Application Specific Integrated Circuit (ASIC) may integrate all the aspects above into a convenient device. The switching output stage may comprise two push-pull transistor pairs, each connected through a respective output inductor to a respective output. The capacitative transducer may be coupled between the outputs . To stabilise the output, a further capacitor may be connected between the outputs which may lessen the effect of capacitative transducer variation or drift on the circuit. The application of switching amplifier technologies, such as class D to audio power amplifiers is well known. Efficiency is gained by reducing the wasted power in the amplifier circuitry, mainly the output stage, by operating the output devices as switches rather than linear "resistive" elements. The efficiency of conventional non- switching amplifiers falls to zero as the load approaches that of a pure reactance; under these conditions switching amplifiers become more suitable as operating efficiencies can remain high. The driving of inductive loads by switching amplifiers is relatively straightforward since a certain amount of inductance is required to integrate the switching amplifier output into a smooth current waveform. More - nductance is simply added if it is required. Driving capacitative loads is not as trivial however as the output inductor will resonate with the effective load capacitance. When considering a practical application it is found that the Q factor of this resonance is often much greater than 1 and tends to fall at the upper frequency end of the operating region. Using open or closed loop voltage drive, precautions must be taken not to drive this LC combination at resonance where a high Q factor would result in significant voltage magnification (across the inductor and load) and significant current flow through the amplifier output. Adding a series resistor will lower the Q factor, but this may drastically reduce the efficiency. The alternative technique, according to the invention, is to operate the switching amplifier as a controlled current source, effectively increasing its output impedance drastically in the operating (audio) band. The Q factor of the indicator and capacitor (LC) combination can therefore be quashed to a low value allowing safe operation. As a side effect, the current source driving of the capacitative load provides a natural roll -off of approximately 6dB/octave with increasing frequency. This may be desirable in some cases, as it will tend to flatten out the rising response associated with such transducers, producing a wider operating bandwidth for audio use. Further equalisation can be provided before the amplifier stage, either actively or passively, if modification of the response is desired. Switching frequencies for the amplifier output stage may be significantly higher than the resonant frequency formed by the LC combination of output inductor and capacitance. Then the high impedance (current) output be extended to a high enough frequency to lower the Q factor of the circuit without causing loop instability. The invention also includes a method of driving a capacitative transducer, including providing an input signal, generating using switches a switched output signal controlled by the input signal, outputting the switched output signal through an output inductor, driving the capacitative transducer through the output inductor, sensing the output current passing through the output inductor and feeding back the sensed output current to provide negative feedback. A specific embodiment of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which Fig .1 shows a block diagram of apparatus according to the invention, and Fig.2 shows a detailed circuit diagram. In general terms, a signal is connected through an input 1, a low pass filter 3, and a high pass filter 5, to an error amplifier 7. The error amplifier 7 has a positive input terminal 9, a negative input terminal 11 and an output terminal 13. The output of the high-pass filter 5 is fed to the positive input terminal 9 of the error amplifier 7. The output terminal 13 of the error amplifier is connected to a modulator 17 which also accepts a clock input signal from a clock 15. The modulator 17 drives a switching output stage 21. The switching output stage contains at least one switch 22 connected between a power supply input 19 and an output inductor 25; the at least one switch 22 is controlled by the modulator 17. The output of the output inductor 25 is passed through a capacitative transducer 27 to earth 29. A current sensor 23 senses the output current through the output indicator 25 and capacitative transducer 27 and feeds a signal representing the sensed current back to the negative terminal 11 of the error amplifier 7. The individual components will now be described in greater detail . The low pass filter 3 is chosen based on the use of switching amplifier technology which requires observation of Nyquist ' s criterion, as the complete system is a sampled data system. The low pass filter has a cut-off frequency which will normally be placed at a value much lower than fs/2 (where fs is the clock frequency of the clock 15) thus helping to prevent aliasing from noise on the input. The filter can be used to limit the audio bandwidth where required and should have good out-of-band rejection. Due to the current source driving of the capacitative load the voltage