MEMS Microphone Bulk Biasing for HD2 Linearization
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Solution Overview
Problem
Single-ended MEMS microphones suffer from prominent 2nd order harmonic distortion (HD2), particularly at low-mid input sound pressure levels, due to the total equivalent parasitic capacitance at the connection node between the MEMS device and the readout ASIC, which affects system linearity.
Innovation Solution
An additional HD2 component is introduced on the ASIC, out of phase with the MEMS intrinsic HD2 component, and the bulk node of the source follower transistor is biased with a nonzero voltage differential to enhance system linearity, allowing for the selection of optimal bulk voltages using a multiplexer to improve total harmonic distortion (THD) performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional source follower configuration is used with zero voltage differential at the bulk node, then the device complexity is low, but the total harmonic distortion (THD) performance is poor with prominent 2nd order harmonic distortion
Solution Approach 1:
The patent applies parameter changes by adjusting the bulk node voltage from zero to a nonzero value. This changes the operating point of the source follower transistor, which modifies the transconductance characteristics and reduces the 2nd order harmonic distortion. The bulk voltage is tuned to optimize linearity while maintaining the existing circuit topology, thus improving THD performance without significantly increasing device complexity.
Solution Approach 2:
The bulk node voltage acts as an intermediary parameter that mediates between the input signal and output signal in the source follower. By controlling the bulk voltage, the patent introduces a degree of freedom that allows optimization of the transistor's operating characteristics, enabling reduction of harmonic distortion without requiring fundamental changes to the circuit architecture.
2Reliability
If the bulk node voltage is adjusted to improve linearity, then the THD cross-point increases, but additional control circuitry is required
Solution Approach 1:
The patent implements self-service by having the bulk node voltage automatically adjusted based on the operating conditions. The circuit uses the existing transistor's bulk terminal to self-regulate its operating point, eliminating the need for external control circuits. The bulk voltage can be set to a fixed nonzero value or dynamically adjusted using simple biasing networks that leverage the transistor's own characteristics.
Solution Approach 2:
The patent changes the bulk voltage parameter from zero to a nonzero value to improve linearity. This parameter change is achieved through simple biasing techniques rather than complex control circuits. The nonzero bulk voltage shifts the transistor's operating point to reduce harmonic distortion, and this can be implemented with basic voltage division networks or current mirrors that are standard in CMOS design.
Data Source
AI summary
A microphone includes a MEMS device having an output node for generating an analog voltage in response to an input sound wave or vibration; a source follower having a control node coupled to the output node of the MEMS device, a current path coupled between a first controlled node and a second controlled node, and a bulk node, wherein the first controlled node is configured for providing a microphone output voltage, and wherein the second controlled node is coupled to a reference voltage; a current source coupled to the first controlled node; and a voltage differential between the first controlled node and the bulk node, wherein a nonzero value of the voltage differential is configured such that a first 1% total harmonic distortion (THD) cross-point of the microphone output voltage is greater than a second 1% THD cross-point of the microphone output voltage using a zero value voltage differential.