WE CLAIM:
1. A preamplifier circuit 400 for a Micro Electro Mechanical Systems (MEMS) capacitive
sensor 405, the preamplifier circuit 400 comprising:
a first N-MOSFET 420 connected in common source configuration, for amplifying an output signal of the MEMS capacitive sensor 405;
a first P-MOSFET 455;
a negative feedback loop for gain control, comprising a first capacitor 417 and a second capacitor 419, wherein the first capacitor 417 is connected between a source terminal of the first P-MOSFET 455 and a bias node of the MEMS capacitive sensor 405, and the second capacitor 419 is connected between the bias node of the MEMS capacitive sensor 405 and a source terminal of the first N-MOSFET 420, to set the gain of the preamplifier circuit 400 greater than one; and
a first current source 440-1 connected to a drain terminal of the first N-MOSFET 420.
2. The preamplifier circuit 400 as claimed in claim 1, further comprising a gate voltage
compensation circuit for eliminating dependence of a voltage at the drain terminal of the first
N-MOSFET 420 on temperature and process gradients across the silicon die of the first N-
MOSFET 420, and for keeping the first N-MOSFET 420 in saturation, the gate voltage
compensation circuit comprising:
a plurality of second current sources 440-2, 440-4, 440-6 and current sinks 440-3, 440-5, 440-7 for establishing a current flow through the first N-MOSFET 420;
a second N-MOSFET 425 designed to carry a current proportional to the current carried by the first N-MOSFET 420, wherein the second N-MOSFET 425 is a scaled-down version of the first N-MOSFET 420;
a first resistor 430 and a second resistor 435 for establishing the voltage at the drain terminal of the first N-MOSFET 420;
a first biasing resistor 445 connected between the second resistor 435 and agate terminal of the first N-MOSFET 420, wherein a resistance of the first biasing resistor 445 is high compared to a resistance of the second resistor 435, wherein the first biasing resistor 445 and a total capacitance at the gate terminal of the first N-MOSFET 420 act as a low pass filter having a cutoff frequency low enough to avoid interferences of the gate voltage compensation circuit in the audio frequency range.
18

3. The preamplifier circuit 400 as claimed in claim 1, further comprising an output buffer
for buffering the voltage at the drain terminal of the first N-MOSFET 420, wherein the output
buffer comprises:
a second P-MOSFET 460, wherein a drain terminal and a gate terminal of the second P-MOSFET 460 are shorted;
a third N-MOSFET 450 connected in source follower configuration, wherein a source terminal of the third N-MOSFET 450 is connected to the source terminal of the first P-MOSFET 455 and a source terminal of the second P-MOSFET 460;
a second biasing resistor 475 connected between a drain terminal and a gate terminal of the third N-MOSFET 450;
a third biasing resistor 480 connected between a gate terminal of the first P-MOSFET 455 and the gate terminal of the second P-MOSFET 460, for biasing the first P-MOSFET 455;
a current sink 485 connected from the gate terminal of the second P-MOSFET 460 to the source terminal of the first N-MOSFET 420; and
a first capacitor 465 connected between the drain terminal of first N-MOSFET 420 and the gate terminal of the third N-MOSFET 450, for driving the third N-MOSFET 450, and a second capacitor 470 connected between the drain terminal of first N-MOSFET 420 and the gate terminal of the first P-MOSFET 455, for driving the first P-MOSFET 455, wherein the first P-MOSFET 455 and the third N-MOSFET 450 together act as a push-pull buffer, and wherein output of the preamplifier circuit is measured at the source terminal of the first P-MOSFET 455.
4. The preamplifier circuit 400 as claimed in claim 1, wherein the first current source 440-1 and the plurality of second current sources 440-2, 440-4, 440-6 and current sinks 440-3, 440-5, 440-7 generate constant currents and sink constant currents respectively.
5. The preamplifier circuit 400 as claimed in claim 1, wherein the first capacitor 417 is configured between an output node of the preamplifier circuit 400 and the gate terminal of the first N-MOSFET 420 for a gain lesser than or greater than one.
6. A preamplifier circuit 400 for a Micro Electro Mechanical Systems (MEMS) capacitive sensor 405, the preamplifier circuit 400 comprising:
19

