Microphone DC Bias Circuit for Faster LPF Startup Settling
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Solution Overview
Problem
Microphone assemblies with MEMS transducers face audible imperfections such as pops or clicks due to the long settling time of low pass filters (LPFs) during startup, which delays the output of electrical signals to speakers.
Innovation Solution
Incorporating an adjustable resistance in the DC bias circuit of the microphone assembly, specifically using a transistor in parallel with a resistance element, reduces the settling time of the LPF by activating the transistor during startup to lower the resistance, allowing faster stabilization of the DC bias voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a low pass filter (LPF) is used to generate and filter DC bias after the microphone is turned on, then the DC bias is properly filtered, but the settling time is too long resulting in audible imperfections
Solution Approach 1:
The patent applies dynamics by making the resistance value time-dependent through a transistor that transitions from conductive to non-conductive state. During startup, the transistor is conductive providing low resistance for fast settling; after startup, it becomes non-conductive providing high resistance for proper filtering. This dynamic adjustment resolves the contradiction between fast settling and proper filtering.
Solution Approach 2:
The patent uses preliminary action by pre-biasing the transistor to a conductive state during startup before switching it to non-conductive. This preliminary conductive state allows the LPF to settle quickly by providing a low resistance path, and the subsequent switching to non-conductive state ensures proper filtering is maintained afterward.
2Loss of time
If the LPF settling time is reduced to enable faster startup, then audible imperfections are reduced, but the filtering quality may be compromised
Solution Approach 1:
The patent resolves this contradiction through dynamic resistance adjustment using a transistor. During startup phase, the transistor is in conductive state providing low resistance that reduces settling time and enables fast startup. After startup completion, the transistor switches to non-conductive state providing high resistance that ensures proper filtering quality is maintained. This time-varying resistance configuration allows both fast startup and high filtering quality.
Solution Approach 2:
The patent applies preliminary action by initially configuring the transistor to be conductive during startup to achieve fast settling, then subsequently switching it to non-conductive state to maintain filtering quality. This two-stage configuration ensures that the system first prioritizes fast startup, then maintains proper filtering operation.
Data Source
AI summary
The disclosure describes devices and methods for starting up a microphone assembly. The device may be implemented on an integrated circuit that includes a direct current (DC) bias circuit. The DC bias circuit may be coupled to a transducer and configured to supply a DC bias signal to the transducer. The DC bias circuit includes a multi-stage charge pump and a low pass filter (LPF) circuit. The LPF circuit includes an adjustable resistance and a capacitor. A resistance of the adjustable resistance may be reduced to reduce the settling time of the LPF while starting (e.g., turning on) the microphone assembly.