Dual-Backplate MEMS Microphone Amplifier With Dynamic Bias Correction
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Solution Overview
Problem
Existing amplifiers for dual backplate MEMS microphones suffer from distortion due to asymmetric current drive capability of source followers, especially when processing signals with large amplitudes and high capacitive loads.
Innovation Solution
The amplifier incorporates a second source follower connected to the second backplate of the MEMS microphone and includes a distortion detection circuit connected to both source followers. This circuit measures distortion and adjusts the bias current of the first source follower through a correctional circuit, ensuring adequate current supply only when needed, thus minimizing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the source follower is required to provide high currents to handle large signal amplitudes and capacitive loads, then the signal distortion is reduced, but the power consumption increases
Solution Approach 1:
The patent implements dynamic bias current adjustment by monitoring the output signal characteristics and automatically modifying the bias current supplied to the source follower. When large signal amplitudes are detected, the bias current is increased to provide sufficient drive capability and reduce distortion. When signals are small, the bias current is reduced to minimize power consumption. This dynamic adaptation resolves the contradiction between maintaining low distortion and reducing power consumption.
Solution Approach 2:
The patent employs a feedback mechanism where the output signal is monitored and used to control the bias current of the source follower. The feedback circuit detects when the source follower is approaching current saturation or exhibiting asymmetric current drive, and automatically adjusts the bias current to maintain optimal operating conditions. This feedback loop ensures that power consumption is minimized while preventing signal distortion under varying signal conditions.
2Reliability
If damping is applied to reduce peak response at resonance frequency, then microphone sensitivity is improved, but noise levels increase
Solution Approach 1:
The patent introduces an intermediary electronic damping mechanism through the controlled adjustment of the source follower's operating point. Rather than applying mechanical or passive electrical damping that directly adds noise, the system uses active current control to achieve damping effect. By dynamically adjusting the bias current, the system achieves the desired damping of resonance peaks while maintaining low noise performance, as the damping is achieved through transistor current modulation rather than resistive or mechanical damping elements.
3Object-generated harmful factors
If a relatively large capacitor is used in the low-pass filter to keep noise low, then noise levels are reduced, but the circuit complexity and power consumption increase
Solution Approach 1:
The patent changes the operating parameters of the source follower dynamically rather than relying on fixed passive filter components. By adjusting the bias current and transconductance of the source follower based on signal conditions, the system achieves effective noise filtering and resonance control without requiring large fixed capacitors. This parameter-based control reduces the need for large passive components and simplifies the overall circuit architecture while maintaining low noise performance.
Data Source
Figure 1
Figure 2A~2B
Figure 3~4
AI summary
An amplifier for a dual backplate MEMS microphone (1) comprising at least a first source follower (6) which has a gate for connection to a first backplate (2) of the dual backplate MEMS microphone (1), and a load circuit(10) connected to a source of the first source follower (6) which provides an output signal of the filter, which filter comprises a second source follower (11) connected to a second backplate (3) of the dual backplate MEMS microphone (1), and a distortion detection circuit(15, 16) which is connected to the source of the first source follower (6) and to the source of the second source follower (11) for measuring distortion of the output signal at the source of the first source follower (6), wherein said distortion detection circuit(15, 16) is connected to a correctional circuit (18) that drives at least a first bias current source (8) for the first source follower (6).