Super Source Follower Compensation for MEMS Input Stability
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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 MEMS device and high input impedance amplifiers.
Innovation Solution
A circuit with a super source follower and a compensation capacitor is used to neutralize the negative real input resistance, reducing self-oscillations by providing a voltage of opposite phase to the internal node of the super source follower, and coupling capacitors between the inputs and internal nodes of the super source followers to stabilize the input impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high input impedance amplifier is used to reduce input attenuation, then the input signal attenuation is reduced, but self-oscillations and underdamped responses occur due to the interaction with MEMS device resonance
Solution Approach 1:
A compensation capacitor is introduced as an intermediary element between the amplifier input and the MEMS device. This capacitor compensates for the negative real part of the input impedance by providing a positive real impedance component, thereby eliminating self-oscillations while preserving the high input impedance benefit of reduced signal attenuation.
Solution Approach 2:
The input impedance characteristics are modified by adding the compensation capacitor. The capacitor changes the frequency-dependent impedance parameters of the amplifier-MEMS interface, specifically counteracting the negative resistance effect that causes oscillations while maintaining the overall high input impedance regime.
2Volume of moving object
If the MEMS device is used to reduce size, then the device size is reduced, but the high output impedance causes susceptibility to electromagnetic interference and power supply disturbances
Solution Approach 1:
The compensation capacitor creates a feedback mechanism that actively counteracts the effects of electromagnetic interference and power supply disturbances on the high-impedance MEMS output. By compensating for the negative real input impedance, the system stabilizes the interface and reduces susceptibility to external harmful factors.
Data Source
Figure 1~2A
Figure 2B~2C
Figure 2D
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.