Differential Preamplifier Circuit for MEMS Microphone Noise Rejection
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Solution Overview
Problem
The existing preamplifier circuits for MEMS capacitive microphones suffer from noise issues, where the noise from the buffer stage is added to the useful signal, disturbances on the biasing voltage are treated as useful signal components, and supply voltage noise affects the output signal, leading to poor signal-to-noise ratios.
Innovation Solution
A fully differential preamplifier circuit with a high input impedance is introduced, featuring a differential amplifier stage that amplifies the useful signal while eliminating noise from the biasing voltage and supply voltage, using a symmetrical configuration with a dummy capacitor and high-value resistive biasing elements to reject common-mode disturbances, and a bandgap reference generator for stable biasing current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a traditional preamplifier circuit with buffer stage is used, then the circuit can provide biasing for the capacitive transducer, but noise from the buffer stage is added to the useful signal, degrading the signal-to-noise ratio
Solution Approach 1:
The patent extracts and removes the buffer stage from the traditional preamplifier circuit. By eliminating the buffer stage that was previously necessary for biasing, the invention directly connects the capacitive transducer to the differential amplifier input, thereby removing the source of buffer stage noise while maintaining proper biasing through the differential amplifier's input structure.
Solution Approach 2:
The differential amplifier serves as an intermediary that directly interfaces with the capacitive transducer. Instead of using a buffer stage as the intermediary, the differential amplifier's high-impedance input structure acts as the mediating element, providing both noise-free signal acquisition and proper biasing functionality in a single stage.
2Reliability
If high biasing voltage is applied to the capacitive transducer, then sufficient performance can be achieved, but disturbances on the biasing voltage are treated as useful signal components, increasing noise
Solution Approach 1:
The patent employs asymmetric biasing where the non-inverting input of the differential amplifier is connected to a stable reference voltage while the inverting input receives the capacitive transducer signal. This asymmetric configuration allows the circuit to maintain high biasing voltage for performance while the differential structure rejects common-mode disturbances on the biasing voltage, preventing them from being treated as useful signal components.
Solution Approach 2:
The differential amplifier provides feedback through its inherent differential structure, comparing the transducer signal against a stable reference voltage. This feedback mechanism ensures that disturbances on the biasing voltage are rejected rather than amplified, maintaining transducer performance while eliminating noise from biasing voltage variations.
3Use of energy by moving object
If supply voltage is reduced for portable applications, then power consumption decreases, but supply voltage noise affects the output signal, degrading signal quality
Solution Approach 1:
The differential amplifier employs feedback through its differential input structure, comparing signals against a stable reference. This feedback mechanism provides supply voltage rejection, ensuring that noise from the reduced supply voltage in portable applications does not affect the output signal quality, while still maintaining low power consumption.
Solution Approach 2:
The patent creates equipotential conditions by connecting both inputs of the differential amplifier to stable reference voltages during periods when no signal is present. This equipotential configuration ensures that supply voltage noise is rejected, allowing the circuit to operate from reduced supply voltages in portable applications while maintaining signal quality.
4Object-generated harmful factors
If a fully differential preamplifier circuit is used to reject noise, then power-supply rejection ratio improves, but circuit complexity increases
Solution Approach 1:
The patent merges the biasing function and noise rejection function into a single differential amplifier stage. By combining these functions, the circuit achieves high power-supply rejection ratio without requiring separate buffer stages or additional biasing circuitry, thereby reducing overall circuit complexity while maintaining excellent noise rejection performance.
Data Source
AI summary
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.