System and method for low distortion capacitive signal source amplifier

• US 9,236,837 B2
• Filed: 08/25/2011
• Issued: 01/12/2016
• Est. Priority Date: 08/25/2011
• Status: Active Grant

• Current Assignee: Infineon Technologies AG
• Original Assignee: Infineon Technologies AG
• Inventors: Kropfitsch, Michael, Ceballos, Jose Luis
• Primary Examiner(s): Goins, Davetta W
• Assistant Examiner(s): Ganmavo, Kuassi
• Application Number:
  US13/217,890
• Publication Number:
  US 20130051582A1
• Time in Patent Office: 1,601 Days
• Field of Search: 381111-114, 381/174, 381/369, 381/175, 381/122, 381/91, 381/92, 330/160, 330/260, 330/144, 330/307, 341130-155
• US Class Current: 1/1
• CPC Class Codes: H03F 1/32 Modifications of amplifiers to reduce non-linear distortion by negative feedback H03F1/34
• CPC Class Codes: H03F 1/56 Modifications of input or output impedances, not otherwise provided for
• CPC Class Codes: H03F 2200/03 the amplifier being designed for audio applications
• CPC Class Codes: H03F 2200/105 A non-specified detector of the power of a signal being used in an amplifying circuit
• CPC Class Codes: H03F 2200/165 A filter circuit coupled to the input of an amplifier
• CPC Class Codes: H03F 2200/211 the input of an amplifier can be attenuated by a continuously controlled transistor attenuator
• CPC Class Codes: H03F 2200/213 A variable capacitor being added in the input circuit, e.g. base, gate, of an amplifier stage
• CPC Class Codes: H03F 3/183 with semiconductor devices only
• CPC Class Codes: H03F 3/187 in integrated circuits
• CPC Class Codes: H04R 19/005 using semiconductor materials
• CPC Class Codes: H04R 19/04 Microphones H04R19/01 takes precedence
• CPC Class Codes: H04R 3/00 Circuits for transducers , loudspeakers or microphones

Abstract

According to an embodiment, a method includes amplifying a signal provided by a capacitive signal source to form an amplified signal, detecting a peak voltage of the amplified signal, and adjusting a controllable impedance coupled to an output of the capacitive signal source in response to detecting the peak voltage. The controllable impedance is adjusted to a value inversely proportional to the detected peak voltage.

Claims

1. A method comprising:

amplifying a signal provided by a capacitive signal source to form an amplified signal;

detecting a peak voltage of a portion of the amplified signal having a frequency higher than a lower corner frequency;

adjusting a controllable impedance of an attenuator coupled to an output of the capacitive signal source in response to detecting the peak voltage of the portion of the amplified signal having the frequency higher than the lower corner frequency, the controllable impedance being adjusted to a value inversely proportional to the detected peak voltage; and

comparing the detected peak voltage to a predetermined threshold, wherein adjusting the controllable impedance comprises

decreasing the controllable impedance at a first rate if the detected peak voltage exceeds the predetermined threshold,

increasing the controllable impedance at a second rate if detected peak voltage does not exceed the predetermined threshold, wherein the first rate is greater than the second rate.

2. The method of claim 1, wherein increasing the controllable impedance comprises increasing the controllable impedance up to a maximum value in a plurality of steps.

3. The method of claim 1, wherein adjusting the controllable impedance comprises adjusting a capacitance coupled to the output of the capacitive signal source.

4. The method of claim 3, wherein the adjusting the capacitance comprises adjusting a capacitance of a capacitor array, adjusting the capacitance of the capacitor array coupling and decoupling capacitors to and from the output of the capacitive signal source.

5. The method of claim 1, wherein adjusting the controllable impedance comprises adjusting a controllable resistance coupled to the output of the capacitive signal source.

6. The method of claim 1, further comprising:

detecting a zero crossing of the signal provided by the capacitive signal source; and

adjusting the controllable impedance comprises adjusting the controllable impedance when a zero crossing is detected.

7. The method of claim 1, wherein:

the capacitive signal source comprises a MEMS microphone; and

the adjustable impedance is controlled such that a total harmonic distortion of the amplified signal is less than 10% for a 140 dBSPL acoustic input to the MEMS microphone.

8. The method of claim 1, wherein:

the capacitive signal source comprises a MEMS microphone; and

the method further comprises biasing the MEMS microphone.

18. The method of claim 1, wherein the amplifier signal comprises an audio signal.

19. The method of claim 1, wherein adjusting the controllable impedance comprises adjusting an adjustable high-pass network coupled to the output of the capacitive signal source.

20. The method of claim 19, wherein adjusting the adjustable high-pass network comprises adjusting a variable resistance.

21. The method of claim 19, wherein adjusting the controllable impedance further comprises increasing a lower corner frequency of the adjustable high-pass network for a predetermined period of time, and then decreasing the lower corner frequency after the predetermined period of time.

9. A system for amplifying a signal provided by a capacitive signal source, the system comprising:

a signal amplifier comprising an input node configured to be coupled to the capacitive signal source;

a controllable attenuation circuit coupled to the input node of the signal amplifier, the controllable attenuation circuit configured to provide a controllable input impedance at the input node of the signal amplifier;

a signal detection circuit coupled to an output of the signal amplifier, the signal detection circuit configured to detect peak signal values of a portion of a signal at the output of the signal amplifier having a frequency higher than a lower corner frequency; and

a control circuit coupled between the signal detection circuit and the controllable attenuation circuit, the control circuit configured to adjust the controllable attenuation circuit in response to changes in detected peak signal values of the portion of the signal at the output of the signal amplifier having the frequency higher than the lower corner frequency, wherein

the control circuit is further configured to command the controllable attenuation circuit to decrease the input impedance if the signal detection circuit detects a peak signal level greater than a threshold and increase the input impedance up to a maximum value in several steps if the signal detection circuit does not detect a peak signal level greater than the threshold,

the control circuit commands the controllable attenuation circuit to decrease the input impedance at a first rate and increase the input impedance at a second rate, and

the first rate is greater than the second rate.

10. The system of claim 9, wherein the signal detection circuit is configured to detect a positive peak signal value and a negative peak signal value.

11. The system of claim 9, wherein:

the signal detection circuit is further configured to detect a zero crossing of a signal at the output of the signal amplifier; and

the control circuit is further configured to adjust the controllable attenuation circuit when the signal detection circuit detects a zero crossing.

12. The system of claim 9, wherein the controllable attenuation circuit comprises a selectable capacitor array coupled to the input node of the signal amplifier.

13. The system of claim 9, wherein the controllable attenuation circuit comprises an adjustable resistance coupled to the input node of the signal amplifier.

14. The system of claim 9, wherein the capacitive signal source comprises a MEMS microphone.

15. The system of claim 9, wherein the portion of the signal at the output of the signal amplifier is an audio signal.

16. The system of claim 9, wherein the controllable attenuation circuit comprises an adjustable high-pass network.

17. The system of claim 16, wherein the control circuit is further configured to increase a lower corner frequency of the adjustable high-pass network for a predetermined period of time, and then decrease the lower corner frequency after the predetermined period of time.