Microphone Interface Circuit for DC Bias Removal Without AC Coupling

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Solution Overview

Problem

Electret microphones generate a large DC bias voltage that is incompatible with the low voltage supply used in modern signal processing circuits, requiring large external AC coupling capacitors which complicate design and increase costs.

Innovation Solution

An interface circuit that uses a sigma-delta ADC with digital feedback and a DAC to control the current through an input resistor, allowing the removal of DC bias voltage without the need for large AC coupling capacitors, thereby simplifying the design and reducing costs.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If high DC bias voltage is used to bias the electret microphone at its most sensitive operating point, then the microphone sensitivity is improved, but the compatibility with low voltage supply circuits deteriorates

Engineering Contradiction:

Improvemicrophone sensitivityVSAvoidvoltage compatibility

Core Design Contradiction:

Measurement precisionVSAdaptability or versatility

Solution Approach 1:

The patent introduces an intermediary circuit between the electret microphone and the low voltage supply circuit. This circuit includes a capacitor coupled to the microphone output and a control circuit that regulates the DC bias voltage, allowing the microphone to operate at its optimal bias point while being compatible with low voltage supply circuits.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent dynamically adjusts the DC bias voltage parameter to match the requirements of different operating conditions. The control circuit monitors the operating point and modifies the bias voltage level accordingly, enabling the system to maintain optimal sensitivity while adapting to low voltage supply constraints.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If large AC coupling capacitors are used to block DC bias voltage, then the DC bias blocking function is achieved, but the device complexity and cost increase

Engineering Contradiction:

ImproveDC bias blockingVSAvoidcircuit complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent extracts the DC bias blocking function from the traditional large capacitor approach and implements it through an active control circuit. By removing the requirement for large external capacitors and replacing them with an integrated control mechanism, the solution reduces device complexity while maintaining reliable DC bias blocking.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent replaces the passive mechanical capacitor-based DC blocking mechanism with an active electronic control system. The control circuit uses electronic components to dynamically regulate and block DC bias voltage, substituting the bulky capacitor-based approach with a more integrated electronic solution that reduces complexity.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Reliability

If large AC coupling capacitors are used to block DC bias voltage, then the DC bias blocking function is achieved, but the manufacturing cost increases

Engineering Contradiction:

ImproveDC bias blockingVSAvoidmanufacturing cost

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent employs cost-effective electronic components in the control circuit that can be easily manufactured and integrated. By replacing expensive large-value capacitors with standard electronic components and integrated circuits, the solution reduces manufacturing costs while maintaining the DC bias blocking function.

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

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

The interface circuit effectively removes DC bias voltage, preventing overload of the ADC and simplifying the design, while reducing the need for external capacitors and associated costs, making it suitable for a wide range of microphones and other sensors.

Implementation Method 1

The series capacitor 104 required to block the DC voltage 107 needs to be large, generally in the uF range (up to 10 uF)

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

a junction field effect transistor (JFET) 106 integrated into the microphone itself, the JFET 106 being configured as a voltage-controlled current source, which has a very high input resistance

Methodology Applied

Scientific EffectField effect transistor operation:

Data Source

PatentUS12184308B2Interface circuit and method for providing DC bias voltage correction

Publication Date: 2024.12.31 NXP BV

AI summary

An interface circuit includes an analogue to digital converter having an input configured to receive an input signal having an unknown DC bias voltage via an input resistance and provide an output signal to an ADC feedback loop. The ADC feedback loop includes a digital filter arranged to digitally filter the fedback output signal. A digital to analogue converter (DAC) forming a DC feedback loop with the ADC and arranged to convert the digitally filtered fedback output signal to an analogue signal that is provided to the input of the ADC, wherein the analogue signal that is provided to the input of the ADC is arranged to include a DC bias component that is comparable to a DC bias component of a current of the input signal passing through the input resistor.