Differential Microphone and Method for Driving a Differential Microphone

The present invention refers to differential microphones and methods for driving such microphones. The invention further refers to means for interfacing differential condenser microphones, e.g. MEMS (MEMS = Micro-Electro-Mechanical

Systems) or ECM' s (ECM = electret condenser microphone) .

Microphones such as MEMS microphones comprise a perforated backplate and a flexible membrane. The backplate and the mem ^¬ brane establish electrodes of a capacitor. Received sound signals induce oscillations of the membrane. Due to corre ^¬ sponding induced oscillations of the capacity, acoustic sig ^¬ nals can be converted into electrical signals. In order to improve the signal quality of MEMS microphones, double back ^¬ plate microphones or double membrane microphones can be cre- ated. In double backplate microphones, the membrane is ar ^¬ ranged between two perforated backplates; in double membrane microphones, a perforated backplate is arranged between two flexible membranes. In each case, a microphone is obtained that comprises two capacitors and provides a differential output port. A differential port comprises two terminals where each terminal mainly provides the same absolute value of a voltage or a current but with opposite polarity. When signals propagate via differential signal ports or signal paths, common mode disturbances can easily be eliminated.

Although microphones with differential ports provide a better signal quality, their use, contrary to simpler microphones with a single capacitor as an acusto-electrical transducer, has not yet been commercialized, thus not much work has been devoted yet to methods to interface such microphones. There ^¬ fore, unsolved problems exist related to receiving and ampli ^¬ fication of the signal from a differential microphone.

Microphones providing differential signals are known from US 4,757,545, US 2008/0310655 Al or US 2010/254544 A.

As a differential MEMS microphone electrically presents two capacitors, its electrodes have to be biased. Accompanying interface circuitry connected to the electrodes of the ca ^¬ pacitor may provide a bias voltage for the capacitor. In pre ^¬ vious works biasing is done by connecting a resistance ele ^¬ ment having a large resistance between the electrodes and ground.

Problems connected with receiving and amplifying a differential microphone signal such as differential microphone bias ^¬ ing, amplifier gain definition, influence of the parasitic capacitances at the interface nodes, differential microphone capacitances impedance conversion, obtaining low noise and low cut-off frequency of the amplifier response exist.

The signal quality depends on the quality of the acoustic ca- pacitors' bias voltage. Further, the signal quality depends on the quality of the common mode output voltage. What is needed is a differential microphone with an improved bias voltage of the microphones acoustic capacitors and a better defined common mode output voltage and a method for driving such a microphone.

It is, thus, an object of the present invention to provide a differential microphone that can process differential signals from the microphone's acoustic capacitors, that provide a stable and well-defined bias voltage for the capacitor's electrodes and that is able to provide well defined common mode output voltage. It is also an object of the present in- vention to provide amplification of the differential micro ^¬ phone signal with well defined gain, i.e. a well defined am ^¬ plification factor.

Therefore, the present invention provides a differential mi- crophone and a method for driving such a microphone according to the independent claims. Dependent claims provide preferred embodiments thereof.

A differential microphone comprises a first microphone elec- trode, a central microphone electrode and a second microphone electrode. The microphone further comprises a differential output port and a differential amplifier stage. The amplifier stage has a differential input port and a differential output port being connected to the output port of the microphone. The microphone further comprises a common mode feedback cir ^¬ cuit having a differential output port being connected to the output port of the microphone.

The common mode feedback circuit provides a well defined com- mon mode output voltage.

Such a microphone provides means for interfacing a differential microphone by the presented electrical circuitry. The microphone solves existing problems connected with receiving and amplification of a differential microphone signal such as differential microphone biasing, amplifier gain definition, influence of the parasitic capacitances at the interface nodes, differential microphone capacitances impedance conver- sion, obtaining low noise and low cut-off frequency of the amplifier response.

Thus, the microphone has an amplifier connected to the micro- phone electrodes providing a bias voltage for the capacitors and a common mode output voltage for further circuits proc ^¬ essing the amplified electrical signals encoding the received acoustical signals. In this context, a connection denotes an electrical connec ^¬ tion between circuit elements.

In one embodiment a first terminal of the amplifier's differ ^¬ ential input port is connected to the first microphone elec- trode and a second terminal of the amplifiers differential input port is connected to the second microphone electrode.

