Preamplifier circuit for a microelectromechanical capacitive acoustic transducer

Patent metadata

Field	Value
Publication number	US-9350305-B2
Application number	US-201314049126-A
Country	US
Kind code	B2
Filing date	Oct 8, 2013
Priority date	Jun 30, 2009
Publication date	May 24, 2016
Grant date	May 24, 2016

Abstract

Described herein is a preamplifier circuit for a capacitive acoustic transducer provided with a MEMS detection structure that generates a capacitive variation as a function of an acoustic signal to be detected, starting from a capacitance at rest; the preamplifier circuit is provided with an amplification stage that generates a differential output signal correlated to the capacitive variation. In particular, the amplification stage is an input stage of the preamplifier circuit and has a fully differential amplifier having a first differential input (INP) directly connected to the MEMS detection structure and a second differential input (INN) connected to a reference capacitive element, which has a value of capacitance equal to the capacitance at rest of the MEMS detection structure and fixed with respect to the acoustic signal to be detected; the fully differential amplifier amplifies the capacitive variation and generates the differential output signal.

First claim

The invention claimed is:

1. A device, comprising:
an acoustic transducer configured to generate a capacitive variation signal as a function of an acoustic signal;
a preamplifier circuit coupled to the acoustic transducer, the preamplifier circuit including:
a differential amplifier having first and second differential inputs and first and second differential outputs, the differential amplifier being coupled to the acoustic transducer and configured to amplify the capacitive variation signal to generate a differential output signal on the first and second differential outputs; and
a reference generator circuit configured to supply a biasing quantity to the differential amplifier and to generate the biasing quantity proportional to a reference voltage,
wherein the differential amplifier includes:
a first input transistor having a control terminal coupled to said first differential input, a first current-conduction terminal configured to receive the biasing quantity, and a second current conduction terminal coupled to a first resistive load element and the first differential output of the differential amplifier; and
a second input transistor having a control terminal coupled to said second differential input, a first current-conduction terminal configured to receive the biasing quantity, and a second current conduction terminal coupled to a second resistive load element and the second differential output of the differential amplifier.

2. The device of claim 1 wherein the acoustic transducer includes a microelectromechanical detection structure, the first differential input is coupled to the microelectromechanical detection structure, and the second differential input is coupled to a reference structure.

3. The device of claim 2 wherein the microelectromechanical detection structure is configured to generate said capacitive variation signal starting from a value of capacitance at rest, said preamplifier circuit including a reference capacitive element having a value of capacitance substantially equal to the value of capacitance at rest of the microelectromechanical detection structure, the reference structure including the reference capacitive element.

4. The device of claim 3 , further comprising:
a first biasing circuit configured to bias said detection structure; and
a second biasing circuit configured to bias said reference capacitive element.

5. The device of claim 1 wherein a value of said biasing quantity is a biasing current correlated to a resistance value of the first and second resistive load elements.

6. The device of claim 5 , wherein the reference generator circuit is configured to generate said biasing current in the form:
I BIAS =V STAB /(α· R L )

where V STAB is the reference voltage that does not vary with temperature variations, a is a proportionality constant, and R L is the resistance value of the first and second resistive load elements.

7. The device of claim 6 wherein said reference generator circuit is a bandgap reference-voltage generator, and said reference voltage is a bandgap reference voltage, said bandgap reference-voltage generator being configured to provide said bandgap reference voltage across a first reference resistive element configured to generate said biasing current, said first reference resistive element having a same value of resistance as, and being matched to, the first and second resistive load elements.

8. The device of claim 7 wherein said bandgap reference-voltage generator comprises:
an operational amplifier having a first input and a second input;
a first transistor in diode configuration coupled to said first input, and a second transistor in diode configuration coupled to said second input via said first reference resistive element;
a second reference resistive element and a third reference resistive element both having a same value of resistance as, and matched to, the first and second resistive load, and coupled respectively to said first and second inputs;
a first reference transistor element and a second reference transistor element, coupled, respectively, to said first and second inputs and having a respective control terminal coupled to an output of said operational amplifier; and
an output transistor element having a respective control terminal coupled to the output of said operational amplifier and a current-conduction terminal configured to supply said biasing current.

9. The device of claim 1 , further comprising a buffer stage coupled to at least one of the first differential output and the second differential output of said preamplifier circuit, the first and second differential outputs each having a low impedance.

10. A device, comprising:
an acoustic transducer configured to generate a capacitive variation signal as a function of an acoustic signal;
first and second biasing circuits; and
a preamplifier circuit coupled to the acoustic transducer, the preamplifier circuit including:
a differential amplifier having first and second differential inputs and first and second differential outputs, the differential amplifier being coupled to the acoustic transducer and configured to amplify the capacitive variation signal to generate a differential output signal on the first and second differential outputs, and
a reference generator circuit configured to supply a biasing quantity to the differential amplifier and to generate the biasing quantity proportional to a reference voltage,

wherein:
the acoustic transducer includes a microelectromechanical detection structure, the first differential input is coupled to the microelectromechanical detection structure, and the second differential input is coupled to a reference structure,
the microelectromechanical detection structure is configured to generate said capacitive variation signal starting from a value of capacitance at rest, said preamplifier circuit including a reference capacitive element having a value of capacitance substantially equal to the value of capacitance at rest of the microelectromechanical detection structure, the reference structure including the reference capacitive element,
the first biasing circuit is configured to bias said detection structure,
the second biasing circuit is configured to bias said reference capacitive element, and
a first terminal of said micromechanical detection structure is coupled to a first terminal of said reference capacitive element, the micromechanical detection structure and the reference capacitive element being configured to receive a first biasing voltage at respective first terminals, said first biasing circuit including a first resistive biasing element having high impedance, the first resistive biasing element being coupled between said first differential input and a second biasing voltage, said second biasing circuit including a second resistive biasing element having said high impedance, the second biasing circuit being coupled between said second differential input and said second biasing voltage.

11. The device of claim 10 wherein said differential amplifier has a capacitive filter element coupled between said first differential output and the second differential output.

12. A readout-interface circuit, comprising:
a preamplifier circuit configured to receive a capacitive variation signal from a MEMS detection structure and to amplify the capacitive variation signal, the preamplifier circuit including:
an amplification stage configured to generate a differential output signal based on the capacitive variation signal, the amplification stage including a fully differential amplifier configured to receive and to amplify the capacitive variation signal, the differ

Assignees

• St Microelectronics Srl

Inventors

• David Filippo
• Padovani Igino

Classifications

• H03F3/45 Primary

  Differential amplifiers (differential sense amplifiers G11C7/062) · CPC title

• H03F3/45183

  Long tailed pairs (H03F3/4521, H03F3/45237 take precedence) · CPC title

• H04R19/01

  characterised by the use of electrets · CPC title

• H03F2203/45628

  the LC comprising bias stabilisation means, e.g. DC level stabilisation means, and temperature coefficient dependent control, e.g. DC level shifting means · CPC title

• G05F3/30

  Regulators using the difference between the base-emitter voltages of two bipolar transistors operating at different current densities (G05F3/26 takes precedence) · CPC title