DESCRIPTION

Technical Field

[0001] The present disclosure relates to a microphone preamplifier circuit.

Background art

[0002] One of the main aspects to take into account in the development of an electronic device, especially mobile devices, is the area occupation. In the field of mobile devices, such as mobile phones, the reduction of the area occupation on the Printed Circuit Board (PCB) is a key point in order to create phones with much more functionalities without altering their dimensions. The focus, during the years, has been to integrate inside a chip, where possible, all those passive components such as resistors, capacitors and inductors which represent the main limit for the area reduction. Inside old generation phones, such passive components were SMD (Surface Mount Devices) mounted directly on the main board. Later on, thanks to improvement in the technology, these devices were placed inside the chip package, a methodology known as PDI (Passive Device Integration) , and in some cases directly integrated inside the chip. However, when it comes to the microphone preamplifying path the implementation of this approach has not been possible due to the huge capacitance value of the decoupling capacitors needed between the microphone and the preamplifier.

[0003] Figures 1 and 2 show two known ways, single- ended and differential, respectively, to bias and connect the microphone circuit MC _S , MC _D to a preamplifier PA _S , PA _D using an RC network. The microphone circuit MC _S , MC _D comprises a microphone 3 and a biasing circuit R _MB I, R _MB 2, R _MB 3, CI, C2, fed by a bias voltage V _BIAS · [0004] The DC bias voltage of the signal coming from the microphone circuit MC _S , MC _D at the output nodes M ₀ , M ₀ ' of the microphone circuit MC _S , MC _D will depend exclusively by the biasing circuit R _MB I, R _MB 2, R _MB 3, CI, C2 and is usually different from the DC bias input voltage of the preamplifier PA _S , PA _D . The level shifting between the microphone 3 and the preamplifier PA _S , PA _D DC biasing voltages is commonly obtained using a decoupling capacitor C _DEC that produces, with the preamplifier PA _S , PA _D input resistance, a first order high-pass filter whose corner frequency is generally lower than 20Hz in order to avoid in-band audio signal perturbation.

[0005] More detailed representations of the differential preamplifier PA _D are shown in Figure 3 (inverting configuration) and Figure 4 (non-inverting configuration) .

[0006] In the inverting case, due to noise generation, input resistors Ri _A and Ri _B cannot have high resistance values (typically from lOkOhm to 50kOhm) , whereas in the non-inverting solution resistors ]¾ _A and R. _3B are used only to bias the amplifiers OA inputs at a common mode voltage V _CM midway between ground and the supply voltage. Accordingly, resistors ]¾ _A and R. _3B don't contribute in noise generation and can be made with larger resistance values with respect to the inverting case (however, not more than some hundreds of kOhms due to area occupation) . In both cases, decoupling capacitors C _DEC of more than lOOnF are needed and such large capacitance values would be difficult to integrate in a chip. In fact, with actual technologies on chip integration of a capacitor having such large capacitance value would require an area greater than 20mm ² and this fact made the integrating approach practically unusable. US 2002/0125949 discloses the above problem of the waste of area due to the integration in the chip of the decoupling capacitor C _DEC , confirming that the integration of the decoupling capacitors C _DEC is practicable only for relatively reduced capacitance values. Also US 7,899,196 addresses the problem of the area occupied by the preamplifier and discloses a digital microphone comprising a microphone element, a preamplifier with a high pass filter function an anti-aliasing filter and an analog to digital converter .

[0007] Moreover, unfortunately, even with the PDI methodology the decoupling capacitors C _DEC can't be realized because of their high capacitance value and the fact that none of their terminals are connected to a fixed potential. This is the reason why all the existing known solutions use SMD capacitors. Since a preamplifier usually has several inputs (voice microphone, mono and stereo audio microphone, mono and stereo line-in, etc.) and each one could be differential, it is clear that on a mobile phone's PCB there are many SMD decoupling capacitors C _D EC-

[0008] The presence of one or more SMD decoupling capacitors is clearly a bottle neck for the area reduction strategy, and there is a strong felt need of trying to find a solution to this problem, till now without success. The same above described problem holds for other consumer devices different from mobile phones, such as portable MP3 players, digital photo cameras, digital audio recorders, video cameras, and in general in devices with audio communication and/or recording and/or processing capabilities.