This solution of an improved microphone can be based on a as ^¬ sembly where the microphone electrodes are electrically cou- pled to a MOS (metal-oxide-semiconductor) integrated circuit.

In one embodiment the amplifier stage has a control port and the common mode feedback circuit is connected to the ampli ^¬ fier stage's control port.

In one embodiment the differential microphone further com ^¬ prises a first resistance element and a second resistance element. The first resistance element is connected between a first terminal of the output port and a first terminal of the input port. The second resistance element is connected be ^¬ tween a second terminal of the output port and a second ter ^¬ minal of the input port. The first and the second resistance elements are part of an amplifier feedback circuit. Further, a first and a second capacitive element may be connected in parallel with the feedback resistance elements. Thus, the feedback circuit is connected to the acoustic ca- pacitors and the microphone comprises a capacitive feedback as the microphone capacitors can be regarded as part of the feedback circuit.

In one embodiment the resistance elements comprise a first diode and a second diode being connected in parallel to and in the opposite direction of the first diode. The second di ^¬ ode's direction may be opposite to the first diode's.

The resistance elements have a resistance R _F greater than 10 GO. This requirement is needed because the resistor should not degrade the noise performance of the microphone preampli ^¬ fier. Additionally to provide that the amplifier is opera ^¬ tional in the whole audio band the cut-off frequency of a mi ^¬ crophone amplifier (inversely proportional to the size of this resistor) should be rather low, e.g. < 20 Hz.

Voltage drop across diodes connected in this way is close to zero and a resistance element is obtained that has a very large resistance. This resistance element can be also a se- ries of diode RE elements or can be implemented in different ways as a series or parallel connection of transistor or di ^¬ ode elements.

In one embodiment the common mode feedback circuit comprises a common mode voltage port for setting and adjusting the common mode voltage at the amplifier stage's output.

Differential microphone comprise a differential signal port connected to its double backplates or double membranes in the case of a double membrane microphone. The central electrode thereof is kept at a constant voltage representing signal ground. The bias voltage is generated by the electronic cir ^¬ cuitry of the microphone's amplifier. A Dickson voltage mul- tiplier followed by a low pass filter might be used for MEMS microphones, ECM microphones do not need this high bias volt ^¬ age .

The amplifier stage can comprise a fully differential opera- tional amp lifier (opamp) . The first and the second acoustic electrodes are connected to the input ports of the opamp. The differential gain of the opamp is typically greater than 1000. The amplifier stage may be a standard MOS topology such as folded cascode amplifier.

The DC voltage at the output of the amplifier is determined by the common mode voltage port by the amplifier's common- mode feedback circuit. Normally the DC voltage at the output of the amplifier determined by the common mode voltage port is set to half of a power supply voltage of the microphone.

The DC bias voltage of the acoustic capacitors may be gener ^¬ ated on the same integrated circuit chip. Via the common mode control port, the DC voltage of the output of the amplifier stage and the microphone's differential output port is set to a value defined by the common mode control port. The same DC voltage will appear at the microphone's capacitors, i.e. the voltage drop on RE is close to zero. In this way, the DC voltage of the microphone capacitor is well defined and fur- ther more can be adjusted.

In one embodiment the amplifier stage's differential input port is connected to gates of a MOS transistors differential pair. Noise of these transistors, connected to the microphone electrodes should be low. Often a P-MOS input stage is used for better noise performance. In one embodiment the microphone comprises a first capaci ^¬ tance element and a second capacitance element. The first ca ^¬ pacitance element is connected between a first terminal of the output port and a first terminal of the amplifier stage's input port. The second capacitance element is connected be- tween a second terminal of the output port and a second ter ^¬ minal of the amplifier stage's input port. The first and the second capacitance elements are part of an amplifier feedback circuit . The capacitance elements may have a capacitance C _F between 0.05 pF and 10 pF.

