Detection and control of diaphragm collapse in condenser microphones

Field of the invention

0001The invention relates to a condenser microphone comprising a detection means adapted to determine a physical parameter value related to a separation between a transducer element diaphragm and back-plate and a collapse control means adapted to control a DC bias voltage of the transducer element based on the determined physical parameter value.

Background of the invention

0002It is well-known that electrostatic actuators and sensors may enter an undesired so-called collapsed state under certain operating conditions such as when exposed to extraordinary high sound pressure levels or mechanical shock.

0003The collapsed state is characterized by a 'collapse' or sticktion between the diaphragm and the back-plate as described in <patcit id="pcit0001" dnum="WO02098166A"><text>PCT patent application WO 02/098166</text></patcit> which discloses a silicon transducer element. When a polarity of an incoming sound pressure is so that the diaphragm, usually the moveable plate, is deflected towards the back-plate, the force originating from an impinging sound pressure is combined with an attractive force originating from a DC electrical field provided between the diaphragm and the back-plate. When a sum of these forces exceeds a predetermined critical value, an opposing force provided by a diaphragm suspension will be insufficient to prevent the diaphragm from approaching and contacting the back-plate and the microphone enters a collapsed state. The diaphragm can only be released from the back-plate once the attractive force originating from the DC electrical field acting on the diaphragm has been removed or at least significantly reduced in magnitude.

0004<patcit id="pcit0002" dnum="US5870482A"><text>US 5,870,482</text></patcit> discloses a prior art silicon microphone wherein mechanical countermeasures have been induded to prevent diaphragm collapse by restricting maximum deflection of the microphone diaphragm to less than a collapse limit which in the disclosed microphone construction is about 1 µm.

0005In silicon condenser microphones where no special means have been applied to prevent collapse of the diaphragm, fully or at least party removing the microphone DC bias voltage will remedy the collapsed state and secure that the transducer element returns to a normal or quiescent state of operation. Usually, the diaphragm and the back-plate condenser plates have both been treated with a non-conducting anti-sticktion coating which will prevent Van der Waal forces keeping the diaphragm sticking even if the DC bias voltage that generates the DC electrical field between the transducer element diaphragm and back-plate has been removed (zeroed).

0006To the knowledge of the inventors, there has not yet been disclosed collapse detection and control circuit adapted for use in condenser microphones.

Summary of the invention

0007In a first aspect the invention provides a condenser microphone comprising: <ul id="ul0001" list-style="dash"><li>a transducer element comprising <ul id="ul0002" list-style="dash"><li>a diaphragm having an electrically conductive portion,</li><li>a back-plate having an electrically conductive portion,</li></ul></li><li>DC bias voltage means operatively coupled to the diaphragm and the back-plate,</li><li>collapse detection means adapted to determine a physical parameter value related to a separation between the diaphragm and the back-plate,</li><li>collapse control means adapted to control the DC bias voltage means based on the determined physical parameter value.</li></ul>

0008The collapse detection is adapted to detect a separation or distance between the diaphragm and back-plate as a measure of the operating condition or state of the transducer element with respect to collapse. In case a collapse has occurred there will be no separation between the diaphragm and the back-plate. A very small separation indicates that the transducer element may be close to a collapse. A large separation or distance between the diaphragm and the back-plate indicates that the transducer element is in a safe operating condition, i.e. it is far from a collapse.

0009The collapse control means is adapted to control the DC bias voltage in order to control the operation state of the transducer element. In case a collapse has occurred it is possible to remedy the collapsed state of the transducer element by reducing or completely removing the DC bias voltage. In case safe operation is detected or determined, the collapse control means will provide a normal or nominal DC bias voltage. In case the collapse detection means determines a too low separation between the diaphragm and the back-plate it may be desirable to reduce the DC bias voltage and thus reduce the DC electrical field strength between the diaphragm and back-plate and hereby prevent an approaching collapse from occurring.

0010The collapse detection means may be adapted to determine an instantaneous value of the physical parameter or short-term average value of the physical parameter. Since a single sound pressure peak may cause a collapse it may be desirable to monitor a peak value, i.e. an instantaneous value of the physical parameter. However, it may be preferred to average the physical parameter value over a short time period, such as a time period in between 1-100 µs or 100 µs and 100 ms.

