Class-D Amplifier Feedback Switching for DC Offset Calibration
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Solution Overview
Problem
Class-D amplifiers operating in closed-loop mode face issues with DC offset in the feedback path due to analogue components, which can result in distortion and unwanted offset in the output signal, particularly when scaling to smaller process nodes and mixed-signal designs.
Innovation Solution
A class-D amplifier circuit with a forward signal path and two feedback paths, allowing selective operation between two modes: one using a digital feedback signal from the output and another using a digital feedback signal from a digital part of the forward signal path, with a calibration module to reduce DC offset in the feedback path during the second mode, enabling dynamic compensation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If closed-loop operation with negative feedback is used to suppress distortion, then output signal quality is improved, but DC offset from analogue feedback components is introduced
Solution Approach 1:
The feedback path is segmented into multiple independent paths: a first feedback path using analogue components and a second feedback path using digital components. This segmentation allows the system to use the advantages of each path (distortion suppression from analogue feedback, offset-free operation from digital feedback) without being constrained by their respective disadvantages.
Solution Approach 2:
The system dynamically changes the parameter of feedback signal source by selecting between two different feedback paths based on operating conditions. The controller can switch between using the analogue feedback path, the digital feedback path, or a combination thereof, thereby adapting the feedback characteristics to minimize both distortion and DC offset.
2Reliability
If analogue feedback components are used to enable closed-loop operation, then distortion suppression is achieved, but device complexity increases
Solution Approach 1:
The feedback system is designed with multi-functionality where the same amplifier circuit can operate in different modes using different feedback paths. The controller universally manages both analogue and digital feedback paths, allowing the system to adapt its complexity level based on operational requirements - using simpler digital feedback when offset is the concern, or more complex analogue feedback when distortion suppression is the priority.
3Object-generated harmful factors
If digital feedback from output is used to avoid analogue offset, then DC offset is reduced, but closed-loop distortion suppression is lost
Solution Approach 1:
The system merges the benefits of both feedback approaches by combining signals from the analogue feedback path and the digital feedback path. The controller can blend these feedback signals in various proportions, thereby achieving both distortion suppression (from the analogue path) and DC offset reduction (from the digital path) simultaneously in a unified feedback mechanism.
Data Source
AI summary
This application describes an amplifier circuit (200) with a forward signal path with a class-D output stage (102) for generating a driving signal (Sout) based on a digital input signal (Sin). The amplifier has a first feedback path for providing a first digital feedback signal (Sfb1) based on the driving signal and a second feedback path for providing a second digital feedback signal (Sfb2) from a digital part of the forward signal path. The digital input signal (Sin) is combined with a selected feedback signal (Sfbs). The amplifier circuit is selectively operable in a first mode, in which the first feedback signal is used as the selected feedback signal, and in a second mode, in which the second feedback signal is used as the selected feedback signal. A calibration module (204) is operable to calibrate the first feedback path to reduce any DC offset when the amplifier circuit is operating in the second mode.