Capacitive Source Follower Amplifier for High-Impedance MEMS Microphones
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Solution Overview
Problem
High impedance MEMS microphones face challenges in signal amplification due to high output impedance, which can lead to signal attenuation and susceptibility to EMI and power supply disturbances, making it difficult to achieve high sound quality in cost-sensitive applications.
Innovation Solution
A two-stage amplifier system is introduced, with a first stage featuring a capacitively coupled gain boosted source follower to reduce output impedance and a second stage using a differential amplifier capacitively coupled to the output of the source follower, allowing for a high-impedance interface and improved noise performance across a wide bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a MEMS microphone with high output impedance is used, then manufacturing cost is reduced and yield is improved, but signal attenuation occurs and susceptibility to EMI and power supply disturbances increases
Solution Approach 1:
A capacitively-coupled interface is introduced between the MEMS microphone and the amplifier input. This interface acts as an intermediary that blocks DC paths (reducing EMI and power supply disturbance coupling) while allowing AC audio signals to pass through to the high-impedance amplifier input, thus protecting against signal attenuation and noise while maintaining the benefits of the high-impedance amplifier architecture
2Reliability
If a high-impedance amplifier interface is used with MEMS microphone, then noise performance is improved, but signal attenuation occurs due to loading effects
Solution Approach 1:
The capacitively-coupled interface serves as a mediator that enables the high-impedance amplifier to operate at full benefit for noise performance while preventing the loading effect from causing signal attenuation. The coupling capacitor blocks DC loading while passing AC signals, allowing the amplifier's high input impedance to benefit noise performance without the penalty of signal loss
3Reliability
If a high-impedance interface is used for MEMS microphone, then power supply rejection ratio is improved, but susceptibility to EMI disturbance increases
Solution Approach 1:
The capacitively-coupled interface acts as a frequency-selective mediator that blocks low-frequency EMI and power supply disturbances (which appear as DC or low-frequency signals) while allowing high-frequency audio signals to pass through to the high-impedance amplifier, thus simultaneously improving PSRR and reducing EMI susceptibility
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
The amplifier system effectively reduces signal attenuation, suppresses EMI disturbances, and enhances noise performance, enabling high-quality sound reproduction while maintaining low manufacturing costs.
Implementation Method 1
a first capacitor having a first end coupled to an output terminal of the voltage follower device, and a second end configured to be coupled to a second terminal of the capacitive signal source
Implementation Method 2
The second stage includes a differential amplifier capacitively coupled to the output terminal of the voltage follower device
Data Source
Figure 1~2a
Figure 2b~3
Figure 4~5
AI summary
According to an embodiment, a system for amplifying a signal provided by a capacitive signal source includes a first stage and a second stage. The first stage has a voltage follower device including an input terminal configured to be coupled to a first terminal of the capacitive signal source, and a first capacitor having a first end coupled to an output terminal of the voltage follower device, and a second end configured to be coupled to a second terminal of the capacitive signal source. The second stage includes a differential amplifier capacitively coupled to the output terminal of the voltage follower device.