0011In some embodiments the collapse control means is adapted to avoid collapse of the transducer element. In alternative embodiments the collapse control means is adapted to allow collapse of the transducer element, and adapted to remedy a collapsed condition by discharge means operatively coupled to the transducer element and adapted to discharge the transducer element for a predetermined discharge time.

0012As described above the first aspect of the invention provides a condenser microphone that can handle high sound pressure levels or drop induced shocks without entering an irreversible collapsed state. This latter condition could require a user to remove a microphone power supply and restart the microphone or the entire apparatus employing the microphone. This can either be achieved by preventing a microphone collapse and thus the transducer will remain operational without interruption of sound. Alternatively, a collapse can be remedied after its occurrence whereby the microphone may malfunction during a certain predetermined period of time before a normal operational state of the transducer element has been re-established. However, such a malfunctional period of time may be acceptable for the user if the sound interruption is sufficiently short, such as shorter than three seconds, or preferably shorter than one second, such as less than 500 ms or 200 ms or most preferably less than 100 ms. A condenser microphone may be exposed to high sound pressure levels at low frequencies by car door slams. However, during such circumstances a short interruption of sound from the microphone may be fully acceptable for the user if normal operation is resumed after for example a few hundred milliseconds.

0013The collapse detection means may be adapted to determine a capacitance of the transducer element. The collapse detection means may be adapted to determine the physical parameter value by applying a probe signal to the transducer element and determine a value of a response to the probe signal. Such probe signal may comprise a signal selected from the group consisting of; DC signals and ultrasonic signals.

0014In some embodiments the collapse detection means comprises a capacitive divider comprising a cascade between a fixed capacitor and the transducer element. In some embodiments the collapse detection means may be responsive to a sound pressure impinging on the diaphragm. In these embodiments the collapse detection means may comprise a sensor microphone positioned in proximity to the transducer element and operatively coupled to the collapse control means.

0015In still other embodiment the collapse detection means is adapted to detect a peak voltage generated by the transducer element, i.e. an instantaneous output signal from the transducer element is directly used as a physical parameter reflecting a sound pressure level to which the transducer element is exposed. In order not to disturb the normal function of the transducer element the detection circuit should have an input buffer that does not load the transducer element significantly, i.e. the input buffer must exhibit a small input capacitance relative to the output capacitance of the transducer element.

0016Preferably, the collapse control means is adapted to reduce a DC bias voltage across the transducer element based on the determined physical parameter value. The collapse control means may comprise bias current monitoring means adapted to detect a DC current flow from the DC bias voltage means to the transducer element. The collapse control means may be adapted to electrically connect the diaphragm and the back-plate upon the detected physical parameter value exceeding a predetermined threshold. Preferably, the collapse control means comprises <ul id="ul0003" list-style="dash"><li>a controllable element adapted to generate an electrical pulse with a predetermined duration and amplitude based on the determined physical parameter value, and</li><li>a switch element adapted to receive said electrical pulse and to electrically connect the diaphragm and the back-plate in response thereto.</li></ul>

0017The collapse control means may be adapted to adaptively reduce the DC bias voltage based on the determined physical parameter value.

0018In preferred embodiments, the transducer element comprises a silicon transducer or MEMS transducer. The silicon transducer may be implemented on a first silicon substrate, while the collapse detection means and the collapse control means are implemented on a second silicon substrate. The collapse detection means and the collapse control means are preferably monolithically integrated on a single die. The die may further comprise a preamplifier operatively coupled to the transducer element.

0019As indicated above the preferred embodiments of the collapse detection means and collapse control means comprises electronic circuits which may make mechanical solutions obsolete and allow a higher degree of freedom in the mechanical construction of the transducer element. This is a significant design advantage with silicon and MEMS based microphones. In addition, electronic solutions offer larger flexibility in a practical setting of a predetermined threshold level associated with a certain sound pressure level or a certain separation between the diaphragm and back-plate where the collapse control means is triggered. Electronic circuit based collapse detection means accordingly allow simple customization to fit needs of any particular application.

