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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
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