a first N-MOSFET 420 connected in common source configuration, for amplifying an output signal of the MEMS capacitive sensor 405; a first P-MOSFET 455;
a first capacitor 417 and a second capacitor 419, wherein the first capacitor 417 is connected between a source terminal of the first P-MOSFET 455 and a bias node of the MEMS capacitive sensor 405, and the second capacitor 419 is connected between the bias node of the MEMS capacitive sensor 405 and a source terminal of the first N-MOSFET 420, wherein the first capacitor 417 and the second capacitor 419 are configured to set the gain of the preamplifier circuit 400 greater than one;
a first current source 440-1 connected to the drain terminal of the first N-MOSFET 420; and
a gate voltage compensation circuit for eliminating dependence of a voltage at the drain terminal of the first N-MOSFET 420 on temperature and process gradients across the silicon die of the first N-MOSFET 420 and for keeping the first N-MOSFET 420 in saturation, the gate voltage compensation circuit comprising:
a plurality of second current sources 440-2, 440-4, 440-6 and current sinks 440-3, 440-5, 440-7 for establishing a current flow through the first N-MOSFET 420;
a second N-MOSFET 425 designed to carry a current proportional to the current carried by the first N-MOSFET 420, wherein the second N-MOSFET 425 is a scaled-down version of the first N-MOSFET 420;
a first resistor 430 and a second resistor 435 for establishing the voltage at the drain terminal of the first N-MOSFET 420; and
a first biasing resistor 445 connected between the second resistor 435 and a gate terminal of first N-MOSFET 420, wherein the resistance of the first biasing resistor 445 is high compared to the second resistor 435, wherein the first biasing resistor 445 and total capacitance at the gate terminal of the first N-MOSFET 420 act as a low pass filter, having a cutoff frequency low enough to avoid interferences of gate voltage compensation circuit in the audio frequency range; and an output buffer for buffering the voltage at the drain terminal of the first N-MOSFET 420.
7. The preamplifier circuit 400 as claimed in claim 6, wherein the output buffer comprises: a second P-MOSFET 460, wherein a drain terminal and a gate terminal of the second P-MOSFET 460 are shorted;
20

a third N-MOSFET 450 connected in source follower configuration, wherein source terminal of the third N-MOSFET 450 is connected to the source terminal of the first P-MOSFET 455 and a source terminal of the second P-MOSFET 460, and wherein a drain terminal and a gate terminal of the second P-MOSFET 460 are shorted;
a second biasing resistor 475 connected between a drain terminal and a gate terminal of the third N-MOSFET 450;
a third biasing resistor 480 connected between a gate terminal of the first P-MOSFET 455 and the gate terminal of the second P-MOSFET 460, for biasing the first P-MOSFET 455;
a current sink 485 connected from the gate terminal of the second P-MOSFET 460 to the source terminal of the first N-MOSFET 420; and
a first capacitor 465 connected between a drain terminal of first N-MOSFET 420 and a gate terminal of the third N-MOSFET 450, for driving the third N-MOSFET 450, and a second capacitor 470 connected between the drain terminal of first N-MOSFET 420 and the gate terminal of the first P-MOSFET 455, for driving the first P-MOSFET 455, wherein the first P-MOSFET 455 and the third N-MOSFET 450 together act as a push-pull buffer, and wherein output of the preamplifier circuit is measured at the source terminal of the first P-MOSFET 455.
8. The preamplifier circuit 400 as claimed in claim 6, wherein the first current source and the plurality of second current sources 440-2, 440-4, 440-6 and current sinks 440-3, 440-5, 440-7 generate constant currents and sinks constant currents respectively.
9. The preamplifier circuit 400 as claimed in claim 6, wherein the first capacitor 417 is configured between an output node of the preamplifier circuit 400 and the gate terminal of the first N-MOSFET 420 for a gain lesser than or greater than one.