Abstract
A MEMS microphone with an improved sensitivity, e.g., a reduced temperature dependence of the sensitivity. The microphone includes a MEMS capacitor, a charging circuit and a bias circuit. The bias circuit includes a closed loop control circuit and creates a bias voltage with a temperature dependence.
Claims
1. MEMS microphone with improved sensitivity, comprising a MEMS capacitor with a variable capacitance, a charging circuit provided for charging the MEMS capacitor, a bias circuit provided for creating a bias voltage V.sub.bias applied to the charging circuit, where the bias circuit comprises a closed loop control circuit and the bias voltage V.sub.bias has a temperature dependence.
2. MEMS microphone according to claim 1, where the bias voltage V.sub.bias has a piecewise linear temperature dependence.
3. MEMS microphone according to claim 1, where the closed loop control circuit comprises a temperature sensor.
4. MEMS microphone according to claim 3, where the temperature sensor provides a Proportional To Absolute Temperature (PTAT) voltage V.sub.sen.
5. MEMS microphone according to claim 1, where the closed loop control circuit comprises a slope generator.
6. MEMS microphone according to claim 5, where the slope generator comprises a comparator and a plurality of serial connections of a resistor and a switch.
7. MEMS microphone according to claim 1, where the closed loop control circuit comprises a slope control circuit.
8. MEMS microphone according to claim 7, where the slope control circuit comprises a plurality of comparators.
9. MEMS microphone according to claim 1, where the charging circuit comprises a charge pump.
10. MEMS microphone according to claim 5, where the slope generator generates a linear voltage V.sub.sg and a slope of V.sub.sg is mainly proportional to the negative slope of the temperature dependence of the sensitivity of the microphone, S(T), at a given temperature.
11. MEMS microphone according to claim 1, where the bias voltage is the sum of a plurality of voltages selected from a plurality of temperature independent voltages and temperature dependent voltages.
12. MEMS microphone according to claim 1, where the bias circuit is a part of an ASIC of the MEMS microphone.
13. MEMS microphone according to claim 1, further comprising a non-volatile memory element for storing linearity parameters.
14. MEMS microphone with improved sensitivity, comprising a MEMS capacitor with a variable capacitance, a charging circuit provided for charging the MEMS capacitor, a bias circuit provided for creating a bias voltage V.sub.bias applied to the charging circuit, where the bias circuit comprises a closed loop control circuit, the bias voltage V.sub.bias has a temperature dependence, and where the bias voltage V.sub.bias has a piecewise linear temperature dependence.
15. MEMS microphone with improved sensitivity, comprising a MEMS capacitor with a variable capacitance, a charging circuit provided for charging the MEMS capacitor, a bias circuit provided for creating a bias voltage V.sub.bias applied to the charging circuit, where the bias circuit comprises a closed loop control circuit which comprises a slope generator, and the bias voltage V.sub.bias has a temperature dependence.
Description
(1) The present invention, basic working principles and preferred embodiments are described in the accompanying drawings, wherein
(2) FIG. 1 shows a basic equivalent circuit diagram of the microphone,
(3) FIG. 2 shows a possible temperature dependent sensitivity curve with decreasing slope,
(4) FIG. 3 shows a possible temperature dependent sensitivity curve with increasing absolute value of the sensitivity,
(5) FIG. 4 shows different piece-wise linear segments of the voltage generated by the slope generator V.sub.sg and the bias voltage V.sub.bias. The bias voltage may be the sum of the voltage generated by the slope generator and an offset voltage,
(6) FIG. 5 shows a possible output of a temperature sensor providing a PTAT voltage,
(7) FIG. 6 shows a more detailed equivalent circuit diagram of the microphone where the bias circuit comprises a temperature sensor, a slope control circuit and a slope generator within the closed loop control circuit,
(8) FIG. 7 shows another embodiment further comprising a voltage buffer in the bias circuit,
(9) FIG. 8 shows another embodiment further comprising an amplifier,
(10) FIG. 9 shows embodiments of a slope control circuit, a temperature sensor, and a slope generator,
(11) FIG. 10 shows a possible embodiment of a charging circuit comprising elements of a charge pump.
(12) FIG. 1 shows an equivalent circuit diagram of the basic embodiment of a MEMS microphone MM. The microphone MM comprises a MEMS capacitor MCAP that may be arranged in a MEMS device, e.g. a MEMS chip MEMS. The microphone further comprises a bias circuit BC and a charging circuit CC. The bias circuit BC provides a control signal controlling the charging circuit CC. The charging circuit CC generates the operation voltage for the MEMS capacitor MCAP and transfers the corresponding electrical charge to the capacitor. The capacitor may comprise two or more electrodes. At least one of the electrodes is connected to a signal output SO where an electrical signal encoding the received audio signal can be obtained for further processing.
(13) The bias circuit BC comprises the closed loop control circuit CLCC creating the control signal controlling the charging circuit. The control signal may be a voltage signal applied to the control circuit CC.
(14) Within the closed loop control circuit, a closed loop circuit monitoring the actual operation temperature and adaptively adjusting the control signal is contained.
(15) FIGS. 2 and 3 show different possible temperature dependent sensitivity curves of a MEMS microphone. In FIG. 2, a sensitivity curve and corresponding piece-wise linear approximations are shown where the sensitivity is reduced with increasing temperature. However, the absolute value of the slope of the sensitivity is reduced with increasing temperature. The threshold temperatures T1, . . . , TN are chosen such that the approximation of the sensitivity curve by the piece-wise linear segments is as good as possible for a given number of threshold temperatures. Thus, at sections where the absolute value of the slope of the temperature dependent sensitivity curve is relatively large, the temperature intervals may be chosen quite narrow. In regions where the absolute value of the slope of the temperature dependent sensitivity curve is quite low, wider temperature intervals can be chosen. Thus, at lower temperatures, the temperature intervals of the approximation in FIG. 2 are narrower than at higher temperatures.
