Super Source Follower Compensation for MEMS Microphone Stability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
MEMS microphones face challenges with high output impedance, leading to signal attenuation, electromagnetic interference, and self-oscillations due to the interaction between the self-resonance of the MEMS device and negative real part input impedance of high input impedance amplifiers.
Innovation Solution
A super source follower circuit with a compensation capacitor is used to neutralize the negative real input resistance by coupling a compensating signal generated by one super source follower to the input of another, reducing or eliminating self-sustained oscillations and underdamped responses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high input impedance amplifiers are used to interface with MEMS microphones, then input attenuation is reduced, but self-oscillations and underdamped responses occur due to interaction with MEMS self-resonance
Solution Approach 1:
A compensation capacitor is introduced as an intermediary element between the amplifier input and ground. This capacitor compensates for the negative real part of the input impedance by providing a positive real part, thereby eliminating the conditions that lead to self-oscillations while preserving the high input impedance benefit of reduced signal attenuation.
Solution Approach 2:
The input impedance characteristics of the amplifier are modified by adding the compensation capacitor. This changes the overall input impedance parameter to have a positive real part instead of a negative real part, transforming the unstable system into a stable one while maintaining high input impedance operation.
2Reliability
If high input impedance amplifiers are used to interface with MEMS microphones, then input attenuation is reduced, but electromagnetic interference and power supply disturbances affect the high output impedance MEMS microphone
Solution Approach 1:
The compensation capacitor acts as an intermediary that modifies the input impedance characteristics, creating a more stable interface that is less susceptible to electromagnetic interference and power supply disturbances while maintaining the high input impedance needed to prevent signal attenuation.
3Object-affected harmful factors
If multiple membrane MEMS devices are used to provide differential outputs, then EMI and power supply disturbances are reduced, but device complexity increases
Solution Approach 1:
Instead of using multiple MEMS devices to achieve EMI rejection, the patent extracts and addresses the root cause of instability by adding a compensation capacitor to the single-ended amplifier circuit. This eliminates the need for complex differential configurations while still achieving EMI and power supply disturbance rejection through the impedance compensation mechanism.
Data Source
AI summary
In accordance with an embodiment, a circuit includes: a first super source follower; a compensation circuit having a compensating node configured to provide a voltage of opposite phase of a voltage of an internal node of the first super source follower; and a first compensation capacitor coupled between an input of the first super source follower and the compensating node of the compensation circuit.