Video Signal Output Circuit Offset Compensation

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Solution Overview

Problem

Conventional video signal output circuits face limitations in raising the maximum output level of video signals due to offset voltages from base currents in differential amplifiers, which reduces the output level further when multiple low-pass filters are connected in series, and require large circuit scales, increasing costs.

Innovation Solution

A video signal output circuit incorporating a sync-tip clamp circuit, a low-pass filter, and a dummy circuit that maintains the sync-tip level at a constant voltage, using resistance and capacitive elements, with feedback control to stabilize the output impedance and reduce circuit complexity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a differential amplifier is used in the low-pass filter to maintain sync-tip level, then the sync-tip level can be maintained, but offset voltage from base current difference reduces the maximum output level

Engineering Contradiction:

Improvesync-tip level stabilityVSAvoidmaximum output level

Core Design Contradiction:

ReliabilityVSPower

Solution Approach 1:

The patent introduces a dummy circuit that copies the configuration of the low-pass filter (including the differential amplifier) but without the capacitive elements. This dummy circuit generates a dummy video signal that has the same offset voltage characteristics as the actual video signal passing through the low-pass filter. By comparing the dummy signal with a reference level and feeding back to adjust the sync-tip clamp voltage, the system compensates for the offset voltage without requiring the differential amplifier to perfectly maintain the sync-tip level, thereby recovering the maximum output level while maintaining sync-tip stability.

Inventive Principle:

Principle #26Copying

2Object-affected harmful factors

If multiple low-pass filters are connected in series to achieve high-order filtering, then the low-pass characteristic improves, but the maximum output level is further reduced

Engineering Contradiction:

Improvesampling noise eliminationVSAvoidmaximum output level

Core Design Contradiction:

Object-affected harmful factorsVSPower

Solution Approach 1:

The patent extends the copying principle to multi-stage filtering by configuring multiple dummy circuits, each corresponding to a low-pass filter stage. Each dummy circuit copies the differential amplifier configuration of its corresponding filter stage without capacitive elements, generating dummy signals that represent the cumulative offset voltage of all filter stages. The feedback system uses these dummy signals to compensate for the total offset voltage accumulation, enabling high-order filtering while maintaining the maximum output level.

Inventive Principle:

Principle #26Copying

3Reliability

If a differential amplifier with large transistors is used to obtain good pair characteristics, then the sync-tip level maintenance improves, but the circuit scale increases

Engineering Contradiction:

Improvepair characteristicVSAvoidcircuit scale

Core Design Contradiction:

ReliabilityVSArea of stationary object

Solution Approach 1:

The patent applies the copying principle by creating dummy circuits that replicate only the essential active components (differential amplifiers with transistors) while omitting the passive capacitive elements. This allows the system to achieve accurate offset voltage compensation using smaller transistors in the dummy circuits, as they don't need to handle full video signal power, thereby reducing the overall circuit scale while maintaining sufficient pair characteristics for accurate offset representation.

Inventive Principle:

Principle #26Copying

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The solution effectively raises the maximum output level of video signals, reduces circuit scale, and increases dynamic range while maintaining stable input levels for output drivers, allowing for cost reduction and improved performance.

Implementation Method 1

a low-pass filter receiving the video signal output from the sync-tip clamp circuit and eliminating a predetermined high-frequency component

Methodology Applied

Scientific EffectLow-pass filtering: Filter (electronic)

Implementation Method 2

a coupling capacitor 102 eliminates a DC component of the video signal input to an input terminal IN, and outputs an AC component

Methodology Applied

Scientific EffectCapacitive coupling: Capacitance

Data Source

PatentUS7940335B2Video signal output circuit and electronic device using the same

Publication Date: 2011.05.10 ROHM CO LTD

AI summary

A video signal output circuit includes a sync-tip clamp circuit fixing a sync-tip level of an input video signal at a constant voltage, a low-pass filter receiving the video signal output from the sync-tip clamp circuit and eliminating a predetermined high-frequency component, a dummy circuit receiving the video signal output from the sync-tip clamp circuit and outputting the video signal having substantially the same sync-tip level as that of the video signal output from the low-pass filter, and an output driver receiving the video signal output from the low-pass filter and outputting the video signal with a low output impedance. The sync-tip clamp circuit controls the sync-tip level of the video signal output from the dummy circuit such that the sync-tip level of the video signal is equal to a predetermined reference voltage.