Microphone preamplifier circuit

Abstract

A microphone preamplifier circuit is adapted to be connected to a microphone circuit, the microphone circuit including a microphone and at least one output node. The microphone preamplifier circuit includes a preamplifier including: an operational amplifier having at least one input and at least one output; at least one input DC decoupling capacitor connected to the at least one input of the operational amplifier; at least one feedback capacitor connected between the input and the output of the operational amplifier in order to set together with the at least one input DC decoupling capacitor a gain value of the preamplifier circuit; and first and second feed nodes adapted to be fed by first and second bias voltages respectively. The preamplifier further includes at least one switched capacitor adapted to be selectively and alternatively connected in response to a clock signal: between the at least one input and the at least one output of the operational amplifier; and between the first and second feed nodes. The microphone preamplifier circuit further includes an anti-aliasing filter having: (i) at least one output terminal connected to the at least one input DC decoupling capacitor and (ii) at least one input terminal adapted to be connected to the at least one output node of the microphone circuit.

Claims

1. Microphone preamplifier circuit adapted to be connected to a microphone circuit, said microphone circuit comprising a microphone and at least one output node, said microphone preamplifier circuit comprising a preamplifier comprising: an operational amplifier comprising at least one input and at least one output; at least one input DC decoupling capacitor connected to said at least one input of said operational amplifier; at least one feedback capacitor connected between said at least one input and said at least one output of said operational amplifier in order to set together with said at least one input DC decoupling capacitor a gain value of said microphone preamplifier circuit; first and second feed nodes adapted to be fed by first and second bias voltages respectively; wherein said preamplifier further comprises at least one switched capacitor adapted to be selectively and alternatively connected in response to a clock signal: between said at least one input and said at least one output of said operational amplifier; and between said first and second feed nodes; wherein said microphone preamplifier circuit further comprises an anti-aliasing filter comprising: (i) at least one output terminal connected to said at least one input DC decoupling capacitor and (ii) at least one input terminal adapted to be connected to said at least one output node of said microphone circuit, and wherein said preamplifier and said anti-aliasing filter are integrated in a same chip.

2. Microphone preamplifier circuit according to claim 1, wherein said at least one switched capacitor comprises: a first plate adapted to be selectively connected to said at least one input of said operational amplifier and said first feed node; a second plate adapted to be selectively connected to said at least one output of said operational amplifier and said second feed node; and wherein said second feed node is configured to be fed with a common mode voltage of said operational amplifier and said first feed node is configured to be fed with a further voltage appropriate for input operation of said operational amplifier.

3. Microphone preamplifier circuit according to claim 2, wherein said operational amplifier is fed with a supply voltage and wherein: said common mode voltage is half of said supply voltage; and said further voltage is lower than said common mode voltage if said operational amplifier has a P-type input and higher than said common voltage if said operational amplifier has a N-type input.

4. Microphone preamplifier circuit according to claim 1, wherein said clock signal has a frequency such that said at least one switched capacitor is adapted to discharge said at least one feedback capacitor at a frequency so that said preamplifier is adapted to implement a high pass filtering function which does not interfere with audio signals of interest for operation of said microphone.

5. Microphone preamplifier circuit according to claim 4, wherein said high pass filtering function has a high pass cut-off frequency of 6Hz.

6. Microphone preamplifier circuit according to claim 4, wherein said clock signal has a frequency of 100 kHz.

7. Microphone preamplifier circuit according to claim 1, wherein said at least one switched capacitor has a capacitance value lower than a capacitance value of said at least one feedback capacitor.

8. Microphone preamplifier circuit according to claim 7, wherein said at least one switched capacitor has a capacitance value of three magnitude orders lower than said capacitance value of said at least one feedback capacitor.

9. Microphone preamplifier circuit according to claim 1, wherein said operational amplifier is a differential amplifier, and said preamplifier has a symmetrical circuital structure including two input DC decoupling capacitors, two feedback capacitors and two switched capacitors.

10. Microphone preamplifier circuit according to claim 1, wherein said operational amplifier is arranged in said microphone preamplifier circuit in order to operate in an inverting configuration.

11. Microphone preamplifier circuit according to claim 1, wherein said anti-aliasing filter comprises a plurality of RC ladder, or serial, cells.

12. Microphone preamplifier circuit according to claim 1, further comprising a multiplexer adapted to selectively connect said microphone preamplifier circuit to one of a plurality of microphone circuits.

13. Microphone preamplifier circuit according to claim 12, wherein said multiplexer, said anti-aliasing filter and said preamplifier are integrated in said same chip.

14. Microphone system comprising said microphone circuit and said microphone preamplifier circuit according to claim 1.

15. Device having at least one of audio processing, recording and communication capabilities, said device comprising said microphone system according to claim 14.