(16) In contrast, FIG. 3 shows the situation where the absolute value of the slope increases with increasing temperature. Correspondingly, the temperature intervals for the piece-wise linear segments can be relatively wide at low temperatures. The width of the temperature intervals may be reduced at higher temperatures.
(17) FIG. 4 shows a possible output V.sub.sg of the voltage generated by the slope generator. Depending on the actual operation temperature and the corresponding temperature interval, respectively, a different slope with a different offset is provided. An additional offset voltage V.sub.offset may be added to the output of the slope generator to obtain the bias voltage V.sub.bias provided to the charging circuit. The additional offset voltage V.sub.offset may be in a range between 8 to 16 V where the variation of the voltage generated by the slope generator may be in a range from 1 to 2 V.
(18) FIG. 5 shows the output of a PTAT temperature sensor providing a voltage scaling with the absolute temperature.
(19) FIG. 6 shows an embodiment of a MEMS microphone where the bias circuit BC comprises a closed loop control circuit CLCC with a temperature sensor TS, a slope control circuit SCC, and a slope generator SG. The temperature sensor TS provides a temperature signal, e.g. a voltage being proportional to the absolute temperature, to the slope control circuit SCC and to the slope generator SG. The slope control circuit SCC provides one or a plurality of control signals to the slope generator SC.
(20) The output of the slope generator SC may be routed to the charging circuit CC. The signal provided to the charging circuit CC may be a control voltage V.sub.sc or a control voltage V.sub.sg in addition to an additional offset voltage V.sub.offset.
(21) FIG. 7 shows a more detailed embodiment of the closed loop control circuit CLCC as the output of the slope generator SC is connected to a voltage buffer VB which feeds a control signal to the charging circuit CC. However, the voltage buffer is optional and can be omitted.
(22) FIG. 8 shows another embodiment where the output of the MEMS capacitor is connected to an amplifier AMP. The amplifier AMP may be integrated into an ASIC chip together with other circuit components of the bias circuit and/or the charging circuit. The voltage buffer shown in FIG. 8 is optional and can be omitted.
(23) FIG. 9 illustrates the closed loop of the closed loop control circuit. The temperature sensor TS may provide a voltage generating element generating a voltage proportional to absolute temperature which may be connected to ground via a resistor R. Further, the output of the PTAT voltage generating element may be connected to the slope control circuit SCC and to the slope generator SC. The slope control circuit comprises a plurality of comparators where the embodiment presented in FIG. 9 has three comparators. Each comparator has two inputs where one of the two inputs is an inverted input. The non-inverted inputs of the comparators are connected to the temperature sensor TS. The output of each comparator is fed via a corresponding signal line to a switch of the slope generator. The slope generator comprises a plurality of serial connections of a resistive element and a switch. The embodiment shown in FIG. 9 comprises three serial connections where the first serial connection has a resistor R1 and a switch S1. The second serial connection has a resistor R2 and a switch S2 and the third serial connection has a resistor R3 and a switch S3. Further, the slope generator has an additional resistive element connected in parallel to the serial connections and a further additional element connected to ground. Connected in parallel to the serial connections, a comparator is comprised in the slope generator SG. The inverted input of the comparator is connected to the serial connections, i.e. to the resistor's side of the serial connections while the non-inverted input of the comparator is connected to the temperature sensor TS.
(24) Reference voltages, e.g. reference voltages V1, V2, V3 in FIG. 9, are applied to the inverted input of the comparators of the slope control circuits. The references voltages V1, V2, V3 are compared to the voltage provided by the temperature sensor TS. Depending on the actual temperature, the voltage provided by the temperature sensor TS causes a certain number of the comparators of the slope control circuit to create an activation signal activating the corresponding switch of the slope generator. Thus, the switching state of the switches of the slope generator depends on the temperature and on the reference voltages applied to the slope control circuit. Accordingly, the output voltage V.sub.sg as shown in FIG. 4 with piece-wise linear segments can be obtained.
(25) FIG. 10 shows a possible implementation of the charging circuit CC which may be realized as a Dickson charge pump comprising a serial connection of basic elements, each comprising a diode and a storage capacitor. Between the output of the slope generator SC and the input of the charging device CC, a further voltage follower VF may be provided. The output of the charging circuit CC may be connected to one or more electrodes of the MEMS capacitor.
(26) The present MEMS microphone is not limited to the embodiments described above or shown in the figures. Microphones comprising further circuit elements such as further amplifier circuits or a higher number of comparators or serial connections in the slope control circuit and in the slope generator, respectively, are also comprised by the present invention.
LIST OF REFERENCE SIGNS
(27) AMP: amplifier BC: bias circuit CC: charging circuit CLCC: closed loop control circuit MCAP: MEMS capacitor MEMS: MEMS component comprising the MEMS capacitor MM: MEMS microphone R: resistor R1, R2, R3: resistor S: sensitivity S1, S2, S3: switch SCC: slope control circuit SG: slope generator SO: signal output T: temperature T.sub.1, T.sub.2, . . . , T.sub.N: threshold temperature TS: temperature sensor V: voltage V.sub.1, V.sub.2, V.sub.3: reference voltages VB: voltage buffer V.sub.bias bias voltage controlling the charging circuit V.sub.offset offset voltage V.sub.sg: output voltage of a slope generator
