Differential Microphone Biasing With Common Mode Feedback
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Solution Overview
Problem
Existing differential microphones face challenges in receiving and amplifying signals due to issues with biasing, amplifier gain definition, parasitic capacitances, impedance conversion, and achieving low noise and low cut-off frequency in amplifier response.
Innovation Solution
A differential microphone with a differential amplifier stage and common mode feedback circuit that provides a stable bias voltage and well-defined common mode output voltage, using MOS integrated circuitry and feedback capacitors to improve signal quality and amplification, while minimizing the impact of parasitic capacitances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a large resistance element is used for biasing the capacitor electrodes, then the bias voltage stability is improved, but the cut-off frequency of the amplifier response increases
Solution Approach 1:
The patent introduces a bootstrap capacitor as an intermediary element that couples the output of the differential amplifier back to its input. This capacitor acts as a mediator that provides a low-impedance path for AC signals while allowing the large biasing resistance to remain in place, thus resolving the contradiction between bias stability and cut-off frequency by decoupling the DC biasing function from the AC signal path.
2Object-affected harmful factors
If the microphone uses a differential port configuration, then common mode disturbances are eliminated, but the device complexity increases
Solution Approach 1:
The patent divides the single capacitor into two separate capacitors arranged in a differential configuration, with each capacitor handling one polarity of the signal. This segmentation allows the system to reject common mode disturbances by processing differential signals, while the complexity is managed through the inherent symmetry and simplicity of the differential architecture.
3Measurement precision
If parasitic capacitances at interface nodes are minimized, then measurement precision is improved, but the ease of manufacture decreases
Solution Approach 1:
The patent employs a bootstrap feedback mechanism where the output of the differential amplifier is fed back to the input through a capacitor. This feedback arrangement actively compensates for the effects of parasitic capacitances at the interface nodes, maintaining high signal quality without requiring complex manufacturing processes to minimize these parasitic effects physically.
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 solution enables stable and well-defined bias voltage and common mode output voltage, improving signal quality and amplification, and achieving low noise and low cut-off frequency, thus enhancing the overall performance of differential microphone signals.
Implementation Method 1
The backplate and the membrane establish electrodes of a capacitor. Received sound signals induce oscillations of the membrane. Due to corresponding induced oscillations of the capacity, acoustic signals can be converted into electrical signals.
Data Source
AI summary
A differential microphone with improved biasing and a well defined common mode output voltage is connected to an amplifier that includes a differential amplifier stage and a common mode feedback circuit. The amplifier is in a feedback configuration.