16. Device comprising: a microphone; a microphone circuit connected to said microphone and having at least one output node; and a microphone preamplifier circuit connected to said at least one output node of said microphone circuit and comprising: an operational amplifier comprising at least one input and at least one output; at least one input DC decoupling capacitor connected to said at least one input of said operational amplifier; at least one feedback capacitor connected between said at least one input and said at least one output of said operational amplifier; first and second feed nodes fed by first and second bias voltages respectively; at least one switched capacitor selectively and alternatively connected in response to a clock signal between: (i) said at least one input and said at least one output of said operational amplifier, and (ii) said first and second feed nodes; and an anti-aliasing filter comprising: (i) at least one output terminal connected to said at least one input DC decoupling capacitor and (ii) at least one input terminal connected to said at least one output node of said microphone circuit, and wherein said microphone preamplifier circuit is integrated in a single chip.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 shows a schematic view of a first example of prior art microphone system comprising a microphone circuit and a preamplifier;

(3) FIG. 2 shows a schematic view of a second example of prior art microphone system comprising a microphone circuit and a preamplifier;

(4) FIG. 3 shows a schematic view of a first example of a known preamplifier for the microphone system of FIG. 2;

(5) FIG. 4 shows a schematic view of a second example of known preamplifier for the microphone system of FIG. 2;

(6) FIG. 5 shows a very schematic view of a device provided with audio recording and/or communication and/or processing capabilities;

(7) FIG. 6 shows an embodiment of a microphone preamplifier circuit;

(8) FIG. 7 shows a part of the circuit of FIG. 6 and the equivalent circuit thereof; and

(9) FIG. 8 shows an embodiment of a microphone system comprising the microphone preamplifier circuit of FIG. 6.

DETAILED DESCRIPTION

(10) In the attached figures identical or similar elements will be indicated with the same reference numbers/symbols.

(11) FIG. 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.sub.D. According to an embodiment, and without for this reason introducing any limitation, the microphone circuit MC.sub.D may be similar to the differential microphone circuit MC.sub.D represented in FIG. 2. The microphone circuit MC.sub.D comprises a microphone 3, a microphone's biasing network and two differential output nodes M.sub.O, M.sub.O (represented in FIG. 6). According to an embodiment, the microphone's biasing network may be identical or similar to the known biasing network of FIG. 2.

(12) 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 FIG. 5, the preamplifier circuit 60, the digital converter 70 and the digital audio processor 80 are integrated on the same chip 7.

(13) 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/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.

(14) FIG. 6 shows an embodiment of microphone preamplifier circuit 60 comprising a preamplifier P.sub.A, the microphone preamplifier circuit 60 being adapted to be connected to the microphone 3, or more precisely to the microphone circuit MC.sub.D.

(15) According to the embodiment shown, the preamplifier P.sub.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.sub.O, M.sub.O of the microphone circuit MC.sub.D. In the particular example shown in FIG. 6, the operational amplifier OA is arranged in said microphone preamplifier circuit 60 in order to operate in an inverting configuration.

(16) Since the structure of the microphone preamplifier circuit 60 and the preamplifier P.sub.A in the embodiment shown is totally symmetrical, in the present description mostly the upper branch of the preamplifier P.sub.A will be detailed, i.e. the branch above the ideal line L1. 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.sub.CD 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.

(17) With reference to FIG. 6, the preamplifier P.sub.A comprises an operational amplifier OA comprising at least one input 20,20 and at least one output 21, 21. Moreover, the preamplifier P.sub.A comprises at least one input DC decoupling capacitor C.sub.D, C.sub.D connected between the input node 10, 10 of the preamplifier P.sub.A and the input 20,20 of the operational amplifier OA.

(18) The preamplifier P.sub.A further comprises at least one feedback capacitor C.sub.2A, C.sub.2A 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.sub.D, C.sub.D a gain value G of the preamplifier circuit 60. Such gain value G is given by:

(19) $G=-\frac{C_D}{C_{2A}}.$

(20) As shown in FIG. 6, the preamplifier P.sub.A comprises at least one first 40, 40 and at least one second feed node 41, 41 adapted to be fed by a first V.sub.CMIN and a second V.sub.CM bias voltage respectively.

(21) The preamplifier P.sub.A further comprises at least one switched capacitor C.sub.2B, C.sub.2B 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.sub.A, i.e. in parallel to the feedback capacitor C.sub.2A, C.sub.2A; and between the above mentioned first 40, 40 and second 41, 41 feed nodes.

(22) According to an embodiment, the switched capacitor C.sub.2B, C.sub.2B comprises: a first plate (in FIG. 6 on the left side of capacitor C.sub.2B, C.sub.2B) adapted to be selectively and alternatively connected to the input 20, 20 and the first feed node 40,40; and a second plate (in FIG. 6 on the right side of capacitor C.sub.2B, C.sub.2B) adapted to be selectively and alternatively connected to the output 21, 21 and the second feed node 41, 41.

