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PN Junction Diode Load Line Analysis

Sep 26, 2024

Load Line Analysis of PN Junction Diode

Introduction

  • Load Line: Used in graphical analysis of nonlinear electronic circuits like diodes and transistors.
  • Nonlinear Circuits: Circuits with nonlinear VI (Voltage-Current) characteristics, not following Ohm's Law.
  • Diode Characteristics: Nonlinear VI characteristics due to not following Ohm's Law.

Circuit Analysis

  • Circuit Components:
    • PN Junction Diode
    • Voltage across diode: $V_d$
    • Load resistance: $R_L$
    • External voltage source: $V$
  • Connections:
    • Negative terminal to n-side
    • Positive terminal to p-side
    • Circuit current: $I_d$

Kirchhoff's Voltage Law (KVL)

  • KVL Equation: $V - I_dR_L - V_d = 0$
  • Rearranged: $V = I_dR_L + V_d$ (Equation 1)

Calculating Diode Current and Voltage

  • When $V_d = 0$:
    • $I_d = \frac{V}{R_L}$
  • When $I_d = 0$:
    • $V_d = V$
  • Points:
    • $(\frac{V}{R_L}, 0)$ and $(0, V)$

Load Line and Operating Point

  • Load Line: Straight line connecting the points $(\frac{V}{R_L}, 0)$ and $(0, V)$
  • Q Point/Operating Point: Intersection of load line and diode characteristics
    • Voltage: $V_{dq}$ (Operating voltage)
    • Current: $I_{dq}$ (Operating current)

Finding the Slope of the Load Line

  • Start from: $V = I_dR_L + V_d$
  • Divide by $R_L$: $\frac{V}{R_L} = I_d + \frac{V_d}{R_L}$
  • Rearranged: $I_d = -\frac{V_d}{R_L} + \frac{V}{R_L}$
  • Comparison with $y = mx + c$:
    • Intercept $c = \frac{V}{R_L}$
    • Slope $m = -\frac{1}{R_L}$ (Negative slope)
    • $V_d$: x-axis, $I_d$: y-axis

Effects of Changing Load Resistance ($R_L$)

  • Changing $R_L$ affects the slope
  • Change in slope shifts the Q point
  • Example: Increasing $R_L$ shifts the operating point