Microphone Amplifier Feedback Charge Compensation for High PSRR
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Solution Overview
Problem
Microphone chips, especially those with MEMS microphones, face challenges in maintaining a high power supply rejection ratio (PSRR) due to parasitic capacitors within the package, leading to audible distortion from supply voltage changes, as the direct coupling of the supply potential to the sensitive input terminal of the amplifier circuit cannot be entirely avoided in compact and cost-optimized designs.
Innovation Solution
An electric amplifier circuit with a differential amplifier and a charge supplying circuit is implemented, featuring a feedback path and variable resistors to set the gain, where the charge supplying circuit compensates for supply potential changes by adjusting the charge in the feedback path, thereby isolating the input terminal from supply potential variations and enhancing PSRR.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a direct coupling from the supply bond pad to the sensitive input terminal is used in compact package design, then cost and compactness are optimized, but the power supply rejection ratio (PSRR) deteriorates due to parasitic capacitor coupling
Solution Approach 1:
A decoupling capacitor is introduced as an intermediary element between the supply bond pad and the sensitive input terminal. This capacitor acts as a mediator that blocks the harmful coupling path while allowing the necessary signal transmission, thereby maintaining both compact package design and high PSRR performance
Solution Approach 2:
The harmful coupling effect of the parasitic capacitor is extracted and compensated for by introducing a separate decoupling capacitor with specifically designed characteristics that counteract the parasitic effects, separating the harmful coupling from the useful signal path
2Ease of manufacture
If the microphone capacitor is kept small (a few pico Farad) for cost optimization, then manufacturing cost is reduced, but the PSRR is limited due to increased susceptibility to supply voltage changes
Solution Approach 1:
A decoupling capacitor is introduced as an intermediary element between the supply bond pad and the sensitive input terminal. This capacitor acts as a mediator that blocks the harmful coupling path while allowing the necessary signal transmission, thereby maintaining both compact package design and high PSRR performance
Solution Approach 2:
The capacitance value of the decoupling capacitor is specifically optimized to counteract the small microphone capacitor value. By carefully selecting the decoupling capacitor parameters, the system achieves high PSRR despite using a small, cost-optimized microphone capacitor
Data Source
Figure 1A~1B
Figure 2
Figure 3
AI summary
An electric amplifier circuit for amplifying an output signal of a microphone comprises a supply input terminal (V10) to apply a supply potential (VDDA) for operating the electric amplifier circuit and a differential amplifier (110) having a first input terminal (E110a) for applying the output signal of the microphone (20), a second input terminal (E110b) and an output terminal (A110) for outputting an amplified output signal (OUT) of the microphone (20). A feedback path (FP) is provided between the output terminal (A110) of the differential amplifier (110) and the second input terminal (E110b) of the differential amplifier (110). A charge supplying circuit (120) is coupled to the feedback path (FP) to supply an amount of the charge to the feedback path (FP) in dependence on the supply potential (VDDA). The amount of charge supplied to the feedback path may be dependent on a change of the supply potential (VDDA).