Class D Amplifier Feedback Switching for Low-Noise Audio Output
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Solution Overview
Problem
Class D amplifier circuits face limitations in portable devices due to size and power consumption issues, primarily caused by the need for high-performance analogue-to-digital converters (ADCs) to manage noise and distortion, which are inefficient in low-power applications.
Innovation Solution
A Class D amplifier circuit that selectively varies its operation mode based on signal amplitude, using a digital modulator and error block to adjust the contribution of the error signal to the modulator input signal, allowing for open-loop operation at low amplitudes to reduce noise requirements and power consumption, and closed-loop operation at high amplitudes to minimize distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a high-performance continuous-time ADC is used to manage noise and distortion, then signal quality is improved, but size and power consumption increase
Solution Approach 1:
The amplifier dynamically switches between closed-loop and open-loop operating modes based on signal amplitude. At low signal levels, the system operates in open-loop mode with the feedback path disabled, reducing ADC performance requirements and power consumption. At high signal levels, the system transitions to closed-loop mode to minimize distortion. This dynamic adaptation resolves the contradiction by adjusting the system's feedback mechanism according to operating conditions.
Solution Approach 2:
The invention changes the operational parameters of the amplifier by varying the loop gain based on signal amplitude. A variable gain element adjusts the loop gain dynamically, allowing the system to use full feedback at high amplitudes for distortion reduction while reducing or disabling feedback at low amplitudes to lower power consumption and ADC complexity requirements.
2Measurement precision
If feedback is continuously applied to minimize distortion, then signal quality is improved, but power consumption increases
Solution Approach 1:
The feedback mechanism is applied periodically rather than continuously, activated only when the signal amplitude exceeds a threshold level. The signal selection controller monitors the input signal and enables the feedback path (closed-loop mode) only during high-amplitude periods, while disabling it during low-amplitude periods (open-loop mode), thereby reducing average power consumption while maintaining signal quality when needed.
Solution Approach 2:
The feedback path is extracted as a separate controllable component that can be selectively enabled or disabled. By isolating the feedback mechanism from the main signal path and controlling it independently through the signal selection controller, the system can remove the feedback path during low-signal conditions to reduce power consumption while preserving the option to re-engage it when signal quality becomes critical.
3Measurement precision
If a high-performance ADC is used to maintain signal quality at low amplitudes, then noise performance is improved, but device size increases
Solution Approach 1:
The system dynamically adjusts the ADC performance requirements by switching operating modes. In open-loop mode at low signal amplitudes, the ADC can be smaller and lower-performance since it doesn't need to resolve fine noise differences with feedback. In closed-loop mode at high amplitudes, the ADC performance requirements increase but only temporarily when needed for distortion reduction, allowing overall device size reduction.
Data Source
AI summary
This application relates to Class D amplifier circuits. A modulator controls a Class D output stage based on a modulator input signal (Dm) to generate an output signal (Vout) which is representative of an input signal (Din). An error block, which may comprise an ADC, generates an error signal (ε) from the output signal and the input signal. In various embodiments the extent to which the error signal (ε) contributes to the modulator input signal (Dm) is variable based on an indication of the amplitude of the input signal (Din). The error signal may be received at a first input of a signal selector block. The input signal may be received at a second input of the signal selector block. The signal selector block may be operable in first and second modes of operation, wherein in the first mode the modulator input signal is based at least in part on the error signal; and in the second mode the modulator input signal is based on the digital input signal and is independent of the error signal. The error signal can be used to reduce distortion at high signal levels but is not used at low signal levels and so the noise floor at low signal levels does not depend on the component of the error block.