0020A second aspect of the invention provides an electronic circuit for condenser microphones, the circuit comprising DC bias voltage means couplable to condenser microphone diaphragm and back-plate, <ul id="ul0004" list-style="dash" compact="compact"><li>collapse detection means adapted to determine a physical parameter value related to a separation between the diaphragm and the back-plate of the associated condenser microphone, and collapse control means adapted to control the DC bias voltage means based on the determined physical parameter value.</li></ul>

0021Such an electronic circuit may be adapted for different types of transducer elements even without any modification, or by means of a limited number of adjustable parameters associated with the function of the collapse control means. The electronic circuit may be integrated on a separate semiconductor substrate or die or it may be monolithically integrated with the microphone transducer element, in particular in case the transducer element comprises a silicon transducer element.

0022The collapse detection means may be adapted to determine a capacitance of the transducer element. Alternatively, the collapse detection means may be adapted to determine the physical parameter value by applying a probe signal to the transducer element. In a simple and advantageous embodiment of the invention, the collapse detection means is adapted to detect a transient peak signal voltage or peak voltage generated by the transducer element. This peak voltage may be reached subsequent to a collapse event so that the collapse event by itself generates a transient signal voltage from the transducer which exceeds a predetermined trigger voltage and activates the collapse control means.

0023The collapse control means may be adapted to adaptively reduce the DC bias voltage based on the determined physical parameter value. In a simple embodiment the collapse control means comprises discharge means operatively coupled to the transducer element and adapted to discharge the transducer element for a predetermined discharge time.

Brief description of drawings

0024In the following the invention will be described with reference to the accompanying figures, of which <ul id="ul0005" list-style="none"><li><figref idref="f0001">Fig. 1</figref> shows a preferred embodiment of a collapse detection and control circuit,</li><li><figref idref="f0002">Fig. 2</figref> shows a preferred embodiment of a DC bias voltage generator,</li><li><figref idref="f0003">Fig. 3</figref> shows a diagram of an embodiment of a collapse detection and control circuit using a probe signal, and</li><li><figref idref="f0004">Fig. 4</figref> shows a diagram of another embodiment of a collapse detection circuit using a sensor microphone and a control circuit implemented using a Digital Signal Processor (DSP).</li></ul>

0025While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

Detailed description of the invention

0026In the following embodiments of a collapse detection and control circuit suitable for integration into miniature silicon based condenser microphones will be described.

0027With regards to a collapse detection circuit for detection of a separation between diaphragm and back-plate several embodiments are envisioned. Physical parameters such as voltage, capacitance and sound pressure can be used as will be described in the following. The detection circuit should preferably not load the transducer element of the condenser microphone with any significant impedance (compared to the generator impedance of the transducer element itself). A silicon transducer element of a MEMS microphone has very large impedance that substantially corresponds to a capacitance between 5 - 20 pF which makes meeting this requirement a significant challenge.

0028Several embodiments of collapse control circuits are also possible according to the invention and some are described in the following in combination with detection circuits. The collapse detection and control circuitry is preferably fabricated on a CMOS semiconductor substrate, such as 0.35µm mixed-mode CMOS process. This technology is flexible with both good analog and digital circuitry capabilities. The bias voltage circuitry for the condenser transducer element and preamplifiers may advantageously be integrated on the same semiconductor substrate. In this latter case the CMOS process preferably comprises high-voltage capabilities. This means semiconductor devices, such as transistors, diodes, capacitors etc., which can withstand respective terminal voltage differences above 10 V, or preferably above 15 or 20 V.

0029<figref idref="f0001">Fig. 1</figref> shows a preferred embodiment of collapse detection and control circuit suitable for integration into a silicon based condenser microphone fabricated by MEMS techniques. A silicon transducer element of this condenser microphone has dimensions of 1,3 * 1.3 mm with an air gap between back-plate and diaphragm of approximately 1 µm and a nominal capacitance of about 5 - 15 pF. The detection circuit comprises a peak voltage detector adapted to determine and flag every generated signal peak with a polarity which corresponds to a sound pressure moving the diaphragm towards the back-plate and which exceeds a predefined threshold level corresponding to a maximum safe sound pressure level.