[0009] Moreover, with reference to figures 3 and 4, a further problem of the prior art microphones preamplifiers, especially if they are intended to be embedded in mobile devices, is their power consumption. A significant contribution to such power consumption is given by the input and feedback resistors provided for setting the gain of the microphone preamplifier. [0010] EP 2 133 993 Al, EP 0 375 017 A2 and US

6,656072 Bl disclose filters and/or gain circuits. However such documents neither refer to microphone systems nor address the above disclosed problem concerning the decoupling capacitor. Summary of the invention

[0011] In view of the above described limitations of the prior art microphone preamplifier circuits, it is an object of the present invention to provide a microphone preamplifier circuit which is adapted to solve the above indicated problems concerning: the impossibility of reducing the PCB area occupation below a desired value due to the presence of one or more decoupling capacitors that cannot be integrated on a chip and the consumption of the preamplifier.

[0012] The above object is reached by a microphone preamplifier circuit adapted to be connected to a microphone circuit, the microphone circuit comprising a microphone and at least one output node. The microphone preamplifier circuit comprises a preamplifier comprising:

- at least one input node adapted to be connected to said output node;

- an operational amplifier comprising at least one input and at least one output;

- at least one input DC decoupling capacitor connected between said input node of the operational amplifier and said input.

The preamplifier comprises at least one feedback capacitor connected between the input and the output of the operational amplifier in order to set together with said input DC decoupling capacitor a gain value of the preamplifier circuit. The preamplifier comprises a first and a second feed node adapted to be fed by a first and a second bias voltage respectively. The preamplifier further comprises at least one switched capacitor adapted to be selectively and alternatively connected under the control of a clock signal: - between said input and said output of the operational amplifier; and

- between said first and said second node.

The preamplifier further comprises an anti-aliasing filter having an output terminal connected to said input node and an input terminal connected/connectable to said output node of the microphone circuit.

In the above circuit, since the high pass filtering function is performed by the operational amplifier, the feedback capacitor and the switched capacitor, the DC decoupling capacitor, apart from contributing to the establishment of the gain of the preamplifier, only performs a DC decoupling function between the microphone circuit's output and the operational amplifier's and does not need to have a high value, as in the above described prior art circuits, in order to implement a high pass filtering function.

Brief description of the drawings

[0013] Further features and advantages of the present invention will become more apparent from the following detailed description of exemplary but non-limiting embodiments thereof, as illustrated in the attached figures, in which: - Fig. 1 shows a schematic view of a first example of prior art microphone system comprising a microphone circuit and a preamplifier;

- Fig. 2 shows a schematic view of a second example of prior art microphone system comprising a microphone circuit and a preamplifier;

- Fig. 3 shows a schematic view of a first example of a known preamplifier for the microphone system of figure 2;

- Fig. 4 shows a schematic view of a second example of known preamplifier for the microphone system of figure 2;

- Fig. 5 shows a very schematic view of a device provided with audio recording and/or communication and/or processing capabilities;

Fig. 6 shows an embodiment of a microphone preamplifier circuit;

- Fig. 7 shows a part of the circuit of figure 6 and the equivalent circuit thereof; and

- Fig. 8 shows an embodiment of a microphone system comprising the microphone preamplifier circuit of figure 6.

Detailed description

[0014] In the attached figures identical or similar elements will be indicated with the same reference numbers/ symbols .

[0015] Figure 5 shows a very schematic view of an embodiment of mobile terminal 5, such as for example a mobile phone 5, comprising a microphone circuit MC _D . According to an embodiment, and without for this reason introducing any limitation, the microphone circuit MC _D may be similar to the differential microphone circuit MC _D represented in figure 2. The microphone circuit MCD comprises a microphone 3, a microphone's biasing network and two differential output nodes M ₀ , M ₀ ' (represented in Figure 6) . According to an embodiment, the microphone's biasing network may be identical or similar to the known biasing network of figure 2.

[0016] The mobile phone 5 comprises a circuit board 6 comprising a chip 7 comprising an integrated microphone preamplifier circuit 60. According to the embodiment shown, the circuit board 6 further comprises an analog to digital converter 70 and a digital audio processor 80. In the example of figure 5, the preamplifier circuit 60, the digital converter 70 and the digital audio processor 80 are integrated on the same chip 7.

[0017] As the general structure and the operation of a mobile terminal, such for example a mobile phone, are well known to a man skilled in the field, for sake of conciseness they will not be detailed further in the following part of the description. On the contrary, the following part of the description will be mainly focused on the microphone preamplifier circuit 60. It is important to remark that such microphone preamplifier circuit 60 can be also employed in systems and and/or devices different from a mobile terminal 5, for example in general in devices having audio communication and/or recording and/or processing capabilities such as, digital audio recorders, MP3 players, photo-cameras etc.

