Differential Microphone Circuit High PSRR JFET Bias
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Solution Overview
Problem
Conventional microphone circuits face challenges in achieving high power supply rejection ratio (PSRR) and low noise, often requiring additional components, higher current consumption, or non-grounded connections, which increase costs and complexity.
Innovation Solution
A differential microphone circuit configuration that measures electrical output across the bias resistor, utilizing a junction gate field-effect transistor (JFET) as a current source with a high output impedance, achieving high PSRR and noise immunity by maintaining a constant current through the bias resistor, and integrating a sensing circuit within the portable electronic device interface to reduce offset caused by connection variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional microphone circuits are used to achieve high PSRR and low noise, then additional components, higher current consumption, or non-grounded connections are required, but this increases costs and complexity
Solution Approach 1:
The circuit is divided into a differential signal path and a separate bias current path. The bias resistor is connected to the differential signal lines through a switch, allowing the bias current to be supplied through a dedicated path that does not interfere with the signal processing. This segmentation enables high PSRR without requiring complex non-grounded connections or additional components.
Solution Approach 2:
A switch is introduced to dynamically connect or disconnect the bias resistor from the differential signal lines. This dynamic control allows the circuit to adapt between different operating states, enabling the bias current to be supplied only when needed while maintaining ground references for the signal paths, thus achieving high PSRR without permanent complexity.
2Reliability
If conventional microphone circuits are used to achieve high PSRR and low noise, then additional components are required, but this increases costs
Solution Approach 1:
The bias resistor serves multiple functions: it provides the bias current for the JFET, acts as a sensing element for the differential output, and can be dynamically connected to ground through the switch. This multi-functionality eliminates the need for separate components for each function, reducing component count and manufacturing cost while achieving high noise immunity.
Solution Approach 2:
The bias resistor is configured to serve the dual purpose of biasing the JFET and providing the sensing path for the differential output signal. The circuit uses the same component for multiple purposes, reducing the need for additional dedicated components and lowering overall cost while maintaining performance.
3Reliability
If conventional microphone circuits are used to achieve high PSRR and low noise, then non-grounded connections are required, but this increases complexity
Solution Approach 1:
The circuit separates the bias current path from the signal ground path by using a switch to connect the bias resistor to the differential signal lines only when needed. This segmentation allows the signal lines to remain grounded for simple connections while the bias current is supplied through a controlled path, achieving high PSRR without non-grounded connections.
Solution Approach 2:
The switch provides dynamic control over the bias resistor connection, allowing the circuit to transition between states where the bias resistor is connected or disconnected from the signal lines. This dynamic operation enables simple grounded connections for the signal paths while providing bias current through a controlled mechanism, eliminating the need for complex non-grounded connections.
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
This configuration achieves improved performance with reduced board space and cost, enhanced noise immunity, and lower external interference, while maintaining acoustic sensitivity and immunity to electromagnetic interference.
Implementation Method 1
The JFET operates as a current source with high output impedance. Since the JFET in the normal bias point works as a current source, any voltage variations and noise from the supply voltage will also happen over the JFET. However, the bias resistor will see an almost completely constant current
Implementation Method 2
Electret microphones have been used for almost half a century since their introduction in 1962. The microphone itself has a very high output impedance due to the capacitance of the electret material
Data Source
AI summary
An apparatus for a portable electronic device for receiving a jack of a headset, the jack including a set of lines, the set of lines including at least one audio line, a ground signal and a microphone signal line, the apparatus comprising a set of switches for receiving the ground signal line and the microphone signal line and a sensing circuit for reducing induced noise from the headset, wherein the sensing circuit is located between the set of switches and the microphone signal line and ground signal line.