Charge Amplifier Biasing Scheme for MEMS Microphone Dynamic Range
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Solution Overview
Problem
MEMS microphone readout circuits experience distortion due to voltage clipping as the amplitude of acoustic signals increases, limiting the dynamic range and introducing high distortion due to the saturation of output voltage at the supply voltage level.
Innovation Solution
The proposed charge amplifier circuit decouples the biasing voltages of the input and output nodes by incorporating a control circuit and a level shifter, using a pair of level-shift generators and transistors to maintain the output biasing voltage proportional to the supply voltage, thereby preventing clipping and increasing the dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the output biasing voltage is set close to the supply voltage to maximize output swing, then the voltage headroom is increased, but the output voltage clips and distortion increases when acoustic signal amplitude increases
Solution Approach 1:
The patent introduces a new dimension by decoupling the output biasing voltage from the supply voltage through an independent control mechanism. The output biasing voltage is set by a dedicated biasing circuit that can be independently adjusted, allowing the output stage to operate with optimal headroom without being constrained by the supply voltage level, thus preventing clipping distortion while maintaining maximum output swing.
Solution Approach 2:
The patent changes the parameter relationship between output biasing voltage and supply voltage. Instead of fixing the output biasing voltage as a function of supply voltage (Vout_bias = f(Vdd)), the invention allows independent adjustment of the output biasing voltage through a separate biasing circuit, enabling optimization of both output swing and distortion performance simultaneously.
2Reliability
If the feedback resistor and transduction resistor have high values to achieve proper biasing, then the input and output biasing voltages are established, but the output dynamic range is limited due to voltage saturation
Solution Approach 1:
The patent segments the biasing function into two independent parts: a feedback biasing network that establishes stable operating points for the amplification stage, and a separate output biasing circuit that independently sets the output voltage level. This segmentation allows the feedback resistors to maintain their high values for stability while the output biasing circuit provides additional voltage headroom to expand the output dynamic range without saturation.
3Device complexity
If the output biasing voltage is coupled to the supply voltage, then the circuit complexity is reduced, but the Acoustic-Overload Point (AOP) is limited due to voltage saturation at supply level
Solution Approach 1:
The patent introduces an intermediary biasing circuit that mediates between the supply voltage and the output biasing voltage. This intermediary circuit decouples the direct relationship between Vdd and Vout_bias, allowing the output biasing voltage to be optimized for maximum AOP while the supply voltage can be independently selected for power consumption considerations. The intermediary biasing network translates and adjusts the voltage levels appropriately.
Data Source
AI summary
A charge amplifier circuit is provided. The charge amplifier circuit is couplable to a transducer that generates an electrical charge that varies with an external stimulus. The charge amplifier circuit includes an amplification stage having an input node, couplable to the transducer, and an output node. The amplification stage biases the input node at a first direct current (DC) voltage. The charge amplifier circuit includes a feedback circuit, which includes a feedback capacitor, electrically coupled between the input and output nodes of the amplification stage. The feedback circuit includes a resistor electrically coupled to the input node, and a level-shifter circuit, electrically coupled between the resistor and the output node. The level-shifter circuit biases the output node at a second DC voltage and as a function of a difference between the second DC voltage and a reference voltage.