[0018] Figure 6 shows an embodiment of microphone preamplifier circuit 60 comprising a preamplifier P _A , the microphone preamplifier circuit 60 being adapted to be connected to the microphone 3, or more precisely to the microphone circuit MC _D .

[0019] According to the embodiment shown, the preamplifier P _A is a differential preamplifier having two input nodes 10, 10' and an operational amplifier OA. The input nodes 10, 10' are adapted to be connected to the output nodes M ₀ , M ₀ ' of the microphone circuit MC _D . In the particular example shown in figure 6, the operational amplifier OA is arranged in said microphone preamplifier circuit 60 in order to operate in an inverting configuration .

[0020] Since the structure of the microphone preamplifier circuit 60 and the preamplifier P _A in the embodiment shown is totally symmetrical, in the present description mostly the upper branch of the preamplifier P _A will be detailed, i.e. the branch above the ideal line LI. Moreover, it should be clear that even if in the present description a microphone preamplifier circuit 60 adapted to be connected to a differential microphone circuit M _C D will be disclosed, the teachings of the present description can be easily extended by a man skilled in the field to the case in which the microphone preamplifier circuit is adapted to be connected to a single-ended microphone circuit.

[0021] With reference to figure 6, the preamplifier P _A comprises an operational amplifier OA comprising at least one input 20,20' and at least one output 21, 21' . Moreover, the preamplifier P _A comprises at least one input DC decoupling capacitor C _D , C _D ' connected between the input node 10, 10' of the preamplifier P _A and the input 20,20' of the operational amplifier OA.

[0022] The preamplifier P _A further comprises at least one feedback capacitor C2R, C2A' connected between the input 20, 20' and the output 21, 21' of the operational amplifier in order to set together with said input DC decoupling capacitor C _D , C _D ' a gain value G of the preamplifier circuit 60. Such gain value G is given by: c

[0023] As shown in figure 6, the preamplifier P _A comprises at least one first 40, 40' and at least one second feed node 41, 41' adapted to be fed by a first VcMi and a second V _C M bias voltage respectively.

[0024] The preamplifier P _A further comprises at least one switched capacitor 2B, C2B' adapted to be selectively and alternatively connected under the control of a clock signal CK: - between the input 20, 20' and the output 21, 21' of the preamplifier P _A , i.e. in parallel to the feedback capacitor C ₂ A, C ₂ A' and

- between the above mentioned first 40, 40' and second 41, 41' feed nodes. [0025] According to an embodiment, the switched capacitor 2B, C2B' comprises: a first plate (in figure 6 on the left side of capacitor C ₂ B, C ₂ B _' ) adapted to be selectively and alternatively connected to the input 20, 20' and the first feed node 40,40'; and

- a second plate (in figure 6 on the right side of capacitor 2B, C2B' ) adapted to be selectively and alternatively connected to the output 21, 21' and the second feed node 41, 41' .

[0026] According to the above embodiment the second feed node 41,41' is configured to be fed with the common mode voltage V _C M of the operational amplifier OA and the first feed node 40, 40' is configured to be fed with a further voltage V _C MIN appropriate for input operation of the operational amplifier OA. For example, if the operational amplifier OA is fed with a supply voltage V _AL :

- the common mode voltage V _C M is half of said supply voltage V _AL ;

- the above mentioned further voltage V _C MIN is lower than said common mode voltage V _C M if the operational amplifier OA has P-type input and higher than said common voltage V _C M if the operational amplifier OA has N-type input .

For example, without for this reason introducing any limitation, if V _AL = 1,5 Volts, V _C M= 0,75 Volts, and V _C MIN = 0,5 Volts if the operational amplifier OA has a P-type input or V _C MIN = 1,0 Volts if the operational amplifier OA has N-type input. [0027] From the above description it is therefore clear that the switched capacitor C2 _B , C2 _B ' is a capacitor provided to bias the input 20, 20' of the operational amplifier OA to the further voltage V _CMIN , since as shown in figure 7 it can be modeled with a resistor having an equivalent resistance R _EQ = 1/ (f _CK -C2 _B ) in series with a battery providing a voltage V _B AT = V _C M ^_ V _C MIN-

[0028] From Fig.7 it is therefore clear that the switched capacitor C ₂ ^, C _2B ' is also adapted to discharge the feedback capacitor C2 _R , C2 _A ' (Figure 6) at a given frequency than can be set to be relatively very low by selecting appropriate values for the frequency f _CK of the clock signal CK and the appropriate capacitance values of the feedback C2R, C2A' and the switched C2B, C2B' capacitors. By selecting the above indicated appropriated values, the preamplifier PA can be designed to be adapted to implement a high pass filtering function which, having a relatively very low high pass cut-off frequency, does not interfere with audio signals of interest for the operation of the microphone 3. According to an embodiment, said high pass filtering function has a high pass cut-off frequency of 6Hz or about 6Hz, for example comprised between 4 Hz and 8 Hz.

