Floating Substrate Electret Microphone Filter Circuit
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Solution Overview
Problem
Miniature electret microphones face sensitivity loss due to stray capacitance, which is exacerbated by the small footprint and manufacturing tolerances, leading to increased circuit noise and reduced gain, especially when using traditional grounded substrate designs.
Innovation Solution
Implementing a floating substrate with junction capacitors that utilize the depletion region of a p-n junction to minimize stray capacitance and reduce die area, while incorporating low pass and high pass filters within the integrated circuit to reject noise, thereby reducing system noise and manufacturing complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a grounded substrate design is used in miniature electret microphones, then the circuit structure is simple, but stray capacitance increases causing sensitivity loss and reduced gain
Solution Approach 1:
The substrate is divided into two independent parts: a grounded substrate providing mechanical support and a floating substrate carrying the buffer circuit. This segmentation allows the floating substrate to be electrically isolated from ground, reducing stray capacitance to ground while maintaining mechanical stability through the grounded substrate.
Solution Approach 2:
A dielectric layer is introduced as an intermediary between the grounded substrate and the floating substrate. This dielectric layer provides electrical isolation, preventing direct capacitive coupling to ground while allowing the floating substrate to maintain its position and reduce stray capacitance effects on the buffer input.
2Manufacturing precision
If the gap between back plate and circuit assembly is increased to minimize stray capacitance, then sensitivity improves, but the microphone footprint increases
Solution Approach 1:
By segmenting the substrate into grounded and floating portions, the invention reduces stray capacitance without increasing the gap between the back plate and circuit assembly. The floating substrate configuration inherently reduces the capacitive coupling path, allowing compact dimensions while maintaining low stray capacitance.
Solution Approach 2:
The electrical state of the substrate is changed from grounded to floating, which fundamentally alters the capacitive coupling characteristics. This parameter change reduces stray capacitance to ground without requiring physical dimensional changes, thus maintaining compact microphone footprint while improving sensitivity.
3Manufacturing precision
If floating substrate is used to guard out stray capacitance, then sensitivity loss is reduced, but circuit noise increases due to feedback
Solution Approach 1:
The segmentation of substrate functions separates the mechanical support function (grounded substrate) from the electrical signal handling function (floating substrate). This separation allows the floating substrate to reduce stray capacitance while the grounded substrate provides a stable reference, thereby reducing the feedback noise that would otherwise occur in a fully floating configuration.
Solution Approach 2:
The invention optimizes the electrical potential of the floating substrate by coupling it to the input signal through the buffer circuit. This parameter adjustment minimizes the voltage difference between the floating substrate and the input, thereby reducing capacitive feedback and associated noise while maintaining reduced stray capacitance benefits.
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 approach effectively minimizes stray capacitance, reduces sensitivity loss, and decreases circuit noise, allowing for more efficient use of die area and improved noise rejection in miniature electret microphones.
Implementation Method 1
the stray capacitance presents a capacitive load on the input to ground reducing circuit gain and overall microphone sensitivity
Implementation Method 2
A low pass filter (LPF) can be used to address the susceptibility of the HI to high amplitude, low frequency disturbances
Implementation Method 3
A high pass filter (HPF) can be used to reject higher frequency disturbances such as ultrasonic interference
Implementation Method 4
The physical screen, usually located in the input port of a microphone of the HI device may be used to perform acoustic damping of the microphone's inherent resonance
Data Source
AI summary
A miniature electret microphone includes a housing, a back plate/diaphragm assembly, an integrated circuit and a filter circuit. The housing for the microphone includes a sound inlet port. The back plate/diaphragm assembly is disposed within the housing and the assembly has an output responsive to sound pressure incident on the diaphragm. The integrated circuit includes a floating ground substrate and a p-n junction disposed within the housing coupled to the output. The filter circuit is formed within the integrated circuit, the filter circuit including a capacitor formed by the p-n junction.