0030In <figref idref="f0001">Fig. 1</figref> a condenser microphone element 1 or transducer element is connected to an integrated microphone preamplifier and microphone biasing and collapse detection and control circuitry indicated by the dashed box 2. A signal amplifier 3 or preamplifier is connected between input terminal IN and output terminal OUT. A DC bias voltage generator 4 provides a DC voltage of VB. A high impedance element and charge monitor circuit 5 with transistor elements A, B and C control the DC bias voltage applied to DC bias voltage terminal BIAS. A collapse control circuitry 6 is indicated within a dashed box. The collapse control circuitry 6 has a voltage generator VP providing a predetermined threshold voltage for collapse control 7 in combination with a voltage drop across resistor R. A comparator 8 compares said threshold voltage for collapse control 7 with the input signal provided by the condenser microphone element 1 at terminal IN. Output from the comparator 8 is connected to a monostable pulse generator 9 that is connected to a bias voltage clamp switch 10, that preferably comprises a high-voltage NMOS transistor, capable of connecting the bias terminal BIAS to ground through a relatively low resistance such as 10 Kohm or less to discharge the transducer element.

0031The high impedance element and charge monitor circuit 5 consists of two anti-parallel, diode-coupled P-channel MOSFETs A and B. The P-channel MOSFET C is a M-fold current mirror ensuring the current passing through the microphone connected to BIAS and IN is multiplied by a factor M. The collapse control circuit 6 compares the input signal at terminal IN with a threshold voltage 7 composed of a predefined portion VP and the voltage drop over the resistor R. The reference voltage 7 is designed so that during charging of the condenser microphone element 1, i.e. during start-up of a DC bias voltage generator VB 4 caused by an approaching collapse event, signal disturbances on terminal IN caused by the microphone charging process, will not be able to trigger the comparator 8 and initiate a pulse for shutting down the bias by the clamp switch 10.

0032When the microphone is fully charged during normal operation, triggering of the clamp switch 10 will only take place if positive signal peaks on IN exceeds VP, reflecting a sound pressure level exceeding the desired predefined threshold voltage or level. If the predefined threshold voltage is selected so that it corresponds to a maximum safe sound pressure level for the transducer element, it is possible to discharge the transducer element prior to collapse and thus prevent a collapse.

0033<figref idref="f0002">Fig. 2</figref> shows a preferred embodiment for the bias voltage generator VB 4 of <figref idref="f0001">Fig. 1</figref> comprising a Dickson voltage multiplier. VB 4 is adapted to provide a DC bias voltage of about 8 - 10 V to node BIAS by multiplying a VBAT voltage between 1.0 and 1.4 Volt. This type of voltage multiplier requires a clock with two, non-overlapping phases φ1 and φ2, as sketched below the diagram of <figref idref="f0002">Fig. 2</figref>. A DC voltage source, for example a battery, applies the DC voltage VBAT to the voltage multiplier. The voltage multiplier consists of a number of separate stages 11 coupled in series. Each stage 11 contains a diode D 12 and a capacitor C 13 where the bottom plate of the e.g. capacitor 13 is coupled to φ1 while a capacitor of the subsequent stage is coupled to φ2 and so forth. An output DC voltage OUT is generated across a final capacitor C 14. All diodes such as diode 12 should preferably be types that show low current leakage and low parasitic capacitances to neighbouring devices and circuit surroundings (substrate, clock, ground or power lines). This means a preferred embodiment of the diodes comprises a substrate-isolated type of diode such as a poly-silicon diode. In other embodiments the diode D 12 may be a PN-junction diode, a Schottky diode or a diode coupled bipolar, or a field-effect transistor.

0034<figref idref="f0003">Fig. 3</figref> shows another embodiment of the invention where a detection circuit, relying on a high-frequency probe signal, transmits the probe signal through the transducer element and detect any significant change in capacitance of the transducer element that would indicate that the transducer element is collapsed or close to collapse.

