Notes on Diode Equivalent Circuits

Introduction to Equivalent Circuits

• Definition: An equivalent circuit is a combination of elements (like resistors, capacitors, inductors, etc.) designed to represent the actual characteristics of a device (e.g., diode, transistor) in a specific operating region.
• Importance: Traditional circuit analysis techniques (Thevenin's theorem, Norton's theorem, superposition theorem) cannot be directly applied to actual devices. Equivalent circuits allow us to replace the device symbol with an equivalent circuit for easier analysis.

Types of Diode Equivalent Circuits

• Three Types of Equivalent Circuits for Diodes:
  1. Piecewise Linear Equivalent Circuit
  2. Constant Voltage Drop (Simplified) Equivalent Circuit
  3. Ideal Equivalent Circuit

1. Piecewise Linear Equivalent Circuit

• Assumption: The curve is assumed to be linear even with small nonlinearity.
• Diode Resistance (Rd):
  • Formula: Rd = 1 / slope
  • Slope Calculation: Slope = tan(θ) = ID / VD
  • Rearranging gives: Rd = VD / ID
• Equivalent Circuit Components:
  • Ideal Diode (barrier potential and diode resistance are considered)
  • Barrier Potential (VB) opposes current flow
  • Diode Resistance (Rd) derived from slope
• Example Calculation:
  • Given: 0.8V (VD), 10mA (ID)
  • Barrier potential for silicon = 0.7V
  • Calculation: Rd = (0.8V - 0.7V) / 10mA = 10Ω

2. Constant Voltage Drop Model

• Assumptions:
  1. Linear curve is considered.
  2. Diode resistance (Rd) is assumed to be zero.
• Characteristics:
  • Slope = ∞ (θ = 90°)
  • Barrier potential remains (VB) for silicon = 0.7V, germanium = 0.3V.
• Equivalent Circuit:
  • Ideal Diode (no diode resistance)
  • Only includes barrier potential (VB)

3. Ideal Equivalent Circuit

• Characteristics:
  • Both diode resistance (Rd) and barrier potential (VB) are equal to zero.
• Plot: Diode current (ID) vs Voltage (VD) without resistance or barrier potential.
• Equivalent Circuit:
  • Only ideal diode, no other elements.

Comparison of Models

• Piecewise Linear: Includes barrier potential (VB) and diode resistance (Rd).
• Constant Voltage Drop: Only barrier potential (VB), Rd = 0.
• Ideal Model: Both VB and Rd = 0.
• Usage: Constant voltage drop model is most commonly used for numerical problems involving diodes.

Important Points for Numerical Problems

• Forward Bias Condition: Replace diode with its barrier potential (e.g., 5V and 0V; for silicon, VB = 0.7V).
• Reverse Bias Condition: Acts as an open circuit (no current flows).

Conclusion

• Summary: Understanding diode equivalent circuits is crucial for analyzing and solving circuits involving diodes effectively.
• Next Steps: Further exploration of practical applications and numerical solving techniques.