Further, the amplification factor is mainly proportional to (V _OUT+ - V _out - ) / ( V _M i - V _M2 ) * C _M /C _F (eqn. 1)

Thus, the microphone's gain is independent of parasitic ca ^¬ pacitances of the capacitor. Here, V _ou t+ and V _ou t- are the out ^¬ put voltages at the differential output terminals. V _M i - V _M2 is the voltage applied to the combination of the capacitors built up by two backplates and a membrane in between. C _M is the capacitance of each single capacitor comprising the membrane and one backplate. A desired property of the microphone described is that the gain can be adjusted through the first and second capacitance elements as feedback capacitors having a capacity C _F . In that case C _F may be connected with some kind of switch arrangement by which the value of C _F can be changed i.e. programmed to give variable gain.

The cut-off frequency o _cut of a microphone amplifier can, then, be denoted as ticut = 1/ (C _F RF) (eqn. 2) where C _F is the capacitance of the first or the second ca- pacitance element and R _F is the resistance of the first or the second resistance element. Thus, with such a feedback configuration, the cut-off frequency o _cut can be 20 Hz or lower. Thus, a low cut-off frequency o _cut is obtained. The first terminal and the second terminal establish the two terminals of a differential signal port of the operational amplifier. In this case, the operational amplifier is con ^¬ nected in a feedback configuration where the feedback network between the operational amplifier's (opamp's) output and the opamp's input comprises a resistance element having a very large resistance in parallel with feedback capacitor.

A purpose of the large resistance is to provide a DC path from the operational amplifier input to the output. At the same time this resistance element provides a DC path from the microphone electrodes through the output of the amplifier to ground. Keeping in mind that the amplifier's DC output volt ^¬ age is set by its common-mode feedback circuitry, a micro ^¬ phone amplifier is provided in which an amplifier stage and a common mode feedback circuit are combined to improve the sig ^¬ nal quality of a microphone. Such a combination enables a stable and well-defined DC bias voltage for a microphone's capacitor. The bias voltage is applied to the capacitor via the input port of the amplifier stage. Further, the common mode feedback circuit provides a stable and well-defined com ^¬ mon mode output voltage to improve further processing of the electric signals.

Further, such a microphone amplifier provides impedance con ^¬ version of capacitive impedances of the microphone's capaci ^¬ tor. As a fully differential topology has been used, a low THD (total harmonic distortion) and a good power supply re- jection, i.e. a good immunity against common mode distur ^¬ bances from the power supply can be achieved.

In one embodiment the first microphone electrode, the central microphone electrode and the second microphone electrode are elements of the acoustically active part of a MEMS microphone or a electret condenser microphone.

The microphone can be produced in MEMS technology on a sili ^¬ con chip and comprise circuit elements being fully integrated in an IC chip in CMOS process, e.g. an ASIC (ASIC = Applica ^¬ tion-Specific Integrated Circuit) chip. The two chips are packaged together. The MEMS microphone and the CMOS inte ^¬ grated circuitry can also be produced on the same silicon substrate i.e. as a single chip. Separate CMOS chip with cir- cuits described can also be connected to an electret con ^¬ denser microphone to form the amplifier described. In all cases the amplifier with the feedback described can provide an amplification factor from 1 to 20. In all cases the differential microphone can have one membrane and two backplates or one backplate and two membranes.

In one embodiment the microphone further comprises a third capacitance element being connected between a first terminal of the amplifier stage's input port and the first microphone electrode and a fourth capacitance element being connected between a second terminal of the amplifier stage's input port and the second microphone electrode (E2) . The third and the fourth capacitance element can have capacitances between 1 pF and 100 pF.

Thus, it is possible to separate, by these DC blocking ca ^¬ pacitors, sensitive circuit elements of the input port of the amplifier from the electrodes of the microphone's capacitor. Especially during manufacturing steps, and when applying high bias voltage on the microphone, the amplifier is protected against high DC voltages at its input nodes which might dam ^¬ age the gate oxide of the input transistors.

In one embodiment a third resistance element is connected between the first microphone electrode and a high bias voltage generated by an integrated circuit. The first micro ^¬ phone electrode and the second microphone electrode are elec ^¬ trically connected. Thus, the acoustical capacitor's elec ^¬ trodes are connected to a bias voltage which may be on-chip generated .