(23) According to the above embodiment the second feed node 41,41 is configured to be fed with the common mode voltage V.sub.CM of the operational amplifier OA and the first feed node 40, 40 is configured to be fed with a further voltage V.sub.CMIN appropriate for input operation of the operational amplifier OA. For example, if the operational amplifier OA is fed with a supply voltage V.sub.AL: the common mode voltage V.sub.CM is half of said supply voltage V.sub.AL; the above mentioned further voltage V.sub.CMIN is lower than said common mode voltage V.sub.CM if the operational amplifier OA has P-type input and higher than said common voltage V.sub.CM if the operational amplifier OA has N-type input.

(24) For example, without for this reason introducing any limitation, if V.sub.AL=1.5 Volts, V.sub.CM=0.75 Volts, and V.sub.CMIN=0.5 Volts if the operational amplifier OA has a P-type input or V.sub.CMIN=1.0 Volts if the operational amplifier OA has N-type input.

(25) From the above description it is therefore clear that the switched capacitor C.sub.2B, C.sub.2B is a capacitor provided to bias the input 20,20 of the operational amplifier OA to the further voltage V.sub.CMIN, since as shown in FIG. 7 it can be modeled with a resistor having an equivalent resistance R.sub.EQ=1/(f.sub.CK.Math.C.sub.2B) in series with a battery providing a voltage V.sub.BAT=V.sub.CMV.sub.CMIN.

(26) From FIG. 7 it is therefore clear that the switched capacitor C.sub.2B, C.sub.2B is also adapted to discharge the feedback capacitor C.sub.2A, C.sub.2A (FIG. 6) at a given frequency than can be set to be relatively very low by selecting appropriate values for the frequency f.sub.CK of the clock signal CK and the appropriate capacitance values of the feedback C.sub.2A, C.sub.2A and the switched C.sub.2B, C.sub.2B 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 6 Hz or about 6 Hz, for example comprised between 4 Hz and 8 Hz.

(27) The time constant of the above mentioned high pass filtering is given by:

(28) $\begin{array}{cc}=\frac{1}{f_{\mathrm{CK}}}.Math.\frac{C_{2A}}{C_{2B}}.& \left(1\right)\end{array}$

(29) 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.sub.CK has a value of the order of magnitude of 100 kHz, for example f.sub.CK=100 kHz and the switched capacitor C.sub.2B, C.sub.2B has a capacitance value significantly lower than a capacitance value of the feedback capacitor C.sub.2A, C.sub.2A. For example, the switched capacitor C.sub.2B, C.sub.2B has a capacitance value of about three magnitude orders lower the capacitance value of said feedback capacitor C.sub.2A, C.sub.2A. In a practical embodiment, a high pass cut-off frequency of about 6 Hz can be for example obtained if f.sub.CK=100 KHz, C.sub.2A=100 pF, C.sub.2A= 1/30 pF.

(30) The above values for the clock frequency f.sub.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 6 Hz (e.g. 0.6 Hz), while paying in performance it can be much higher than 6 Hz (e.g. 60 Hz).

(31) It is clear that the above condition (1) fixes the limit for the minimum value of the capacitor C.sub.2A and in turn of C.sub.2B. 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.

(32) Moreover, it is to be noticed that since the high pass filtering function is performed by the operational amplifier OA, the feedback capacitor C.sub.2A, C.sub.2A and the switched capacitor C.sub.2B, C.sub.2B, the DC decoupling capacitor C.sub.D, C.sub.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.sub.D, C.sub.D can have a value of about 100 pF and therefore it can be easily integrated without wasting a significant amount of silicon area.

(33) 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.

(34) As shown in FIG. 6, according to an embodiment, the microphone preamplifier circuit 60 comprises an anti-aliasing filter INT.sub.C at the input of the preamplifier P.sub.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.sub.O, M.sub.O of the microphone circuit M.sub.CD. The anti-aliasing filter INT.sub.C advantageously prevents the production of undesired replicas in the output signal of the preamplifier P.sub.A due to the presence of the switched capacitor C.sub.2B, C.sub.2B.

(35) According to an embodiment, the anti-aliasing filter INT.sub.C 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.sub.C comprises four RC ladder cells.

(36) 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.sub.D, C.sub.D having a relatively reduced capacitance value such that said capacitors can be integrated in the preamplifier's P.sub.A chip. The advantage in terms of area occupation is even greater when the preamplifier P.sub.A is shared between a plurality of n sources, such as for example n microphone circuits M.sub.CD, M.sub.CD.sup.1, M.sub.CD.sup.n. In this case, as shown in FIG. 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.sub.D,C.sub.D in order to allow the selective connection among the different n sources and the preamplifier P.sub.A. In this case, only two relatively small decoupling capacitors C.sub.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.

(37) 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.