Table of Contents

Overview
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Active matrix driving uses thin-film transistors (TFTs) at each pixel to maintain voltage between refresh cycles. This enables higher resolution, faster response, and better image quality than passive matrix.

Architecture
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              Data Lines
         D1   D2   D3   D4
          │    │    │    │
    ┌─────┼────┼────┼────┼─────┐
G1 ─┤    ⊏●   ⊏●   ⊏●   ⊏●    │
    ├─────┼────┼────┼────┼─────┤
G2 ─┤    ⊏●   ⊏●   ⊏●   ⊏●    │  Gate Lines
    ├─────┼────┼────┼────┼─────┤
G3 ─┤    ⊏●   ⊏●   ⊏●   ⊏●    │
    ├─────┼────┼────┼────┼─────┤
G4 ─┤    ⊏●   ⊏●   ⊏●   ⊏●    │
    └─────┴────┴────┴────┴─────┘
         ⊏ = TFT, ● = Pixel

Pixel Circuit
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Basic TFT-LCD Pixel
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Gate Line ──┬──[TFT]──┬── Data Line
            │         │
           ═╪═       ═╪═
           ═╪═ Cst   ═╪═ Clc
           ═╪═       ═╪═
            │         │
         Common ──────┘

Components:

  • TFT: Thin-film transistor (switch)
  • Clc: Liquid crystal capacitance
  • Cst: Storage capacitor

Operating Principle
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Write Phase
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  1. Gate line activates TFT
  2. Data voltage charges pixel capacitor
  3. TFT turns off, moves to next row

Hold Phase
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When scan line closes and moves to next row:

  • Storage capacitor maintains voltage
  • Electric field persists across liquid crystal
  • Image remains stable until next refresh

Voltage Retention
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$$ V_{pixel}(t) = V_{data} \cdot e^{-t/\tau} $$

Where \(\tau = R_{TFT(off)} \cdot C_{total}\)

High TFT off-resistance ensures minimal voltage decay.

Why This Matters
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Human Perception
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Since data updates occur discretely row by row:

  • Without storage: flickering image
  • With storage: stable, static appearance

The capacitor bridges the gap between discrete updates and continuous perception.

Resolution Scaling
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As displays increase in resolution:

  • More rows to scan
  • Less time per row
  • Storage becomes critical

TFT Types
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TypeMaterialMobilityApplication
a-SiAmorphous SiLowStandard LCD
LTPSLow-temp poly-SiHighMobile, OLED
IGZOOxideMedium-HighHigh-res, large

Comparison with Passive Matrix
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AspectPassiveActive
Voltage holdingNoneCapacitor
CrosstalkSignificantMinimal
Resolution limit~256 rowsUnlimited
Contrast ratio10:11000:1+
Response timeSlowFast

Timing Parameters
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Frame Period
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For 60 Hz display:

$$ T_{frame} = \frac{1}{60} = 16.67 \text{ ms} $$

Line Time
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For 1080 rows:

$$ T_{line} = \frac{T_{frame}}{1080} \approx 15.4 \text{ μs} $$

Storage Capacitor Design
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Purpose
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  1. Increase total capacitance
  2. Reduce voltage droop
  3. Stabilize pixel voltage

Sizing
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$$ C_{st} \approx C_{lc} \times (2 \sim 3) $$

Trade-off:

  • Larger Cst → Better holding, slower charging
  • Smaller Cst → Faster charging, more droop

Connection to DRAM
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The same principle applies to Dynamic RAM:

  • TFT ≈ Access transistor
  • Storage capacitor ≈ Memory cell
  • Refresh needed ≈ Periodic data refresh
DRAM Cell:          TFT-LCD Pixel:
    │                    │
  ──┼── Word Line     ──┼── Gate Line
    │                    │
  [Tr]                 [TFT]
    │                    │
  ═╪═ Capacitor       ═╪═ Cst + Clc
  ═╪═                 ═╪═
    │                    │
    ─ Bit Line          ─ Data Line

Advanced Pixel Circuits
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OLED Active Matrix (AMOLED)
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Additional transistors for current control:

2T1C Structure:
- T1: Switching TFT
- T2: Driving TFT
- C1: Storage capacitor

Compensation Circuits
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Address TFT variation:

  • Current sensing
  • Voltage compensation
  • Multiple TFTs per pixel (4T, 6T, etc.)