Triangular Waveform Integrator Feedback for Low Audio Band Noise
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Solution Overview
Problem
Class D audio amplifiers generate PWM output signals with undesirable spectral components and audio band noise due to the reproduction of noise present in the triangular wave oscillator ramp input, which conventional negative feedback techniques cannot effectively mitigate due to low loop gain at high audio frequencies.
Innovation Solution
A triangular waveform generating apparatus comprising a square waveform clock circuit, an active integrator, and an active feedback network that reduces audio band noise content by configuring the feedback network for high gain at audio frequencies and low gain at switching frequencies, maintaining linearity and system stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional negative feedback techniques are used to reduce noise, then noise reduction is attempted, but the loop gain is low at high audio frequencies which limits effectiveness
Solution Approach 1:
The patent applies feedback by adding an active feedback network to the integrator circuit. This feedback network feeds a portion of the integrator output back to the inverting input of the integrator, creating a closed-loop system that reduces audio band noise in the triangular waveform without requiring high loop gain across the entire audio bandwidth.
Solution Approach 2:
The patent changes the noise reduction approach by targeting the triangular waveform generation stage rather than the final amplifier output. By reducing noise at the oscillator ramp input stage through the active feedback network, the system prevents noise propagation through the PWM modulation process, achieving effective noise reduction without needing high loop gain at high frequencies.
2Object-affected harmful factors
If feedback network is added to reduce noise, then audio band noise is reduced, but device complexity increases
Solution Approach 1:
The active feedback network serves multiple functions: it reduces audio band noise in the triangular waveform, maintains DC balance in the integrator circuit, and preserves linearity of the waveform output. By combining these functions into a single circuit block, the patent avoids the complexity that would result from adding separate circuits for each function.
Solution Approach 2:
The active feedback network uses the integrator's own output signal to generate the feedback, creating a self-regulating system. The feedback automatically adjusts to maintain low audio band noise and DC balance without requiring external control circuits or additional components beyond the operational amplifier and associated passive elements.
3Object-affected harmful factors
If feedback network is configured for high gain at audio frequencies, then noise reduction is improved, but linearity may be compromised
Solution Approach 1:
The patent optimizes the feedback network parameters (resistor values, capacitor values) to achieve high gain specifically in the audio band frequency range while maintaining appropriate gain characteristics at higher frequencies. This frequency-selective parameter optimization allows noise reduction in the audio band without introducing nonlinearity that would occur with uniform high gain across all frequencies.
Solution Approach 2:
The active feedback network provides localized high gain specifically where needed (in the audio band frequency range) rather than uniformly across all frequencies. The feedback network's transfer function is designed to have frequency-dependent gain characteristics, applying strong feedback action where noise reduction is most beneficial while maintaining linearity at higher switching frequencies.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution significantly reduces audio band noise in the triangular waveform output, improving the quality of the Class D amplifier's output signal without compromising linearity, and is suitable for use in Class D amplifiers and other applications requiring low noise triangular waveforms.
Implementation Method 1
an active integrator receives input from the square waveform clock circuit and generates a triangular waveform output
Implementation Method 2
An active feedback network is operatively added to the active integrator to reduce the audio band noise content in the triangular waveform output. The feedback network acts as a DC balance without significant sacrifice in the linearity of the triangular waveform output.
Data Source
AI summary
A triangular waveform generator includes a square waveform clock circuit and an active integrator. The active integrator receives input from the square waveform clock circuit and generates a triangular waveform output. An active feedback network is operatively added to the active integrator to reduce the audio band noise content in the triangular waveform output. The feedback network acts as a DC balance without significant sacrifice in the linearity of the triangular waveform output.