Self-Oscillating Amplifier Loop Filter for Low-Frequency Stability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current switching Class D audio amplifiers face challenges such as zero power supply rejection, distortion due to MOSFET parasitics, non-linear output filters, and electromagnetic interference, which complicate achieving robust stability and high-definition audio performance.
Innovation Solution
A self-oscillating amplifier system with a high-order forward filter including a second-order pole pair and zero pair is introduced, along with a DC-servo loop and additional feedback paths to reduce phase turn at low frequencies, increase loop gain, and compensate for demodulation filter poles, thereby enhancing stability and audio performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high-order forward filter including second-order pole pair and zero pair is introduced, then loop gain within audio band is increased and phase turn at low frequencies is reduced, but device complexity increases
Solution Approach 1:
The patent changes the parameters of the forward filter by introducing a high-order filter with specific second-order pole and zero pairs. This modifies the frequency response characteristics to achieve reduced phase turn at low frequencies and increased loop gain within the audio band, directly resolving the stability issue while accepting increased filter complexity as a necessary trade-off.
Solution Approach 2:
The patent employs feedback mechanisms where the high-order forward filter is integrated into the control loop. The filter's output feeds back to influence the switching modulator, creating a closed-loop system that leverages the filter's phase and gain characteristics to improve overall system stability and performance.
2Manufacturing precision
If a DC-servo loop is added to minimize DC-offset and startup pops, then audio performance is improved, but phase turn at low frequencies increases which affects stability
Solution Approach 1:
The patent applies preliminary anti-action by designing the forward filter with specific pole-zero configurations that preemptively counteract the destabilizing phase turn introduced by the DC-servo loop. The filter's characteristics are chosen to compensate for the DC-servo's low-frequency phase effects before they can compromise system stability.
Solution Approach 2:
The patent applies local quality by making different parts of the frequency spectrum have different characteristics. The forward filter provides enhanced phase compensation specifically in the low-frequency region where DC-servo operates, while maintaining appropriate gain and phase characteristics in the audio band, thus addressing the stability issue locally without compromising overall performance.
3Object-affected harmful factors
If passive demodulation filter is used, then EMI is reduced, but distortion from non-linear output impedance increases
Solution Approach 1:
The patent uses feedback to compensate for the non-linear output impedance effects introduced by the passive demodulation filter. The control loop continuously monitors the output and adjusts the switching modulator's control signal to counteract the filter's non-linearities, maintaining better voltage control of the speaker load while retaining the EMI benefits of passive filtering.
Solution Approach 2:
The patent replaces the mechanical/non-linear filtering approach with an active control system. Instead of relying solely on the passive filter's physical characteristics, the system uses electronic control through the switching modulator and control loop to achieve linear output impedance, substituting mechanical filtering with electronic regulation.
Data Source
Figure 1~2
Figure 3~4
Figure 5
AI summary
A self-oscillating amplifier system is disclosed. The system comprises a pulse modulator, a switching power amplification stage and a demodulation filter. Moreover, the system comprises a compensator including a forward filter which is a high order filter including a second order pole pair and a second order zero pair. Hereby it is possible to decrease the phase turn at low frequencies for better stability and increasing the gain of the control loop within the desired bandwidth.