Differential Amplifier Current Control for Symmetric Output Waveforms
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Differential amplifiers used in source drivers of liquid crystal display panels exhibit asymmetrical output waveforms due to differing rising and falling speeds, affecting display quality by causing variations in grayscale representation across different colors despite identical input data.
Innovation Solution
A differential amplifier design incorporating a variable current source that controls the charging and discharging of a parasitic capacitor at the source of the differential pair, ensuring equal current usage for both rising and falling edges, thereby improving the symmetry of the output waveform.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional differential amplifier is used, then the circuit structure is simple, but the output waveform becomes asymmetrical due to different rising and falling speeds
Solution Approach 1:
The patent intentionally introduces asymmetry into the circuit by adding a variable current source that selectively adjusts the current during rising edge versus falling edge operations. This compensates for the inherent asymmetry in the parasitic capacitor charging/discharging behavior, thereby achieving symmetrical output waveforms through controlled asymmetrical current modulation
Solution Approach 2:
The patent dynamically changes the current parameter supplied to the differential pair by incorporating a variable current source. The current magnitude is adjusted based on whether the output is transitioning high or low, allowing optimization of both rising and falling edge speeds to achieve waveform symmetry
2Stability of the object's composition
If the rising and falling speeds are made equal, then the output waveform symmetry is improved, but the circuit complexity increases due to additional current control mechanisms
Solution Approach 1:
The patent introduces a variable current source as an intermediary component between the power supply and the differential pair. This mediator dynamically adjusts the bias current based on the output transition state, enabling precise control over charging and discharging speeds without requiring direct modification of the differential pair transistors themselves
Solution Approach 2:
The patent transforms the static bias current into a dynamic variable current that adapts to the operational state of the amplifier. The current source varies its output based on whether the output node is transitioning high or low, enabling real-time optimization of edge speeds for symmetrical waveforms
Data Source
AI summary
An input stage of a differential amplifier includes a differential pair formed by an N-channel MOS transistor MN1 having a gate connected to an INM and an N-channel MOS transistor MN2 having a gate connected to an INP, both having sources connected to each other, a constant current source connected to the sources of the MN1 and MN2, and a variable current source connected to the sources of the MN1 and MN2. A subsequent-stage processing circuit having an intermediate stage and an output stage includes a phase compensation capacitor and outputs an output responsive to a change in the differential inputs by charging and discharging the phase compensation capacitor through the constant current source. The variable current source turns ON when the change reaches a level causing a parasitic capacitor at the sources of the differential pair to be discharged, and supplies a current for discharging the parasitic capacitor.