Inverting Headphone Amplifier Startup Noise Suppression Circuit
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Solution Overview
Problem
Audio signal amplifier circuits experience noise issues due to voltage differences between the non-inverting input and output terminals of operational amplifiers during startup, especially when using N-channel MOSFETs, leading to increased circuit area and current consumption with rail-to-rail amplifiers.
Innovation Solution
An audio signal amplifier circuit with an inverting amplifier configuration, including a discharging path and second switch to equalize output and non-inverting input terminal potentials, and a control unit to manage the second input differential pair between active and inactive states, reducing noise and current consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a rail-to-rail amplifier is used to eliminate noise during startup, then noise suppression is improved, but circuit area and current consumption increase
Solution Approach 1:
The patent applies preliminary action by equalizing the potentials at the non-inverting input and output terminals of the operational amplifier before the amplifier is activated. This is achieved through a discharging path with a resistor and switch that pre-charges or discharges the output terminal to match the bias voltage level at the non-inverting input terminal. By performing this potential equalization in advance, the patent eliminates the need for rail-to-rail amplifier architecture, thereby reducing current consumption and circuit area while still preventing noise during startup transitions.
2Object-affected harmful factors
If a rail-to-rail amplifier is used to eliminate noise during startup, then noise suppression is improved, but circuit area increases
Solution Approach 1:
The patent applies preliminary action by equalizing the potentials at the non-inverting input and output terminals of the operational amplifier before the amplifier is activated. This is achieved through a discharging path with a resistor and switch that pre-charges or discharges the output terminal to match the bias voltage level at the non-inverting input terminal. By performing this potential equalization in advance, the patent eliminates the need for rail-to-rail amplifier architecture, thereby reducing current consumption and circuit area while still preventing noise during startup transitions.
3Object-affected harmful factors
If a discharging resistor is arranged in parallel with the electroacoustic transducer to provide a charging/discharging path for the coupling capacitor, then noise suppression is improved, but current consumption increases continuously
Solution Approach 1:
The patent applies dynamics by making the discharging path controllable rather than always active. A switch element is introduced to enable or disable the discharging path based on the operational state of the amplifier. When the amplifier is off, the switch closes to provide the discharging path and equalize potentials. When the amplifier is on, the switch opens to disconnect the discharging path, preventing continuous current consumption through the resistor while maintaining noise suppression capability during state transitions.
Data Source
AI summary
An inverting amplifier which drives headphones via an output capacitor includes: an operational amplifier, an input resistor having a first terminal via which an audio signal to be amplified is received, and a second terminal connected to an inverting input terminal of the operational amplifier; and a feedback resistor having a first terminal connected to an inverting input terminal of the operational amplifier, and a second terminal connected to the output terminal. A reference voltage source generates a bias voltage Vb, and supplies it to the non-inverting input terminal. A discharging path includes a discharging resistor and a first switch arranged in series between an output terminal of the reference voltage source and an fixed voltage terminal. A second switch is arranged between the output terminal of the operational amplifier and a node on the discharging path where the electric potential is higher than it is at the discharging resistor.