MEMS Microphone Buffer Circuit for Low-Noise, Low-Power Sensing
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Solution Overview
Problem
Capacitive microphones, such as MEMS microphones, face challenges with high power consumption and noise due to the limitations of existing two-stage buffer-amplifier circuits, which affect the signal-to-noise ratio and introduce loading issues.
Innovation Solution
The proposed electrical circuit for capacitive sensor assemblies includes a two-stage buffer-amplifier configuration with a source-follower topology, a band-shaping filter, and a feedforward circuit, along with a DC bias circuit, to reduce noise and improve signal quality by biasing the second stage output to the first stage output, and utilizing a charge pump for DC biasing when necessary.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a two-stage buffer-amplifier circuit is used in capacitive microphones, then impedance matching and frequency band shaping are achieved, but power consumption increases and noise is introduced
Solution Approach 1:
The patent changes the operating parameters of the buffer circuit by implementing a correlated double-sampling technique that samples the sensor output at two different times and subtracts the samples to remove low-frequency noise. This parameter-based approach (temporal sampling and subtraction) reduces noise without requiring additional power-consuming amplification stages, thereby improving signal-to-noise ratio while controlling power consumption
Solution Approach 2:
The patent introduces an intermediary processing stage between the sensor and output that performs correlated double-sampling. This intermediary mechanism processes the signal in the time domain by taking multiple samples and subtracting them, effectively removing low-frequency noise components without requiring high-power amplification, thus resolving the contradiction between noise reduction and power consumption
2Manufacturing precision
If the resistance of the filter is reduced to improve frequency band shaping, then loading issues such as slew rate or gain degradation occur
Solution Approach 1:
The patent replaces the traditional resistive filter mechanism with a digital or active filter implementation that uses capacitive coupling and active amplification stages. Instead of relying on resistive loading to achieve frequency band shaping, the system uses active circuit elements that can provide the same filtering function without the harmful loading effects, thereby maintaining both frequency shaping precision and signal integrity
Solution Approach 2:
The patent segments the filtering function into separate stages: a first stage for impedance matching and a second stage for frequency band shaping. By dividing the buffer circuit into multiple independent stages, each optimized for its specific function, the patent avoids the need for low resistance in a single stage, thereby preventing slew rate and gain degradation while achieving proper frequency band shaping
3Ease of manufacture
If electret microphones are used, then low cost and small size are achieved, but they are being supplanted by capacitive MEMS microphones indicating performance limitations
Solution Approach 1:
The patent optimizes the electrical parameters of the capacitive MEMS microphone by implementing correlated double-sampling and optimized buffer circuitry. These parameter optimizations enable capacitive MEMS microphones to achieve performance levels comparable to or exceeding electret microphones, justifying the transition despite the slightly increased complexity. The parameter changes in the electrical circuit allow capacitive sensors to deliver low noise and high sensitivity while maintaining small size and competitive cost
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration significantly reduces noise across the audio frequency band, improving the signal-to-noise ratio and dynamic range, while minimizing power consumption and avoiding slew rate or gain degradation.
Implementation Method 1
capacitive transduction element or motor (also referred to herein as a 'capacitive sensor') like a MEMS die
Implementation Method 2
a source-follower buffer-amplifier circuit 305, 405, e.g., a two-stage source-follower buffer circuit
Data Source
AI summary
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.