Differential amplifier circuit for a capacitive acoustic transducer and corresponding capacitive acoustic transducer

Patent metadata

Field	Value
Publication number	US-2016352294-A1
Application number	US-201514960155-A
Country	US
Kind code	A1
Filing date	Dec 4, 2015
Priority date	May 29, 2015
Publication date	Dec 1, 2016
Grant date	—

Abstract

An amplifier circuit, for a capacitive acoustic transducer defining a sensing capacitor that generates a sensing signal as a function of an acoustic signal, has a first input terminal and a second input terminal, which are coupled to the sensing capacitor and: a dummy capacitor, which has a capacitance corresponding to a capacitance at rest of the sensing capacitor and a first terminal connected to the first input terminal; a first buffer amplifier, which is coupled at input to the second input terminal and defines a first differential output of the circuit; a second buffer amplifier, which is coupled at input to a second terminal of the dummy capacitor and defines a second differential output of the circuit; and a feedback stage, which is coupled between the differential outputs and the first input terminal, for feeding back onto the first input terminal a feedback signal, which has an amplitude that is a function of the sensing signal and is in phase opposition with respect thereto.

First claim

1 . An amplifier circuit, comprising:
a first input terminal and a second input terminal designed to be electrically coupled to a first terminal and a second terminal of a sensing capacitor configured to generate a sensing signal as a function of an acoustic signal and being part of a sensing structure of a capacitive acoustic transducer; a dummy capacitor having a capacitance corresponding to a respective capacitance at rest in the absence of said acoustic signal, of the sensing capacitor and a respective first terminal connected to the first input terminal; a first buffer amplifier having its non-inverting input terminal coupled to the second input terminal and its inverting input terminal connected to a respective output terminal, which defines a first differential output of said amplifier circuit; a second buffer amplifier having its non-inverting input terminal coupled to a second terminal of the dummy capacitor and its inverting input terminal connected to a respective output terminal, which defines a second differential output of said amplifier circuit, an output signal as a function of said sensing signal being present in use between said first and second differential outputs; and a feedback stage coupled between said first and second differential outputs and said first input terminal and configured to feed back onto said first input terminal a feedback signal, which has an amplitude that is a function of said sensing signal and is in phase opposition with respect to said sensing signal.

2 . The amplifier circuit according to claim 1 , wherein said first and second buffer amplifiers are unity-gain voltage followers.

3 . The amplifier circuit according to claim 1 , wherein said feedback signal has an amplitude equal to half the amplitude of said sensing signal and is phase-shifted by 180° with respect to said sensing signal.

4 . The amplifier circuit according to claim 1 , wherein said feedback stage comprises:
a resistive divider which is connected between said first differential output and said second differential output and defines a feedback node; a feedback amplifier which has a first input terminal connected to the feedback node, a second input terminal receiving a reference voltage, and an output terminal; and a feedback capacitor connected between the output terminal of said feedback amplifier and said first input terminal.

5 . The amplifier circuit according to claim 4 , wherein said reference voltage is a common-mode voltage between said first and second differential outputs.

6 . The amplifier circuit according to claim 4 , comprising a feedback resistor connected between the first input terminal and the output terminal of said feedback amplifier.

7 . The amplifier circuit according to claim 6 , wherein said feedback resistor has a resistance R B ; and wherein said resistive divider comprises a first division resistor, which is connected between said first differential output and said feedback node and has a resistance R 1 , and a second division resistor, which is connected between said second differential output and said feedback node and has a resistance R 2 ; wherein:
R 1 =R 2 //R B .

8 . The amplifier circuit according to claim 4 , wherein said feedback capacitor has a capacitance C B , said sensing capacitor has a capacitance C MIC , and said dummy capacitor has a capacitance C DUM ; wherein:
C B >>C MIC +C DUM .

9 . The amplifier circuit according to claim 4 , comprising a first high-impedance element that couples said first input terminal to a first biasing line set at a biasing voltage; and a second high-impedance element and a third high-impedance element, which couple, respectively, said second input terminal and said second terminal of said dummy capacitor, to a second biasing line set at said reference voltage.

10 . A capacitive acoustic transducer, comprising:
a sensing structure which defines a sensing capacitor and is configured to generate a sensing signal as a function of an acoustic signal; and an amplifier circuit configured to process said sensing signal and supply an output signal, the amplifier circuit including,
a first input node and a second input node coupled first and second nodes of the sensing capacitor, respectively;
a dummy capacitor having a first node coupled to the first input node, the dummy capacitor having a capacitance value that is approximately equal to a capacitance value of the sensing capacitor in the absence of the acoustic signal;
a first buffer amplifier circuit having a non-inverting input coupled to the second input node and an output that defines a first differential output of the amplifier circuit;
a second buffer amplifier circuit having a non-inverting input coupled to a second node of the dummy capacitor and an output that defines a second differential output of the amplifier circuit; and
a feedback circuit coupled between the first and second differential outputs and the first input node, the feedback circuit configured to generate on the first input node a feedback signal having an amplitude that is a function of the sensing signal and having a phase in phase opposition with the sensing signal.

11 . The capacitive acoustic transducer of claim 10 , wherein the capacitive acoustic transducer comprises a MEMS type transducer.

12 . The capacitive acoustic transducer of claim 10 , wherein the feedback circuit comprises a resistive voltage divider coupled across the first and second differential outputs and

13 . An electronic device, comprising:
a capacitive acoustic transducer provided with a sensing structure which defines a sensing capacitor and is configured to generate a sensing signal as a function of an acoustic signal; an amplifier circuit including,
a first input terminal and a second input terminal designed to be electrically coupled to a first terminal and a second terminal of the sensing capacitor;
a dummy capacitor having a capacitance corresponding to a respective capacitance at rest, in the absence of the acoustic signal, of the sensing capacitor, and a respective first terminal connected to the first input terminal;
a first buffer amplifier having a non-inverting input terminal coupled to the second input terminal and an inverting input terminal connected to a respective output terminal that defines a first differential output of the amplifier circuit;
a second buffer amplifier having a non-inverting input terminal coupled to a second terminal of the dummy capacitor and an inverting input terminal connected to a respective output terminal that defines a second differential output of the amplifier circuit, wherein an output signal, as a function of the sensing signal, being present in use between the first and second differential outputs; and
a feedback stage coupled between the first and second differential outputs and the first input terminal, and configured to feed back onto the first input terminal a feedback signal having an amplitude that is a function of the sensing signal and is in phase opposition with respect to the sensing signal; and
a microprocessor unit coupled to the amplifier circuit of the capacitive acoustic transducer.

14 . The electronic device according to claim 13 , wherein the electronic device comprises one of a cellphone; a personal digital assistant; a portable computer; a digital audio player with voice-recording capacity; a photographic camera or video camera; and a control device for videogames.

15 . The electronic device according to claim 14 further comprising an input/output interface and a memory coupled to the microprocessor unit.

Assignees

• St Microelectronics Srl

Inventors

• Nicollini Germano
• Barbieri Andrea

Classifications

• H04R2410/03

  Reduction of intrinsic noise in microphones · CPC title

• H04R3/00

  Circuits for transducers (arrangements for producing a reverberation or echo sound G10K15/08; amplifiers H03F) · CPC title

• H03F3/187

  in integrated circuits · CPC title

• H04R3/007 Primary

  Protection circuits for transducers · CPC title

• H04R19/005 Primary

  using semiconductor materials · CPC title