MEMS Charge Amplifier Input Stage With Separate DC Control Loop
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Solution Overview
Problem
MEMS interface circuits face instability in their feedback loop due to increasing input current, leading to a high-pass roll-off frequency that is too high, which hinders fast start-up settling of MEMS microphones.
Innovation Solution
The implementation of a separate DC control loop independent of the signal path, featuring a front-end charge amplifier and a second amplifier with high resistance circuit elements and anti-parallel diodes, allows for stable operation and faster settling by decoupling the frequency characteristics from input current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the feedback loop in the amplifier circuit is used to amplify the input signal, then the signal amplification function is achieved, but the feedback loop becomes unstable as input current increases
Solution Approach 1:
The patent divides the amplifier circuit into two separate loops: an AC signal path for amplification and a DC control loop for stability. The AC path handles signal amplification while the DC path manages bias current control, preventing interference between the two functions and resolving the instability issue.
Solution Approach 2:
The patent introduces a separate DC control loop that acts as an intermediary to manage the bias current independently. This DC control loop includes a second amplifier and high resistance elements that regulate the input bias current without affecting the AC signal path, thereby maintaining feedback loop stability.
2Reliability
If the input impedance of the amplifier is increased to achieve high-pass roll-off frequency below audible range, then the frequency response is improved, but the circuit becomes more sensitive to input current variations
Solution Approach 1:
The patent segments the impedance control function from the signal path by placing high resistance elements in the separate DC control loop. This allows the AC signal path to maintain high input impedance for good frequency response while the DC loop independently manages current variations.
Solution Approach 2:
The DC control loop with high resistance elements (R1, R2) and integrator acts as an intermediary that buffers the AC signal path from input current variations. This mediator structure allows the main signal path to maintain high impedance without being directly affected by current changes.
3Speed
If the bias voltage rises quickly from zero to steady-state voltage, then the start-up speed is improved, but the MEMS membrane settling time increases
Solution Approach 1:
The DC control loop is designed to preemptively manage the bias voltage rise during start-up. The integrator and high resistance elements in the DC loop control the rate of voltage change, preventing sudden jumps that would cause membrane oscillation, while still achieving fast settling through proper compensation.
Solution Approach 2:
The DC control loop provides feedback control during the start-up phase, monitoring and regulating the bias voltage to ensure smooth transitions. This feedback mechanism prevents the bias voltage from rising too quickly, thereby reducing membrane settling time while maintaining fast start-up performance.
Data Source
AI summary
An interface circuit comprises a signal path including a front-end charge amplifier coupling an input of the interface circuit to an output of the interface circuit, and a DC control loop separate from the signal path. In some implementations, the interface circuit is part of a MEMS sensor that includes a MEMS transducer having an output coupled to the input of the interface circuit. The interface circuit can, in some cases, allow faster settling of the circuit to its steady-state operating point.