In one embodiment the amplifier stage is a folded-cascode amplifier comprising a first, a second, a third, a fourth, a fifth, a sixth, a seventh, an eighth, a ninth, a tenth and an eleventh transistor, where the first, the second and the third transistor are connected to a power supply. The fourth transistor is connected between the second and the sixth transistor and the fifth transistor is connected between the third transistor and the seventh transistor. The eighth transistor is connected between the sixth transistor and ground and the ninth transistor is connected between the seventh transistor and ground. The tenth transistor is connected between the first transistor and the eighth transistor and the eleventh transistor is connected between the first transistor and the ninth transistor. The eighth transistor and the ninth transistor are connected to a control port.

This circuit is presented as an example, whereas it is possi ^¬ ble to use many other high-gain amplifier stage implementations that can be found in the literature describing the field of art. The amplifier stage can be designed to have the optimal low noise performance when connected together with the microphone. The amplifier is usually designed to be op ^¬ erational under low-voltage and with low current consumption. In one embodiment the amplifier stage comprises a first, a second, a third, a fourth, a fifth, a sixth, a seventh, an eighth, a ninth, a tenth, an eleventh and a twelvth transistor. The first transistor and the second transistor are connected to a power supply. The third transistor is connected between the first transistor and the seventh transistor and the fourth transistor is connected between the second transistor and the ninth transistor. The fifth transistor is connected between the first transistor and the eighth transistor and the sixth transistor is connected between the second transistor and the eighth transistor. The seventh transistor is connected between the third transistor and the tenth transistor and the ninth transistor is connected between the fourth transistor and the twelvth transistor. The eighth transistor is connected to the eleventh transistor. The tenth transistor, the eleventh transistor and the twelvth transistor are connected to ground. The tenth transistor, the eleventh transistor and the twelvth transistor are connected to a control port. In one embodiment the common mode feedback circuit comprises a first, a second, a third, a fourth, a fifth, a sixth, a seventh and an eighth transistor. The first and the second transistor are connected to a power supply. The third tran ^¬ sistor is connected between the first transistor and the seventh transistor. The fourth transistor is connected between the second transistor and the seventh transistor. The fifth transistor is connected between the first transistor and the eighth transistor. The sixth transistor is connected between the second transistor and the eighth transistor. The seventh transistor and the eighth transistor are connected to ground.

This circuit is presented as an example, whereas it is possi- ble to use many other common-mode feedback implementations that can be found in the literature describing the field of art. Switched capacitor common-mode feedback might be used as well . In one embodiment all circuit elements of the amplifier are fully integrated in a CMOS ASIC chip. The chip may be

manufactured in a standard CMOS process.

The CMOS circuit chip is assembled together with the MEMS mi- crophone chip in a package either by soldering to a PCB

(printed circuit board) like substrate (ceramic or similar) or by wire bonding the two chips together.

Further, the amplifier and the microphone can be produced starting from the same silicon substrate forming a single chip solution. A method for driving differential microphone, e.g. one of the above mentioned microphones, comprising the steps

- receiving an acoustical signal,

- converting the acoustical signal into an electrical signal, - adjusting the bias voltage of the first and second micro ^¬ phone electrode by adjusting a common mode voltage via a com ^¬ mon mode voltage port (VCOM) of the common mode feedback cir ^¬ cuit .

In one embodiment of the method the electrical signal is amplified with a adjustable and well defined gain insensitive to parasitic capacitances.

The basic principles and exemplary embodiments thereof are shown in the schematic figures.

Short description of the figures shows an equivalent circuit diagram of a differen tial microphone, shows an embodiment of a resistance element, shows an equivalent circuit diagram of an ampl stage, shows an equivalent circuit diagram of another amplifier stage,

FIG. 4A shows an equivalent circuit diagram of a common

mode feedback circuit that may be in use with the amplifier stage shown in FIG. 3A, FIG. 4B shows an equivalent circuit diagram of another com ^¬ mon mode feedback circuit that may be in use with the amplifier stage shown in FIG. 3B, FIG. 5 shows an equivalent circuit diagram of a micro ^¬ phone,

