News Column

Patent Application Titled "Microphone Preamplifier Circuit" Published Online

June 26, 2014

By a News Reporter-Staff News Editor at Politics & Government Week -- According to news reporting originating from Washington, D.C., by VerticalNews journalists, a patent application by the inventors Nicollini, Germano (Piacenza, IT); Barbieri, Andrea (Casalpusterlengo, IT), filed on July 13, 2012, was made available online on June 12, 2014.

The assignee for this patent application is St-ericsson Sa.

Reporters obtained the following quote from the background information supplied by the inventors: "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.

"FIGS. 1 and 2 show two known ways, single-ended and differential, respectively, to bias and connect the microphone circuit MC.sub.S, MC.sub.D to a preamplifier PA.sub.S, PA.sub.D using an RC network. The microphone circuit MC.sub.S, MC.sub.D comprises a microphone 3 and a biasing circuit R.sub.MB1, R.sub.MB2, R.sub.MB3, C1, C2, fed by a bias voltage V.sub.BIAS.

"The DC bias voltage of the signal coming from the microphone circuit MC.sub.S, MC.sub.D at the output nodes M.sub.O, M.sub.O' of the microphone circuit MC.sub.S, MC.sub.D will depend exclusively by the biasing circuit R.sub.MB1, R.sub.MB2, R.sub.MB3, C1, C2 and is usually different from the DC bias input voltage of the preamplifier PA.sub.S, PA.sub.D. The level shifting between the microphone 3 and the preamplifier PA.sub.S, PA.sub.D DC biasing voltages is commonly obtained using a decoupling capacitor C.sub.DEC that produces, with the preamplifier PA.sub.S, PA.sub.D input resistance, a first order high-pass filter whose corner frequency is generally lower than 20 Hz in order to avoid in-band audio signal perturbation.

"More detailed representations of the differential preamplifier PA.sub.D are shown in FIG. 3 (inverting configuration) and FIG. 4 (non-inverting configuration).

"In the inverting case, due to noise generation, input resistors R.sub.1A and R.sub.1B cannot have high resistance values (typically from 10 kOhm to 50 kOhm), whereas in the non-inverting solution resistors R.sub.3A and R.sub.3B are used only to bias the amplifiers OA inputs at a common mode voltage V.sub.CM midway between ground and the supply voltage. Accordingly, resistors R.sub.3A and R.sub.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.sub.DEC of more than 100 nF 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 20 mm.sup.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.sub.DEC, confirming that the integration of the decoupling capacitors C.sub.DEC is practicable only for relatively reduced capacitance values. Also U.S. Pat. No. 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.

"Moreover, unfortunately, even with the PDI methodology the decoupling capacitors C.sub.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.sub.DEC.

"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.

"Moreover, with reference to FIGS. 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.

"EP 2 133 993 A1, EP 0 375 017 A2 and U.S. Pat. No. 6,656,072 B1 disclose filters and/or gain circuits. However such documents neither refer to microphone systems nor address the above disclosed problem concerning the decoupling capacitor."

In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventors' summary information for this patent application: "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.

"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.


"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 FIG. 2;

"FIG. 4 shows a schematic view of a second example of known preamplifier for the microphone system of FIG. 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 FIG. 6 and the equivalent circuit thereof; and

"FIG. 8 shows an embodiment of a microphone system comprising the microphone preamplifier circuit of FIG. 6."

For more information, see this patent application: Nicollini, Germano; Barbieri, Andrea. Microphone Preamplifier Circuit. Filed July 13, 2012 and posted June 12, 2014. Patent URL:

Keywords for this news article include: St-ericsson Sa.

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Source: Politics & Government Week

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