Folded-Cascode Differential Amplifier for Rail-to-Rail LCD Drivers

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Solution Overview

Problem

Conventional differential amplifiers face challenges in achieving high accuracy and rail-to-rail operation while maintaining cost-effectiveness, as they struggle with offset voltage cancellation and input voltage range limitations, particularly in LCD source drivers.

Innovation Solution

A differential amplifier configuration using a folded-cascode load circuit with depletion-type N-channel MOS transistors, which includes a differential pair of depletion-type transistors, current mirror circuits, and constant current sources, allowing for input voltages from negative supply voltages and improved offset cancellation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If conventional differential amplifiers are used in LCD source drivers, then cost reduction is achieved through simplified circuit configurations, but high accuracy is compromised due to offset voltage cancellation issues and limited input voltage range

Engineering Contradiction:

Improveoutput accuracyVSAvoidcircuit configuration complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent changes the key parameter of the differential pair transistors from enhancement-type to depletion-type. This parameter change enables the differential amplifier to accept input voltages from negative supply voltages and achieve rail-to-rail operation, thereby improving output accuracy without requiring complex additional circuitry for offset cancellation

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent segments the input voltage range into multiple regions (negative voltage region, intermediate region, and positive voltage region) that are handled by different transistor pairs. The depletion-type N-channel differential pair handles the negative and intermediate regions, while enhancement-type P-channel differential pair handles the positive region, enabling comprehensive coverage with improved accuracy

Inventive Principle:

Principle #1Segmentation

2Adaptability or versatility

If enhancement-type MOS transistors are used in differential pairs, then manufacturing simplicity is maintained, but input voltage range is limited and cannot achieve rail-to-rail operation

Engineering Contradiction:

Improveinput voltage rangeVSAvoidtransistor fabrication difficulty

Core Design Contradiction:

Adaptability or versatilityVSEase of manufacture

Solution Approach 1:

The patent changes the threshold voltage parameter of the N-channel MOS transistors from positive (enhancement-type) to negative (depletion-type). This enables the transistors to conduct at negative gate-source voltages, extending the input voltage range to include negative supply voltages and achieve rail-to-rail operation

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent employs dynamic switching between different differential pairs based on the input voltage level. The depletion-type N-channel differential pair is activated for negative and intermediate voltages, while the enhancement-type P-channel differential pair is activated for positive voltages, providing adaptive coverage across the entire supply voltage range

Inventive Principle:

Principle #15Dynamics

3Measurement precision

If offset voltage cancellation is implemented in conventional differential amplifiers, then accuracy is improved at intermediate gray-scale levels, but amplitude difference deviation deteriorates near supply voltages

Engineering Contradiction:

Improveoffset voltage cancellation accuracyVSAvoidamplitude difference deviation characteristic

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent segments the operating voltage range into distinct regions handled by different differential pairs. The depletion-type N-channel differential pair specifically handles the negative and intermediate voltage regions where offset cancellation is critical, while the enhancement-type P-channel differential pair handles the positive voltage region, ensuring reliable amplitude difference deviation characteristics near supply voltages

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent converts the typically harmful effect of threshold voltage variations into a beneficial feature by using depletion-type transistors whose negative threshold voltage enables them to operate correctly in the negative voltage region, turning what would be a limitation into an advantage for achieving rail-to-rail operation with improved amplitude difference deviation characteristics

Inventive Principle:

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

Data Source

PatentUS7683714B2Differential amplifier and display device using the same

Publication Date: 2010.03.23 RENESAS ELECTRONICS CORP

AI summary

Disclosed is a differential amplifier which comprises a differential pair comprising depletion-type first and second N-channel MOS transistors, a first current source that supplies a current for the differential pair, a current mirror circuit formed by transistor pairs connected in cascode fashion in two stages, for connecting an output pair of the differential pair in folded connection, second and third current sources connected to an input terminal of the current mirror circuit and an output terminal of the current circuit, respectively, and a buffer amplifier with that has an input terminal connected to the output terminal of the current mirror circuit and has an output terminal connected to an output terminal of the differential amplifier.