Capacitive sensor assemblies and electrical circuits therefor
A sensor assembly including a capacitive sensor, like a microelectromechanical (MEMS) microphone, and an electrical circuit therefor are disclosed. The electrical circuit includes a first transistor having an input gate connectable to the capacitive sensor, a second transistor having an input gate coupled to an output of the first transistor, a feedforward circuit interconnecting a back-gate of the second transistor and the output of the first transistor, and a filter circuit interconnecting the output of the first transistor and the input gate of the second transistor.
1. An electrical circuit for a capacitive sensor assembly, the circuit comprising:
a first transistor having an input gate connectable to a capacitive sensor;
a second transistor having an input gate coupled to an output of the first transistor, the second transistor including a back-gate;
a feedforward circuit interconnecting the back-gate of the second transistor and the output of the first transistor; and
a filter circuit interconnecting the output of the first transistor and the input gate of the second transistor,
wherein the feedforward circuit bypasses the filter circuit.
2. The circuit of claim 1 , wherein the feedforward circuit is a direct electrical connection between the back-gate of the second transistor and the output of the first transistor.
3. The circuit of claim 1 , wherein the filter circuit is an audio bandpass filter.
4. The circuit of claim 1 ,
the first transistor configured as a source-follower comprising a source coupled to the output of the first transistor,
the second transistor configured as a source-follower comprising a source coupled to an output of the second transistor.
5. The circuit of claim 4 having an input capacitance on the order of 0.1 pF.
6. The circuit of claim 5 further comprising a sensor bias circuit connectable to the capacitive sensor, and a DC bias circuit having a reference signal output coupled to the input gate of the first transistor by a resistance element, the reference signal output also coupled to the input gate of the second transistor by a resistance element.
7. The circuit of claim 6 , the first transistor including a back-gate coupled to the source of the first transistor.
8. The circuit of claim 6 , wherein the filter circuit is an audio band filter.
9. The circuit of claim 8 is an integrated circuit die.
10. The circuit of claim 9 in combination with a capacitive microelectro-mechanical systems (MEMS) die having a capacitance between approximately 0.1 pF and approximately 5.0 pF.
11. An integrated circuit for a capacitive microelectromechanical systems (MEMS) microphone assembly, the circuit comprising:
a first transistor configured as a source-follower comprising an input gate connectable to an output of a capacitive MEMS die, and a source coupled to an output of the first transistor;
a second transistor configured as a source-follower comprising an input gate coupled to the output of the first transistor, a source coupled to an output of the second transistor, and a back-gate electrically coupled to the output of the first transistor;
an audio band filter circuit interconnecting the output of the first transistor and the input gate of the second transistor;
a DC bias circuit having a reference signal output coupled to the input gate of the first transistor by a resistance element, the reference signal output of the DC bias circuit coupled to the input gate of the second transistor by a resistance element,
the circuit having an input capacitance on the order of 0.1 pF.
12. The circuit of claim 11 , wherein the back-gate of the second transistor is directly connected to the output of the first transistor.
13. The circuit of claim 11 in combination with a capacitive microelectro-mechanical systems (MEMS) die having a capacitance between approximately 0.1 pF and approximately 5.0 pF.
14. A microphone sensor assembly comprising:
a housing having an external-device interface and a sound port;
a capacitive microelectromechanical systems (MEMS) die disposed in the housing and acoustically coupled to the sound port;
an integrated circuit disposed in the housing and electrically coupled to contacts on the external-device interface, the circuit comprising:
a first transistor having an input gate coupled to a first node of the capacitive MEMS die;
a second transistor having an input gate coupled to an output of the first transistor, the second transistor including a back-gate;
a feedforward circuit interconnecting the back-gate of the second transistor and the output of the first transistor;
a filter circuit interconnecting the output of the first transistor and the input gate of the second transistor; and
a MEMS die bias circuit coupled to a second node of the capacitive MEMS die.
15. The sensor of claim 14 , wherein the feedforward circuit is a direct electrical connection between the back-gate of the second transistor and the output of the first transistor.
16. The sensor of claim 14 ,
the first transistor configured as a source-follower comprising a source coupled to the output of the first transistor,
the second transistor configured as a source-follower comprising a source coupled to an output of the second transistor.
17. The sensor of claim 16 , the MEMS die having a capacitance between approximately 0.1 pF and approximately 5.0 pF and the buffer-amplifier having an input capacitance on the order of 0.1 pF.
18. The sensor of claim 17 further comprising a DC bias circuit having a reference signal output coupled to the input gate of the first transistor by a resistance element, the reference signal output of the DC bias circuit coupled to the input gate of the second transistor by a resistance element.
19. The sensor of claim 18 is a microphone and the filter circuit is an audio band filter.
20. The circuit of claim 19 , the feedforward circuit is a direct electrical connection between the back-gate of the second transistor and the output of the first transistor.