[0029] The time constant of the above mentioned high pass filtering is given by:

[0030] In view of the above, the high pass cut ^¬ off frequency can be made as to have a value of about 6 Hz if for example the clock frequency f _C K has a value of the order of magnitude of 100 kHz, for example f _C K = 100 kHz and the switched capacitor -2^, _t ^-iw has a capacitance value significantly lower than a capacitance value of the feedback capacitor 2A, C ₂ A _' · For example, the switched capacitor C ₂ B, C ₂ B _' has a capacitance value of about three magnitude orders lower the capacitance value of said feedback capacitor C ₂ A , C _2A ' · In a practical embodiment, a high pass cut-off frequency of about 6 Hz can be for example obtained if f _CK = 100 KHz, C _2A =100pF, C _2A =l/30 pF.

[0031] The above values for the clock frequency f, CK and the cutoff frequency represent a good compromise between area occupation and audio performance and represent a preferred choice, but it should be clear that a broad range of values is possible for both cut-off frequency and clock frequency. For example, paying in area the cutoff frequency can be much lower than 6Hz (e.g. 0,6 Hz), while paying in performance it can be much higher than 6Hz (e.g. 60 Hz) .

[0032] It is clear that the above condition (1) fixes the limit for the minimum value of the capacitor C2A and in turn of C2B- In any case it is clear from the example above that such values can be easily integrated without wasting a significant amount of silicon area.

[0033] Moreover, it is to be noticed that since the high pass filtering function is performed by the operational amplifier OA, the feedback capacitor C2R, C2A' and the switched capacitor C2B, C2B' , the DC decoupling capacitor C _D , C _D ' , apart from contributing to the establishment of the gain G, only performs a DC decoupling function between the microphone circuit's output and the operational amplifier' s and does not need to have a high value, as in the above described prior art circuits, in order to implement a high pass filtering function. For example, the DC decoupling capacitor C _D , C _D ' can have a value of about lOOpF and therefore it can be easily integrated without wasting a significant amount of silicon area.

[0034] Moreover, a microphone preamplifier circuit 60 as described above has the additional advantage of having a relatively reduced power consumption, due to the fact that no resistors are required for biasing and setting the gain of the preamplifier, since no physical feedback and biasing resistors are directly connected to the input 20,20' of the operational amplifier.

[0035] As shown in figure 6, according to an embodiment, the microphone preamplifier circuit 60 comprises an anti-aliasing filter INT _C at the input of the preamplifier P _A having an output terminal connected to the input node 10, 10' and an input terminal 61, 61' connected/connectable to the output node M ₀ , M ₀ ' of the microphone circuit M _CD - The anti-aliasing filter INT _C advantageously prevents the production of undesired replicas in the output signal of the preamplifier P _A due to the presence of the switched capacitor C2B, C2 _B ' ·

[0036] According to an embodiment, the anti-aliasing filter INTc is a passive filter comprising a plurality of RC ladder cells (in the differential embodiment, otherwise a plurality of RC serial cells in the single- ended embodiment) . In the particular example shown the anti-aliasing filter INT _C comprises four RC ladder cells.

[0037] As is clear from the above description, the above described microphone preamplifier circuit 60 has the advantage of requiring one or more decoupling capacitors C _D , C _D ' having a relatively reduced capacitance value such that said capacitors can be integrated in the preamplifier's P _A chip. The advantage in terms of area occupation is even greater when the preamplifier P _A is shared between a plurality of n sources, such as for example n microphone circuits M _CD , M _CD ¹ , Men" . In this case, as shown in figure 8, it is possible to place, or better to integrate, a multiplexer 50 (for example, realized with CMOS transfer gates) just before the decoupling capacitor C _D , C _D ' in order to allow the selective connection among the different n sources and the preamplifier P _A . In this case, only two relatively small decoupling capacitors C _D (if the architecture of the preamplifier is differential) are required instead of 2n external SMD capacitors, obtaining a large reduction in the area/space occupation.

[0038] Naturally, in order to satisfy contingent and specific requirements, a person skilled in the art may apply to the above-described microphone preamplifier circuits many modifications and variations, all of which, however, are included within the scope of protection of the invention as defined by the following claims.