Audio Amplifier Path Ramp Control for POP Noise Suppression
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Solution Overview
Problem
Existing audio processing circuits generate POP noise during operational amplifier path establishment due to offset voltages, which causes discomfort to human ears due to sudden voltage changes.
Innovation Solution
An audio processing circuit is designed with a secondary operational amplifier and a variable-resistor circuit that gradually changes output resistance to control voltage at the output node, preventing sudden voltage changes and reducing POP noise. The circuit includes a ramp signal controller to manage resistance changes, ensuring a smooth path establishment by cutting off or connecting the path between the operational amplifier circuit and the output node based on offset voltage polarity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the operational amplifier path is established quickly, then the circuit responds faster, but POP noise is generated due to sudden voltage changes
Solution Approach 1:
The patent applies preliminary action by establishing the operational amplifier working point before establishing the operational amplifier path. The working point establishment procedure sets the output voltage to a predetermined value (e.g., ground potential or common-mode voltage) before the path establishment begins. This preliminary preparation ensures that when the path is subsequently established, no sudden voltage transitions occur, thereby preventing POP noise while maintaining efficient circuit response.
2Loss of time
If the output resistance changes suddenly, then the circuit establishes the path faster, but the voltage at the output node changes abruptly causing POP noise
Solution Approach 1:
The patent applies dynamics by making the output resistance of the second operational amplifier circuit changeable over time during the path establishment procedure. The output resistance is configured to gradually decrease from a high initial value to a low final value, rather than switching abruptly. This dynamic adjustment allows the voltage at the output node to transition smoothly, preventing POP noise while completing the path establishment within an acceptable time frame.
3Power
If the operational amplifier circuit directly drives the output node, then the driving capability is strong, but offset voltage causes sudden voltage changes and POP noise
Solution Approach 1:
The patent applies preliminary action by establishing the working point of the operational amplifier circuit before connecting it to drive the output node. The working point establishment procedure ensures that the output voltage is already at the correct predetermined value (such as ground or common-mode voltage) before the driving connection is made. This preliminary preparation eliminates sudden voltage changes caused by offset voltages, preventing POP noise while maintaining the strong driving capability of the operational amplifier circuit.
Data Source
Figure 1~2
Figure 3~4a
Figure 4b~5
AI summary
An audio processing circuit and a terminal device are provided. The circuit includes a cascade operational amplifier circuit, an output node, and a pull-down circuit. The cascade operational amplifier circuit is configured to amplify an input voltage, and output the amplified voltage to the output node. The cascade operational amplifier circuit includes a first operational amplifier circuit and a second operational amplifier circuit. The first operational amplifier circuit is a front-stage operational amplifier of the second operational amplifier circuit. The first operational amplifier circuit includes a main operational amplifier and a secondary operational amplifier that are connected in parallel. One end of the pull-down circuit is coupled to the output node, and the other end of the pull-down circuit is grounded or connected to a common mode voltage module. The pull-down circuit is configured to pull down a voltage at the output node after the first operational amplifier circuit is turned on. The second operational amplifier circuit is configured to: after the secondary operational amplifier is turned on, control a voltage gain of the secondary operational amplifier to change gradually from low to high, so as to control the voltage at the output node to change gradually from low to high with gradual changing of output resistance. With the circuit provided in this solution, POP noise generated in an operational amplifier path establishment procedure can be effectively suppressed.