0035In <figref idref="f0003">Fig. 3</figref> a transducer element 1 of a condenser microphone is shown coupled to an output terminal Out via preamplifier Amp. A reference voltage Ref V is generated and supplied to an oscillator 30. This is done, so that the output of the oscillator 30 is well-defined. A voltage pump or voltage multiplier is operated on a clock frequency generated by the oscillator. VP increases the reference voltage to the DC bias voltage of transducer element 1 of a MEMS microphone, typically in the range 10-20 V.

0036A portion of the AC voltage from the oscillator 30 is used as a high-frequency probe and fed to the transducer element 1 through a cascade coupled capacitor 31, Cx. The probe voltage drop across the capacitive transducer element 1 will be modulated by any incoming sound pressure due to the varying capacitance thereof.

0037In case of a collapse of the microphone diaphragm, the average separation between the diaphragm and the back-plate of the transducer element 1 will be significantly smaller than the nominal separation i.e. the quiescent distance between the back-plate and diaphragm. Since the distance between these two plates is zero during collapse, the capacitance of the transducer element 1 will be substantially larger so as to result in a lower probe voltage across the transducer element 1 of the microphone. Likewise, a larger probe voltage will exist across the external capacitor 31. This latter signal is high pass filtered by high pass filter 32, HPF, to remove any audio information and eliminate DC-offset. The high frequency component is fed to an electronic multiplier X, which may comprise an analog multiplier such as a Gilbert cell, and multiplied by the direct output of the oscillator 30.

0038The multiplication will result in sum and difference products of the angular oscillator frequency ω, in mathematical terms:<maths id="math0001"><math display="block"><msub><mi mathvariant="normal">A</mi><mn mathvariant="normal">0</mn></msub><mo mathvariant="normal">*</mo><mi>cos</mi><mfenced><mi mathvariant="normal">ωt</mi></mfenced><mo mathvariant="normal">*</mo><msub><mi mathvariant="normal">B</mi><mn mathvariant="normal">0</mn></msub><mo mathvariant="normal">*</mo><mi>cos</mi><mfenced><mi mathvariant="normal">ωt</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">φ</mi></mfenced><mo mathvariant="normal">=</mo><msub><mi mathvariant="normal">½A</mi><mn mathvariant="normal">0</mn></msub><mo>⁢</mo><msub><mi mathvariant="normal">B</mi><mn mathvariant="normal">0</mn></msub><mrow><mo mathvariant="normal">(</mo><mfenced><mi>cos</mi><mfenced><mn mathvariant="normal">2</mn><mo>⁢</mo><mi mathvariant="normal">ωt</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">φ</mi></mfenced><mo mathvariant="normal">+</mo><mi>cos</mi><mfenced><mi mathvariant="normal">φ</mi></mfenced></mfenced></mrow><mspace width="1em" /><mi>wherein</mi></math><img file="EP1599067B1_D0001.tif" /></maths>

0039A₀ is the magnitude of the probe signal across the transducer element 1 and B₀ a constant associated with the multiplication process. After lowpass filtering LPF, output is: ½ A₀B₀ cos(φ), where φ is a small phase difference (φ&lt;&lt;1) between the high frequency probe signal across the transducer element 1 and the probe signal of the oscillator 30. The DC component of the demodulated probe signal is thus proportional to the probe voltage across the transducer element 1 and can be utilized to determine the state of the transducer element 1 by a simple threshold circuit or procedure with a predetermined threshold level.

0040By comparing the above described detection scheme to a scheme based on detection of the collapsed condition only based on a threshold trigger mechanism relative to the acoustic output possible advantages are visible. Detecting collapse by measuring the acoustic level from the microphone will cause difficulties in measuring collapse, if this occurs near the maximum acoustic level that is desirable to measure. Under these conditions a collapse may go undetected If the trigger level is set too high, or a collapse is detected while inside the normal working range. One way to ensure completely safe collapse prevention, even when the collapse level is close to the maximum acoustic level desirable to measure, is by setting the corner frequency to a lower frequency than the highpass filter 32, e.g. to a frequency of about 10-30Hz.

