Two-Transistor Model

Two-Transistor Model (BJT / Thyristor)

1. Basic Idea

The two-transistor model represents a device (especially a thyristor/SCR) using:

• One PNP transistor (T₁)
• One NPN transistor (T₂)

The transistors are connected so that the collector of one feeds the base of the other, creating positive feedback.

2. Structure

         T₁ (PNP)        T₂ (NPN)
       -----------     -----------

    

Collector of T₁ → Base of T₂
Collector of T₂ → Base of T₁

3. Current Relations

Let:

• α₁ = current gain of T₁
• α₂ = current gain of T₂

Total current through device:

I = I_input / (1 - (α₁ + α₂))

4. Turn-ON Condition

α₁ + α₂ → 1

When this condition is met:

• Denominator → 0
• Current rises rapidly
• Device switches ON (latches)

5. Working Principle

• OFF State: α₁ + α₂ < 1 → small current flows
• Triggering: Small input current increases α₁ and α₂
• ON State: Positive feedback loop causes rapid current rise → full conduction

6. Intuition

Think of it like two people pushing each other harder and harder. Once it starts, it keeps going (device latches ON).

7. Importance

• Explains latching behavior of SCR
• Shows positive feedback mechanism
• α₁ + α₂ ≈ 1 → device ON

Summary

• Two-transistor model = PNP + NPN pair
• Positive feedback controls switching
• α₁ and α₂ are internal transistor gains
• Turn-ON occurs when α₁ + α₂ ≈ 1