Cross-Coupled Transconductor Circuit Without Common-Mode Feedback

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Solution Overview

Problem

Conventional fully differential Gm-C filters require large common-mode feedback circuits to stabilize the common-mode potential, leading to increased power consumption, chip area, and deteriorated noise characteristics due to the complexity of the feedback loop and additional noise-generating components.

Innovation Solution

A fully differential transconductor design that connects the inversion input terminal of one transconductor with the non-inversion input terminal of another, eliminating the need for a common-mode feedback circuit by using a current mirror circuit and variable/constant current sources to stabilize the common-mode operating point, thereby reducing power consumption and improving noise characteristics.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Stability of the object's composition

If a large common-mode feedback circuit is provided to stabilize the common-mode potential in fully differential Gm-C filters, then the common-mode operating point stability is improved, but power consumption and chip area increase

Engineering Contradiction:

Improvecommon-mode operating point stabilityVSAvoidpower consumption

Core Design Contradiction:

Stability of the object's compositionVSUse of energy by stationary object

Solution Approach 1:

The invention extracts and eliminates the common-mode feedback circuit from the fully differential Gm-C filter structure. By reconfiguring the transconductor to use cross-connected inversion and non-inversion input terminals, the patent removes the need for separate common-mode feedback circuitry while maintaining stability through the inherent symmetry of the differential structure.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The invention merges the common-mode stabilization function into the differential signal path itself. By cross-connecting the inversion input terminal of one transconductor with the non-inversion input terminal of another, the differential structure inherently provides common-mode rejection and stabilization without requiring additional feedback components.

Inventive Principle:

Principle #5Merging (Combining)

2Stability of the object's composition

If a large common-mode feedback circuit is provided to stabilize the common-mode potential, then the common-mode operating point stability is improved, but chip area increases

Engineering Contradiction:

Improvecommon-mode operating point stabilityVSAvoidchip area

Core Design Contradiction:

Stability of the object's compositionVSArea of stationary object

Solution Approach 1:

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The differential transconductor structure performs multiple functions simultaneously: it provides voltage-to-current conversion, differential signal amplification, and common-mode stabilization all within a single integrated circuit block, eliminating the need for separate common-mode feedback circuitry.

Inventive Principle:

Principle #6Universality (Multi-functionality)

3Stability of the object's composition

If a common-mode feedback circuit is provided to stabilize the common-mode potential, then the common-mode operating point stability is improved, but noise characteristics deteriorate due to additional noise-generating components

Engineering Contradiction:

Improvecommon-mode operating point stabilityVSAvoidnoise characteristics

Core Design Contradiction:

Stability of the object's compositionVSObject-generated harmful factors

Solution Approach 1:

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The invention converts the potential instability of the common-mode voltage into a benefit by using the differential structure's inherent common-mode rejection ratio (CMRR). The cross-connected configuration ensures that common-mode variations are rejected rather than amplified, turning a potential harm into a beneficial noise-rejection mechanism.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

4Power

If conventional fully differential transconductors with high common-mode gain are used, then voltage-to-current conversion is achieved, but the common-mode potential changes largely with small input changes requiring feedback stabilization

Engineering Contradiction:

Improvevoltage-to-current conversion capabilityVSAvoidcommon-mode potential stability

Core Design Contradiction:

PowerVSStability of the object's composition

Solution Approach 1:

The invention inverts the conventional connection approach by cross-connecting the inversion input terminal of one transconductor with the non-inversion input terminal of another. This inversion of the standard configuration reduces the common-mode gain and stabilizes the common-mode output voltage without requiring additional feedback circuitry.

Inventive Principle:

Principle #13The other way round (Inversion)

Data Source

PatentUS7605645B2Transconductor, integrator, and filter circuit

Publication Date: 2009.10.20 PANASONIC SEMICON SOLUTIONS CO LTD

AI summary

A transconductor for receiving a differential voltage signal and outputting a differential current signal, includes two transconductors for receiving the differential voltage signal and outputting a single-end current signal. An inversion input terminal of one of the two transconductors is connected with a non-inversion input terminal of the other. The transconductor outputs a current signal output from each of the two transconductors as the differential current signal.