Rail-to-Rail Operational Amplifier With Offset Scattering for LCD Drivers
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Solution Overview
Problem
Conventional operational amplifiers used in LCD drivers face challenges in minimizing offset voltage differences between positive and negative sides, leading to amplitude difference deviation and vertical stripes in LCD displays, especially when dealing with 2H inversion driving methods where current discharging and sucking capabilities are insufficient.
Innovation Solution
The operational amplifier design includes a first and second differential pair with corresponding active loads, bias circuits, and switches that allow for offset voltage cancellation by spatially scattering the offset voltage through controlled switching functions, enabling operation across a wide input voltage range from VSS to VDD and constituting a Rail-to-Rail amplifier.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional operational amplifiers are used with single differential pair configuration, then the circuit structure is simple, but the offset voltage difference between positive and negative sides cannot be minimized
Solution Approach 1:
The operational amplifier is divided into two independent differential pairs (first and second differential pairs) with separate active loads and bias circuits. Each differential pair handles one polarity (positive or negative), allowing independent optimization and offset cancellation for each side, thereby minimizing the offset voltage difference between positive and negative outputs.
Solution Approach 2:
Switches are introduced to dynamically select between different differential pair configurations and connect them to appropriate output terminals. The switching mechanism enables the amplifier to adaptively cancel offset voltages by selecting the appropriate differential pair configuration based on the required output polarity, thereby minimizing offset voltage differences.
2Use of energy by moving object
If conventional operational amplifiers are used, then power consumption is reduced, but current discharging and sucking capabilities are insufficient for 2H inversion driving
Solution Approach 1:
The operational amplifier is designed with dual differential pairs and switching mechanisms that enable it to perform both current sourcing and current sinking functions effectively. The first and second differential pairs, along with their respective active loads and bias circuits, provide balanced current capabilities in both directions, making the amplifier suitable for 2H inversion driving methods that require symmetric current discharge and suck capabilities.
3Measurement precision
If offset voltage cancellation is implemented through switching functions, then amplitude difference deviation is improved, but the circuit complexity increases
Solution Approach 1:
The offset cancellation function is merged into the main signal path by integrating switches directly within the differential pair configurations and active load connections. Rather than adding separate offset cancellation circuits, the switches are combined with the signal processing elements, allowing offset cancellation to occur naturally during normal operation without requiring additional dedicated cancellation circuitry.
Data Source
AI summary
To reduce the apparent effect of offset voltage by making the offset voltage spatially scattered. An operational amplifier 1 includes: a first differential pair having N-channel MOS transistors MN1 and MN2; a second differential pair having P-channel MOS transistors MP1 and MP2; an output stage having output transistors NP3 and MN3; first and second active loads 11 and 12 having a switch, adapted to convert differential outputs from the first and second differential pairs to a single output, respectively; first and second bias circuits 13 and 14 having a switch, adapted to select one of two outputs of each of the first and second active loads 11 and 12 having the switch and to determine idling current of the MP3 and MN3, respectively; first and second switches SW1 and SW2 adapted to respectively connect an output terminal and an input terminal to one of the gates of each of the first and second differential pairs; and a terminal 17 adapted to receive an offset cancellation signal for controlling switching functions and the switches in linkage with each other.