FIG. 6 shows a cross section of a microphone. Detailed description

FIG. 1 shows an equivalent circuit diagram of a differential microphone MIC comprising an amplifier stage AS being connected to the mechanical elements, i.e. the acoustical elec- trodes, of a MEMS microphone MEM. The mechanical elements MEM comprise a first electrode El and a second electrode E2. A central electrode EC is arranged between the first electrode El and the second electrode E2. The first electrode El and the second electrode E2 can be established by perforated backplates of a double backplate or by membranes of a double membrane microphone. The amplifier stage AS comprises a dif ^¬ ferential input port DIP. The differential output of the am ^¬ plifier stage AS is connected to the differential output DOP of the microphone MIC. The differential input port DIP com- prises two terminals, each terminal receiving a signal of mainly the same absolute value but of different polarity com ^¬ pared to the respective other terminal's signal. A first re ^¬ sistance element RE1 is connected between an input terminal and an output terminal. A second resistance element RE2 is connected between the respective other input terminal and the respective other output terminal. The electric potential of the input terminal and the output terminal connected to one resistance element have an opposite polarity, i.e. the ampli- fier stage is in a negative feedback configuration. The first and second capacitance elements and the first and second re ^¬ sistance elements establish, thus, an amplifier feedback cir ^¬ cuit AFC .

Further, a first capacitance element CE1 is connected between the first output terminal and the first input terminal. A second capacitance element CE2 is connected between the sec ^¬ ond output terminal and the second input terminal.

Embodiments of amplifier stages are shown in FIGs. 3A and 3B. Embodiments of common mode feedback circuits are shown in FIGs. 4A and 4B.

FIG. 2 shows an embodiment of a resistance element RE com- prising diodes being connected in parallel but with opposite polarity with respect to each other. Thus, a large resistance for low voltages can be obtained.

FIG. 3A shows a more detailed circuit equivalent diagram of an amplifier stage AS comprising 11 transistors Tl - Til. A power supply PS is connected to the respective source of a first transistor Tl, of a second transistor T2, and of a third transistor T3. The gate of first transistor Tl is connected to the gate of the second transistor T2 and the third transistor T3. The drain of the first transistor Tl is connected to the sources of the tenth transistor T10 and the eleventh transistor Til. The gates of the tenth transistor T10 and the eleventh transistor Til establish the respective input terminals of the differential input port DIP. The drains of the second transistor T2 and of the third transis ^¬ tor T3 are connected to the sources of the fourth transistor T4 and the fifth transistor T5. The gate of the fourth transistor T4 is connected to the gate of the fifth transistor T5. The drains of the fourth transistor T4 and the fifth transistor T5 are connected to the differential output port DOP of the common mode feedback circuit. The ports' respec ^¬ tive terminals are connected to the drains of the sixth tran- sistor T6 and the seventh transistor T7 both the gates of which are connected to each other. The drains of the tenth transistor T10 and the eleventh transistor Til are connected to the drains of the eighth transistor T8 and the ninth transistor T9, respectively. The sources of the eighth transistor T8 and the ninth transistor T9 are connected to ground GND. The gates of the eighth transistor T8 and the ninth transis ^¬ tor T9 are connected to a control port VCNT .

3B shows an equivalent circuit diagram of another embodiment of an amplifier stage AS comprising 12 transistors Tl - T12. A power supply PS is connected to the sources of the first transistor Tl and of the second transistor T2. The gates of the first transistor Tl and of the second transistor T2 are electrically connected to each other. The drains of the first transistor Tl and of the second transistor T2 are connected to the sources of the third transistor T3 and of the fourth transistor T4, respectively. Further, the drains are con ^¬ nected to drains of the fifth transistor T5 and of the sixth transistor T6, respectively. The gates of the fifth transis- tor T5 and of the sixth transistor T6 establish the respective first and second input terminals TIN1, TIN2 of the am ^¬ plifier stage. The sources of the fifth transistor T5 and of the sixth transistor T6 are connected to the drain of the eighth transistor T8. Further, the drains of the third tran- sistor T3 and of the fourth transistor T4 are connected to drains of the seventh transistor T7 and of the ninth transistor T9 respectively and are connected to the output termi ^¬ nals TOUT1, TOUT2 of the output port. The gate of the third transistor T3 is connected to the gate of the fourth transis ^¬ tor T4. The gate of the seventh transistor T7 is connected to the gate of the eighth transistor T8 and to the gate of the ninth transistor T9. The sources of the seventh transistor T7 and of the ninth transistor T9 are connected to the drains of the tenth transistor T10 and of the twelvth transistor T12. The sources of the tenth transistor T10 and of the twelvth transistor T12 are connected to ground GND, as is the source of the eleventh transistor Til. The gate of the tenth

transistor T10 is connected to the gate of the eleventh transistor Til and to the gate of the twelvth transistor T12 and to a control port VCNT . The gate of the seventh transistor T7 is connected to the gate of the ninth transistor T9. The first transistor Tl and the second transistor T2 are connected to a bias terminal and the third transistor T3 and the fourth transistor are connected to a bias terminal.

