I-V Characteristics: Resistor vs Diode
Overview
The current-voltage (I-V) characteristic is the fundamental fingerprint of any circuit element — it tells you exactly how much current flows for any applied voltage.
This MicroSim plots two I-V curves side-by-side so you can directly compare their shapes:
| Component | Relationship | Shape |
|---|---|---|
| Resistor (R) | \(I = V/R\) — Ohm's Law | Straight line through origin |
| Diode | \(I = I_S(e^{V/V_T} - 1)\) | Exponential — nearly zero for V < 0, sharp rise above ~0.6 V |
Why Linearity Matters
Because the resistor's I-V curve is a straight line, every technique from Chapter 2 — superposition, Thévenin equivalents, node-voltage analysis — applies without restriction.
The diode's curve is nonlinear: doubling the voltage does not double the current. This means simple scaling and superposition break down, requiring more advanced methods (piecewise-linear models, iterative solvers) covered in later chapters.
The Diode Equation
where:
- \(I_S \approx 10^{-12}\) A — saturation current
- \(V_T = kT/q \approx 25.85\) mV at 25 °C — thermal voltage
- Knee ≈ 0.6 V — the voltage at which forward current rises steeply
Interactive Controls
| Control | Effect |
|---|---|
| R slider (50 – 500 Ω) | Changes the slope of the resistor line; shallower for larger R |
| Hide Resistor | Isolate the diode curve |
| Hide Diode | Isolate the linear relationship |
| Hide Knee | Remove the 0.6 V marker |
| Hover | Tooltip shows exact V and I values for each curve |
Learning Objectives
After using this simulation, students will be able to:
- Compare the linear I-V characteristic of a resistor with the nonlinear characteristic of a diode (Bloom L2 — Understand)
- Explain why linearity is a simplifying assumption that enables superposition and Thévenin analysis
- Read an I-V plot and identify whether a device is linear or nonlinear
- Describe how resistance controls the slope of the linear I-V curve
Key Observations
- Slope = 1/R: Drag the R slider from 50 Ω to 500 Ω and watch the resistor line rotate around the origin. Steeper = lower R = more current per volt.
- Diode is off for negative voltages: The diode passes essentially zero current (≈ −10⁻¹² A) for any negative voltage — this is reverse bias.
- The knee at 0.6 V: Current jumps from microamps to milliamps in a very narrow voltage range — about 0.1 V. This is unlike anything a resistor does.
- 20 mA clamp: The chart clips at ±20 mA. The diode current continues to rise exponentially beyond this; the resistor current keeps rising linearly.
Lesson Plan
Duration: 20 minutes
Bloom Level: Understand (L2)
| Phase | Activity |
|---|---|
| Predict (3 min) | "What shape do you expect if I = V/R?" — students sketch before seeing the chart |
| Explore (5 min) | Show both curves; identify the knee; hover to read off exact values at V = 0.6 V |
| Contrast (7 min) | Hide one curve at a time; drag R slider; answer: "What changes, and what stays the same?" |
| Discuss (5 min) | Why can we use Kirchhoff's laws directly for resistors but not raw diodes? |
References
- Sedra & Smith, Microelectronic Circuits, §3.1 — The Ideal Diode
- Razavi, Fundamentals of Microelectronics, Ch. 2 — Diode Models
- Horowitz & Hill, The Art of Electronics, §1.2 — Nonlinear Devices