across the load will be rising at 6dB/octave as the frequency decreases. Thus, a high pass filter 5 to limit the low frequency extension may therefore be included. The high pass filter should have at least 6dB/octave slope below a high-pass cut-off frequency and preferably at least 12dB/octave. Headroom will be maximised in this way. The error amplifier 7 forms the heart of the current output system. The output current is sampled by a sensor 23 and compared to the input 9, thus converting the amplifier to transconductance operation. Loop gain and compensation settings should be carefully set to ensure stability given the clock frequency/output stage supply voltage/load capacitance etc. At this point the analogue signal is converted to switching waveforms by the modulator 17 according to the chosen scheme. Various approaches are possible but simple, naturally sampled, class-D works well and is easily generated. In practice, the switching waveforms may be generated by a dedicated integrated circuit . A degree of hysteresis is usually required and the amount may be balanced against the increase in total harmonic distortion (THD) , particularly at near-clip levels. Too little and instability may set in. The switching output stage 21 may include a single- ended switching push-pull circuit. To maintn .,ι low THD the output waveform should be clean with fast transitions. To increase available voltage swing a bridge-tied output can be used (as in the circuit of Fig.2) . Two inductors will then be required and additional capacitors from the load terminals to ground may be needed to minimise stray noise. Most of the presently available capacitative transducers require significant voltages from the power supply 19 to drive them to their full potential. Typically the output stage may have to switch up to 50V for basic piezo materials and possibly more for others. Using the switching amplifier technique power consumption will be very low however. This makes it feasible to use a miniature step-up inverter to power the output stage with the few mA it generally requires. Although much more current will generally be flowing through the output stages and load, the energy associated with this is simply moving back and forth between a power supply decoupling capacitor and the output components. This process is what allows the switching amp to be so efficient, generating high output currents without high supply current demand. The current sensing may be carried out in any convenient manner. In particular, the voltage across a low-value resistor, for example 1Ω, may be sensed. The choice of inductor value of the output inductor 25 is a compromise between the resonance frequency value and the amount of switching current to be handled. Smaller values push the resonance frequency out of the operating band but also lead to a higher ripple current at the switching frequency. Losses due to the increased current in both the inductor and the output switching devices will increase accordingly. However, most of the extra output current will not be seen by the power supply directly due to the return of energy to the supply decoupling capacitor. The load also deserves attention. Irregularities in the load impedance can often be caused through mechanical resonance in the transducer. Although generally small, to minimise their effect on the frequency response an added capacitor can be connected across the output terminals, thus more closely approximating an ideal load. Adding a dominant capacitor here will also make the amplifier much more immune to the effect of changing the transducer. However, care must be taken not to reduce the LC resonance frequency of output inductor 25 and capacitor significantly into the operating region as output voltage capability rapidly diminishes above this resonant frequency by LC low- pass filter action. Work has been done to check that it is indeed feasible to combining all the required circuitry for this amplifier onto an application specific integrated circuit. Output stages working at up to 100V are possible within the same device package. As a result it is possible to produce a very compact amplifier design using one IC and a handful of passive components. A single 5V supply is provided and the built-in power supply electronics provide the higher output stage voltage. Using a device with a fabrication process allowing 50V, low distortion output swings of up to +/-45V are possible with a bridge-tied output. On board filters are configured with a small quantity of external passive components and loop compensation is achieved in a similar way. Applications of the device are many and various. Fig.2 shows a full component diagram for the capacitative transducer drive circuit. The input 1 is a headphone socket SK102. This feeds through a first op-amp, IC102C, and surrounding components which makes the op-amp a high-pass filter 5. This then feeds into a second op-amp IC102D configures as a low-pass filter 3. The output from the high-pass filter is fed to the error amplifier 7 IC103B. The error amplifier 7 is provided with a capacitor across it to give it some integration capacity to smooth out fluctuations in its output. The error amplifier feeds into a commercially available modulator 17 in the form of a switching controller, IC101 which is an HIP4080A. The output of this IC drives a switching output stage 21 including four field effect transistors 