FIG. 4A shows a detailed circuit equivalent diagram of a com- mon mode feedback circuit CMFBC comprising 8 transistors Tl - T8. A power supply PS is connected to the sources of the first transistor Tl and the second transistor T2, the gates of which are connected to each other. The drains of the first transistor Tl and of the second transistor T2 are connected to the sources of a third transistor T3 and of a fifth tran ^¬ sistor T5 and to the sources of a fourth transistor T4 and a sixth transistor T6, respectively. The gates of the fifth transistor T5 and of the sixth transistor T6 establish the terminals of the common mode feedback circuit's output port DOP . Further, the drains of the third transistor T3 and of the fourth transistor T4 are connected to the gate of a sev ^¬ enth transistor T7 and to the drain of the seventh transistor T7. The gate of the seventh transistor T7 is further connected to a control port VCNT . The source of the seventh transistor T7 is connected to ground and to the source of an eighth transistor T8. Further, the drains of the fifth transistor T5 and of the sixth transistor T6 are connected to the gate and to the drain of the eighth transistor T8. Gates of the third transistor T3 and the forth transistor t4 are connected together and connected to a port VCOM.

FIG. 4B shows an equivalent circuit diagram of another em- bodiment of a common mode feedback circuit CMFBC . The common mode feedback circuit CMFBC comprises four capacitance ele ^¬ ments CE where each two capacitance elements are connected in series and two series of capacitance elements are connected in parallel. Switches SW can be utilized to electrically connect or disconnect capacitance elements. The common mode feedback circuit CMFBC comprises a first output terminal TOUT1 and a second output terminal TOUT2 forming a differential output port, control port VCNT, and a common mode volt ^¬ age port VCOM.

FIG. 5 shows an equivalent circuit diagram of a microphone MIC comprising a third capacitance element CE3 and a fourth capacitance element CE4 and a third resistance element RE3. The third capacitance element CE3 is connected to an input terminal of the differential input port DIP. The fourth ca ^¬ pacitance element CE4 is connected to the respective other input terminal of the differential input port DIP. Further the third resistance element RE3 is connected between the mi ^¬ crophone electrodes (or membranes) and an on-chip generated bias voltage source. The other side of the resistance element is connected to the third and fourth capacitance element which are also connected to the terminals of the amplifier stage AS. FIG. 6 shows a cross section of a microphone assembly MIC comprising a MEMS chip MC containing the microphone's acous ^¬ tical elements and an ASIC chip AC containing the circuit elements. The microphone chip MC and the ASIC chip AC are arranged on a substrate SU. It is, however, possible that the acoustical and the electrical elements of a microphone are integrated in a single chip, e.g. a silicon chip.

A differential microphone is not limited to the embodiments described in the specification or shown in the figures.

Amplifiers comprising further elements, e.g. such as

capacitance elements, resistance elements, transistors, electrodes, or further input or output ports are also

comprised by the present invention.

List of reference signs:

AC: ASIC chip

AMP: amp1i fier

AS : amplifier stage

CE: capacitance element

CE1, CE2: first, second capacitance element

CE3, CE4: third, fourth capacitance element

CMFBC : common mode feedback circuit

DIP: differential input port of the amplifier stage

DOP: differential output port of the microphone

El, E2: first, second electrode of the microphone's ca ^¬ pacitor

EC: central electrode of the microphone's capacitor

GND: ground

MEM: mechanical elements of a MEMS microphone

MIC: microphone

PS : power supply

RE : resistance element

RE1, RE2 : first, second resistance element

RE3 : third resistance element

SU: substrate

SW: switch

T1-T12 : transistors

TIN1, TIN2: first, second terminal of a differential input port

TOUT1, TOUT2: first, second output terminal of a differential output port

VCNT : control port

VCOM: common mode voltage port