0041The optimum noise margin for reliable detection of the collapsed state without generating false positive collapse detection events can be found as described in the following. If the capacitance of the microphone in quiescent operating is designated Cn, and in the collapsed condition Cc, a maximum sensitivity is obtained by choosing the value of the external feed capacitor Cx, integrated on-chip, as follows:<maths id="math0002"><math display="block"><mi>Cx</mi><mo>=</mo><mi>½</mi><mo>⁢</mo><mfenced><mi>Cn</mi><mo>+</mo><mi>Cc</mi></mfenced></math><img file="EP1599067B1_D0002.tif" /></maths>

0042It is preferred that respective manufacturing tolerances of Cn and Cc can be kept smaller than about 10-20%, in order to reliably and accurately detect a collapsed state of the transducer element 1. The high-frequency probe voltage across the transducer element 1 at the frequency of the oscillator 1, will have an amplitude larger than U/2, where U is the AC voltage provided by oscillator 30 during normal operation and an amplitude lower than U/2 during a collapsed state.

0043As a numerical example, consider Cc=15 pF and Cn=5 pF. An optimal feed-forward capacitor is then Cx=10 pF.

0044It will finally be noted, that power is consumed due to the charging/discharging of the capacitors. During normal operation this power loss is:<maths id="math0003"><math display="block"><mi mathvariant="normal">P</mi><mo mathvariant="normal">=</mo><mi mathvariant="normal">f</mi><mo mathvariant="normal">*</mo><mi mathvariant="normal">U</mi><mo mathvariant="normal">*</mo><mi mathvariant="normal">U</mi><mo mathvariant="normal">*</mo><mfenced><mi>Cn</mi><mo mathvariant="normal">*</mo><mi>Cx</mi></mfenced><mo mathvariant="normal">/</mo><mfenced><mi>Cn</mi><mo mathvariant="normal">+</mo><mi>Cx</mi></mfenced><mo mathvariant="normal">,</mo></math><img file="EP1599067B1_D0003.tif" /></maths> If U=1 Volt, f=250 kHz and with the values above, power loss P will be:<maths id="math0004"><math display="block"><mi mathvariant="normal">P</mi><mo mathvariant="normal">=</mo><mn mathvariant="normal">0.25</mn><mo mathvariant="normal">*</mo><mn mathvariant="normal">6</mn><mspace width="1em" /><mi mathvariant="normal">μW</mi><mo mathvariant="normal">=</mo><mn mathvariant="normal">1.5</mn><mspace width="1em" /><mi mathvariant="normal">μW</mi><mn mathvariant="normal">.</mn></math><img file="EP1599067B1_D0004.tif" /></maths>

0045This value is acceptable also for low-power applications such as portable and battery operated mobile terminals and hearing prostheses.

0046In the case that the oscillator frequency is considerably higher than 250 kHz, it may be of advantage to divide it down with a fixed integer number N, and use this frequency instead for the multiplication outlined above. It is an advantage to main the same frequency for testing and mixing and that this frequency is placed outside the audible range. Also, it should preferably not be placed right at a high frequency resonance of the silicon microphone. Preferably, the high-frequency probe passed through the transducer element 1 has the same frequency as pump frequency used for the voltage pump 34, VP, that generates the DC bias voltage of across condenser plates of the transducer element 1. This choice is to avoid any unwanted mixing products between these two frequencies.

0047In yet another embodiment of the invention several portions of the detection circuit of <figref idref="f0003">Fig. 3</figref> is used and this embodiment is likewise based on a detecting parameters derived from a capacitive voltage divider. In the present embodiment, a change in DC voltage across the transducer element 1 is directly measured and used to indicate or detect which state the transducer element 1 has. This embodiment relies on detecting a collapsed state of the transducer element 1 by detecting a large DC shift of the signal voltage across the transducer element 1 caused by an abrupt change of capacitance of the transducer element 1. This abrupt change of capacitance changes a division of DC voltage between fixed capacitor 31 and the transducer element 1. The threshold detector TD 35 of <figref idref="f0003">Fig. 3</figref> can detect the change of DC voltage. If the transducer element 1 and the microphone preamplifier 3 (<figref idref="f0003">Fig. 3</figref>) has a long settling time, it means that a collapse produces a long DC pulse.

