Differential amplifier circuit for a capacitive acoustic transducer and corresponding capacitive acoustic transducer
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.
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 to 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.
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.
16. The electronic device according to claim 14 , wherein each of the first and second buffer amplifiers comprises an operational amplifier configured in a voltage-follower configuration.
17. The electronic device according to claim 16 , wherein the feedback stage comprises a resistive voltage divider coupled between the first and second differential outputs and a feedback amplifier circuit having an input coupled between a node of the voltage divider and an output coupled through feedback capacitor to the first input terminal.
18. The electronic device according to claim 17 , wherein the feedback amplifier comprises an operational transconductance amplifier.
19. A processing method, comprising:
providing a capacitive acoustic transducer with a sensing structure which defines a sensing capacitor and is configured to generate a sensing signal as a function of an acoustic signal;
providing an amplifier circuit having a first input terminal and a second input terminal which are designed to be electrically coupled to a first terminal and a second terminal of said sensing capacitor the amplifier circuit comprising:
a dummy capacitor which has 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; and
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 which is a function of said sensing signal being present between said first and second differential outputs and
feeding 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 the sensing signal thereby implementing a fully differential processing of said sensing signal.
20. The method according to claim 19 further comprising:
generating a feedback voltage that is equal to a common-mode voltage between the first and second differential outputs; and
generating the feedback signal based on the feedback voltage, wherein said feedback signal has an amplitude equal to half of the amplitude of said sensing signal and is phase-shifted by 180° with respect to said sensing signal.