Programmable Gain Amplifier Feedback Switching for Low-Distortion Audio
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Solution Overview
Problem
Audio microphones, particularly MEMS microphones, face challenges in handling high dynamic range audio signals due to amplifier optimization for specific ranges, leading to distortion and issues with high impedance and EMI disturbances, which are exacerbated by poor power supply rejection ratios and loading effects.
Innovation Solution
A programmable gain amplifier with a switchable feedback capacitor system that adjusts gain settings by redistributing charge between capacitors, minimizing DC errors and distortion, and compensating for gain changes in later stages to maintain a constant signal path.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If amplifiers are optimized for a particular dynamic range, then they can handle signals within that range effectively, but they cannot handle the full audio range without adding significant distortion
Solution Approach 1:
The patent implements a programmable gain amplifier with multiple gain settings that can be dynamically adjusted based on the input signal level. The amplifier switches between different gain configurations (e.g., 26dB, 32dB, 38dB, 44dB) to adapt to different dynamic ranges, allowing it to handle both low-level and high-level audio signals without distortion while maintaining optimization for each specific range.
2Power
If a preamplifier is used to interface with high impedance MEMS microphones, then signal amplification is achieved, but loading effects attenuate the microphone's output signal and high resistance makes it prone to EMI disturbance
Solution Approach 1:
The patent employs capacitive feedback in the amplifier design where a feedback capacitor is connected between the output and inverting input of the amplifier. This feedback mechanism creates a virtual ground at the inverting input, minimizing the loading effect on the high-impedance MEMS microphone while maintaining signal amplification. The feedback also improves the amplifier's ability to drive capacitive loads and reduces sensitivity to EMI by maintaining a stable virtual ground potential.
3Adaptability or versatility
If feedback capacitors are switched to change gain settings, then programmable gain is achieved, but perturbations in operating points cause distortion in the signal
Solution Approach 1:
The patent implements a two-stage switching mechanism where switches are activated in a specific sequence. Before the feedback capacitor is switched to change the gain setting, a first switch is activated to prepare the circuit, and then the main feedback switch is activated. This preliminary action allows the circuit to settle into the new gain configuration smoothly, minimizing perturbations in the operating point and reducing signal distortion during gain transitions.
Data Source
AI summary
In accordance with an embodiment, a system includes a programmable gain amplifier having a switchable feedback capacitor coupled in parallel with a first capacitor and a controller. The controller is configured to couple the feedback capacitor between an input node of the programmable gain amplifier and an output node of the programmable gain amplifier in a first gain setting, and switch a first terminal of the feedback capacitor from the output of the programmable gain amplifier to a reference node while a second terminal of the feedback capacitor remains coupled to the input node of the programmable gain amplifier for a first time period when transitioning from the first gain setting to a second gain setting.