0048Based on the detected threshold by threshold detector TD, a reset circuit 36, Res C, which may comprise a semiconductor switch of low impedance, such as lower than 25 Kohm or 10 Kohm, when activated. The active semiconductor switch serves to reduce or even null any DC voltage between the plates of the transducer element 1 for a predetermined period of time. A timer 37, T, is preferably including to provide a reduction or null of the DC bias voltage during a predetermined period of time, such as 1-100 ms, after which a collapsed state of the transducer element 1 can be assumed remedied.

0049<figref idref="f0004">Fig. 4</figref> shows an embodiment based on detecting a physical parameter value associated with a separation between diaphragm and back-plate of a silicon condenser microphone 41, MMIC, by sensing a sound pressure to which the condenser microphone is exposed by a dedicated sensor microphone, 40, S MIC. The sensor microphone 40 and preamplifier 2 are added to the silicon substrate and amplifier circuit that already comprises the main microphone 41 and its associated preamplifier for which collapse detection and control are to be implemented.

0050The sensor microphone 40 is preferably substantially smaller than the main microphone 41 and may have a lower sensitivity. Preferably, the sensor microphone 40 has a collapse point or threshold which is around 10 - 30 dB higher in sound pressure level than the collapse threshold of the main microphone 41 so as to ensure that the sensor microphone 40 behaves in substantially linearly in the collapse region of the main microphone 41 for all envisioned main microphone variants. The output of the sensor microphone 40 is provided to the collapse control means 42, BC, that preferably operates by providing gradual decrease of DC bias voltage of a condenser transducer element (not shown) of the main microphone 41. It is preferred to hold the DC bias voltage of the sensor microphone 40 substantially constant.

0051According to the present embodiment of the invention, the main microphone 41 is supplied by bias voltage controlled by the bias voltage control means 42 that is supplied with a DC voltage which could be a battery voltage from a 1.30 Volt Zinc-air battery. The collapse detection and control means may comprise a DSP 43 adapted to control the bias voltage control circuit 42 based on an output signal of the sensor microphone 40. A control algorithm implemented in the DSP 43 may be adapted to either reduce the DC bias voltage to the main microphone once a threshold sound pressure level is reached, or the DSP 43 may be adapted to reduce or even completely null the DC bias voltage if the instantaneous or short-term average incoming sound pressure level exceeds threshold sound pressure level to indicate a potential collapse of the main microphone 41.

0052The collapse control circuit may be based on a more sophisticated control of the DC bias voltage of the transducer element than the ones shown. Instead of clamping the DC bias voltage across the transducer element of the main microphone 41, the DC bias voltage may be gradually decreased in response to detecting an approach of collapse. This dynamic adoption of DC bias voltage based on the detected incoming sound pressure level will also be able break a positive feedback loop that causes the collapse. A safe operation region of the transducer element can be maintained. After an intermittent reduction of DC bias voltage, the DC bias voltage may advantageously be increased toward a nominal of DC bias voltage with a suitable predetermined release time constant. Such type of adaptive gradual control of the DC bias voltage could be implemented by a suitable piece of software or set of program instruction in the DSP 43.

0053This type of dynamic adoption of the DC bias voltage based on the detected incoming sound pressure level could also be added to any of the detection circuits shown in <figref idref="f0001">Figs. 1</figref> and <figref idref="f0003">3</figref>.

0054In general, it may be desirable to implement at least parts of the collapse detection and control means using a DSP. It may be advantageous to utilise a DSP means already present in the associated apparatus, for example a programmable DSP of a mobile phone or a hearing aid. In this way it is possible to minimize the need for additional components to implement the collapse detection and control. Using a DSP enables implementation of complex algorithms for both collapse detection and control.

0055The solutions according to the invention could be implemented either integrated into the microphone or, as shown in <figref idref="f0001">Fig. 1</figref>, the collapse detection and control circuits could be arranged on a separate Application Specific Integrated Circuit. DC bias voltage circuits may be integrated with the collapse control circuit. If preferred, separate ASICs may be provided for the collapse detection circuit and the collapse control circuit.

0056The invention has a wide range of applications within miniature condenser microphones suited for portable communication devices such as mobile phones and hearing prostheses.