Electret Microphone Circuit Source Follower Impedance Matching
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Solution Overview
Problem
Conventional electret microphone circuits suffer from signal distortion and reduced audio signal levels due to parasitic capacitances and resistances in FETs, which are exacerbated by the Miller effect and high apparent input capacitance, leading to degradation of audio signals.
Innovation Solution
The proposed electret microphone circuit employs a floating voltage source and a voltage follower with a coupling capacitor and resistor to maintain constant relative voltage values across the FET, minimizing parasitic component involvement and achieving a high dynamic load resistance, thus reducing distortion and maintaining signal integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional FET preamplifier circuit is used to transform high impedance to low impedance, then impedance transformation is achieved, but signal distortion increases and audio signal levels are reduced due to parasitic capacitances and the Miller effect
Solution Approach 1:
The patent extracts and removes the problematic Miller effect feedback path by using a source follower configuration instead of a common-source amplifier. The source follower topology eliminates the voltage gain that causes Miller multiplication of gate-drain capacitance, thereby extracting the harmful capacitance effect from the signal path and achieving low distortion performance.
Solution Approach 2:
The patent changes the operating parameters of the FET by biasing it in the source follower configuration with a high value gate resistor (greater than 1 Gigaohm). This parameter change optimizes the trade-off between input impedance, noise performance, and distortion by maintaining the FET in a specific operating region where parasitic effects are minimized.
2Reliability
If a high value gate resistor is used to bias the FET gate, then input impedance is maintained high, but the circuit becomes more sensitive to noise and signal levels are reduced
Solution Approach 1:
The source follower configuration provides self-service by inherently providing impedance transformation without requiring additional active amplification stages. The FET's transconductance naturally provides the impedance transformation from high gate impedance to low source impedance, eliminating the need for external feedback networks that would introduce additional noise and signal loss.
3Power
If the FET is operated with varying voltage between source and gate to amplify signals, then signal amplification is achieved, but distortion increases due to nonlinear variation of parasitic gate-source capacitance
Solution Approach 1:
Instead of using the conventional common-source amplifier configuration where the gate voltage varies to provide amplification, the patent inverts the approach by using source voltage variation in a source follower configuration. This inversion of the amplification mechanism eliminates the Miller effect and reduces the impact of nonlinear capacitance variations, achieving low distortion while maintaining signal levels.
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 effectively transforms the high impedance of the electret microphone to a low impedance without attenuation or distortion, significantly improving the dynamic range and reducing total harmonic distortion, as demonstrated in the improved circuit of FIG. 4.
Implementation Method 1
The diaphragm moves in response to incident acoustic waves, thus modulating the capacitance of the parallel plate capacitor
Implementation Method 2
A polarizing voltage must be applied via a high value load resistor to charge or polarize the parallel plate capacitor
Implementation Method 3
An electret microphone is a variation of an electrostatic microphone in which at least one of the fixed plate and the diaphragm include a permanently charged dielectric layer
Data Source
AI summary
There is disclosed a microphone, a circuit, and a method. A microphone capsule may include an electret microphone and a field effect transistor (FET). A floating DC voltage source may have a first end connected to a drain terminal of the electret microphone capsule and a second end. A load resistor may be connected between the second end of the floating DC voltage source and a source terminal of the electret microphone capsule. A voltage follower may have an output connected to the source terminal of the electret microphone capsule and the first end of the floating DC voltage source. A coupling capacitor may couple an audio signal from the source terminal of the electret microphone capsule to an input of the voltage follower.