22 FET102, FET101, FET104 and FET103 which are arranged in two push-pull pairs each connected to an output terminal through 1 mH inductors . The output current is sensed across 1Ω resistors R131 and R132. The current sense outputs, labelled II and 12 are fed into a further op-amp IC103A which compares the currents passing through the two resistors to provide a measure of the current passing through the capacitative transducer. The resistors R131 and R132 in combination with the further op-amp constitute a current sensor 23. The output from this op-amp is fed through an electrolytic capacitor to the error amplifier IC103B. Also shown in the diagram are circuits that provide the reference voltage (top left) , the clock is that supplies an oscillator voltage, (to the right of the reference circuit element) . At the bottom right a start-up mute and over current trip circuit is shown that produces a disabled signal for feeding to the modulator IC IC101 and for illuminating a LED D104 for indicating a fault condition. The circuit can run with a high voltage supply up to +80V DC, typically +50V DC. The load capacitance can be up to lμF. It should be noted that the invention is not limited to the specific circuit arrangement shown. Indeed, the skilled person will be able to envisage many changes that will retain the functionality of the circuit. For example, the low- and high-pass filters can be switched round, different modulators can be used which will require different circuits for the switching output stage, and parts of the circuit such as the reference voltage supply or the oscillator can be omitted from the apparatus and separate reference voltages or oscillator signals provided, as appropriate in individual applications. Advantages provided by the present approach include a miniaturised solution and a full integrati n of low and high voltage sections with one convenient supply; low cost, and a simple solution to driving capacitative transducer. The types of devices that may be used as transducers are known as "smart materials". There are two main types: Piezoelectric and Electrostrictive . Piezoelectric materials are most often used because of their linearity, but it is possible to drive both types in the way described as they both provide a capacitative load. Both ceramic (PZT) and polymer (PVDF) piezoelectric materials are suitable and various dopings are available to suit the application. The amplifier is also applicable to electrostatic type loudspeakers where with high voltage switching the amplifier could be used to drive the plates directly.

Claims: CLAIMS 1. Capacitative transducer drive apparatus including a signal input for accepting an input signal, power inputs for accepting electrical power, an output inductor connected to an output, a switching output stage having at least one switch connected between the power inputs and the output inductor, a modulator for controlling the switching output stage in accordance with the input signal, a current sensor for sensing the output current supplied through output inductor, and a negative feedback loop for negatively feeding back the sensed output current through the modulator. 2. Apparatus according to claim 1 wherein the negative feedback loop includes an error amplifier having an output connected to the modulator, and a positive input and a negative input connected to the output of the current sensor and to the signal input . 3. Apparatus according to claim 2 further comprising a filter between the signal input and the error amplifier. 4. Apparatus according to claim 1 integrated on an application specific integrated circuit. 5. Apparatus according to claim 1, wherein the switching output stage comprises two push- pull transistor pairs, each push-pull transistor pair being connected through a respective output inductor to a respective output, between which outputs the capacitative transducer may be coupled . ~~ " 6. Apparatus according to claim 6 further comprising a capacitor connected between the outputs . 7. Loudspeaker drive apparatus including a capacitative transducer drive apparatus according to any preceding claim, and a piezoelectric loudspeaker transducer connected to the output of the capacitative transducer drive apparatus. 8. A method of driving a capacitative transducer, including providing an input signal, generating using switches a switched output signal controlled by the input signal, outputting the switched output signal through an output inductor, driving the capacitative transducer through the output inductor, sensing the output current passing through the output inductor and feeding back the sensed output current to provide negative feedback. 9. A method of driving a capacitative transducer according to claim 8, including providing an error amplifier having positive and negative inputs and an output feeding the input signal to the positive input, feeding the sensed output current to the negative input , and controlling " the switched output current using the output of the error amplifier. 10. A method according to claim 9 including filtering the input signal before feeding it to the error amplifier.

Inventors: AZIMA HENRY, GAMBLE DAVID, AZIMA, HENRY, GAMBLE, DAVID
Applicants: AZIMA HENRY, GAMBLE DAVID, NEW TRANSDUCERS LTD, AZIMA, HENRY, GAMBLE, DAVID, NEW TRANSDUCERS LIMITED
Application year: 2000
Main classification: H04R
IPC codes: H02M3, H03F3, H04R3, H04R17