Capacitive Signal Amplifier Buffering for MEMS Microphone PSRR
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Solution Overview
Problem
MEMS microphones face challenges with high output impedance, leading to signal attenuation and susceptibility to EMI and power supply disturbances due to poor power supply rejection ratio (PSRR) when interfaced with electrical systems.
Innovation Solution
A capacitive signal amplifier system using a gain boosted source follower stage with a high-impedance interface and reduced parasitic gate-drain capacitance, achieved through buffering and in-phase output signal application, allowing for larger transistor sizes without signal degradation and improved noise performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a MEMS microphone is used to reduce manufacturing cost and improve yield, then manufacturing precision and productivity are improved, but the high output impedance causes signal attenuation and poor power supply rejection ratio
Solution Approach 1:
A buffer amplifier stage is introduced as an intermediary between the MEMS microphone and the subsequent circuitry. This buffer has high input impedance to avoid loading the microphone and low output impedance to drive subsequent stages, thereby maintaining signal integrity while allowing the use of cost-effective MEMS microphones
Solution Approach 2:
The circuit parameters are optimized by selecting specific transistor sizes and biasing conditions that maximize the buffer amplifier's input impedance and minimize its output impedance. This parameter optimization ensures that the buffer effectively isolates the high-impedance MEMS microphone from low-impedance subsequent stages
2Reliability
If the preamplifier loading is reduced to avoid signal attenuation, then signal quality is improved, but the high resistance nature of MEMS microphone makes it prone to EMI disturbance
Solution Approach 1:
The buffer amplifier serves as a protective intermediary that provides galvanic isolation between the high-impedance MEMS microphone and the external environment. Its low output impedance stage acts as a shield against EMI while maintaining the high input impedance needed to prevent signal attenuation
3Reliability
If larger transistor sizes are used to improve noise performance, then noise performance is improved, but parasitic gate-drain capacitance increases causing signal degradation
Solution Approach 1:
The amplification function is segmented into multiple stages: a first stage with optimized transistor sizing for low noise, and a second stage that compensates for parasitic effects. This segmentation allows each stage to be optimized for its specific function without compromising overall performance
Solution Approach 2:
Feedback mechanisms are employed to compensate for the effects of parasitic gate-drain capacitance. By using feedback, the circuit can maintain stability and signal integrity even with larger transistor sizes that provide better noise performance
Data Source
AI summary
In accordance with an embodiment, a system for amplifying a signal provided by a capacitive signal source includes a first voltage follower device, a second voltage follower device, and a first capacitor. The first voltage follower device includes an input terminal configured to be coupled to a first terminal of the capacitive signal source, and the second voltage follower device includes an input terminal coupled to the first output terminal of the first voltage follower device, and an output terminal coupled to a second output terminal of the first voltage follower device. Furthermore, first capacitor has a first end coupled to a first output terminal of the first voltage follower device, and a second end configured to be coupled to a second terminal of the capacitive signal source.