Super Source Follower Compensation for Stable MEMS Inputs
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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, which degrade their performance.
Innovation Solution
A super source follower circuit with a compensation capacitor is used to neutralize the negative real input resistance, reducing self-oscillations and improving signal integrity by coupling a compensating signal generated by one super source follower to the input of another, thereby stabilizing the input impedance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high input impedance amplifiers are used to reduce input attenuation, then signal 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 high input impedance amplifier and the MEMS device. This capacitor acts as a mediator that filters out the problematic self-resonance frequencies of the MEMS device, preventing them from interacting with the amplifier's negative real part input impedance. The result is that the amplifier maintains its high input impedance for signal amplification while the compensation capacitor blocks the oscillatory interactions, thus resolving the contradiction between signal integrity and system stability.
Solution Approach 2:
The patent employs a feedback mechanism where the output of the amplifier is fed back through the compensation capacitor to the input. This feedback loop is designed to provide phase compensation that counteracts the underdamped responses and self-oscillations. By carefully selecting the compensation capacitor value, the feedback signal cancels out the oscillatory components, allowing the system to maintain both high input impedance and stability simultaneously.
2Object-affected harmful factors
If multiple membrane MEMS devices providing differential outputs are used to reduce EMI and power supply disturbances, then electromagnetic interference resistance is improved, but device complexity increases
Solution Approach 1:
The single-ended amplifier circuit with compensation capacitor is designed to perform multiple functions: it provides high input impedance for signal amplification, filters MEMS self-resonance, and can be configured in differential pairs for EMI rejection. By making this basic circuit topology multi-functional, the patent avoids the need for completely separate circuits for each function, thereby reducing overall complexity while maintaining EMI resistance through differential configuration when needed.
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.