Audio Op-Amp Input Bias Canceller for Offset and Noise Reduction

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Solution Overview

Problem

Audio operational amplifiers face challenges in canceling out input bias current, which can lead to voltage offsets, pop-and-click noise, and other performance issues, particularly in applications with high common input impedance.

Innovation Solution

The implementation of an input bias cancellation stage that includes an input differential pair, a current mirror, and a bias duplicator transistor, which duplicates and mirrors the input bias current to subtract it from the bases of the transistors, effectively canceling out the input bias current.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If conventional operational amplifier input stages are used, then the circuit is simple, but input bias current causes voltage offsets and noise

Engineering Contradiction:

Improveinput bias current effectsVSAvoidinput stage complexity

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The input stage is segmented into multiple functional blocks: a first differential pair for signal amplification, a second differential pair for bias current sensing, and a current mirror for bias current replication. This segmentation allows independent optimization of signal processing and bias current cancellation functions, effectively reducing input bias current effects while maintaining manageable complexity through modular design.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A bias duplicator transistor is introduced as an intermediary element that replicates the bias current from the first differential pair transistors. This intermediary component enables the bias current to be sensed in one differential pair and applied to cancel the bias current in the other differential pair, effectively eliminating input bias current effects without requiring direct modification of the signal path transistors.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Measurement precision

If bias cancellation circuits are added, then voltage offsets and noise are reduced, but the circuit complexity increases

Engineering Contradiction:

Improvevoltage offset accuracyVSAvoidcircuit structure complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The bias cancellation function is merged with the signal amplification function by using matched differential pairs where the same transistor types and configurations are used for both signal processing and bias current sensing. This merging allows the circuit to perform dual functions with minimal additional components, reducing voltage offsets and noise while limiting the increase in circuit complexity through functional integration.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The bias current is copied from the first differential pair transistors to the second differential pair transistors through a bias duplicator transistor and current mirror. This copying mechanism allows the bias current characteristics to be replicated precisely, enabling accurate bias current cancellation that reduces voltage offsets and noise without requiring complex active cancellation circuits.

Inventive Principle:

Principle #26Copying

3Adaptability or versatility

If high common input impedance is required, then audio preamplification performance improves, but input bias current effects worsen

Engineering Contradiction:

Improvehigh impedance application suitabilityVSAvoidbias current induced voltage offset

Core Design Contradiction:

Adaptability or versatilityVSObject-affected harmful factors

Solution Approach 1:

The circuit applies preliminary anti-action by pre-canceling the input bias current effects before they can cause voltage offsets in high impedance applications. The bias duplicator transistor and current mirror replicate the bias current in advance, and the second differential pair is configured to subtract this replicated bias current from the signal path, preventing bias current induced voltage offsets before they affect the high impedance input nodes.

Inventive Principle:

Principle #9Preliminary anti-action

Solution Approach 2:

The circuit changes the bias current parameter by using different transistor configurations and doping levels in the first and second differential pairs. The first differential pair uses transistors optimized for signal amplification with high input impedance, while the second differential pair uses matched transistors optimized for bias current sensing and cancellation. This parameter differentiation allows high common input impedance to be maintained while effectively canceling bias current effects.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS7649417B1Apparatus and method for input stage and bias canceller for an audio operational amplifier

Publication Date: 2010.01.19 NAT SEMICON CORP

AI summary

An input bias cancellation stage for an audio operational amplifier is provided. The input bias cancellation stage includes an input differential pair, a current mirror, and a bias duplicator transistor that substantially duplicates the input bias current. The bias duplicator transistor receives substantially the same emitter current as the transistors in the input differential pair, and has substantially the same Vce as the transistors in the input differential pair. The current mirror mirrors the duplicated bias current and subtracts it from the bases of the transistors in the input differential pair so that the input bias current is substantially cancelled.