Diode I-V Curve and Static or Dynamic Resistance
For a nonlinear diode characteristic, static resistance at an operating point is V/I, whereas dynamic resistance is the local differential ratio dV/dI, approximated by a small Delta V divided by Delta I around that point.
Why this shows up in the exam
Comparing diode operating points · Forward-to-reverse resistance comparison · Energy analysis across a junction barrier
Learn the idea
A diode is nonlinear, so resistance may mean V/I at a point or the local slope dV/dI. The forward curve becomes steep after the knee while reverse current stays tiny before breakdown. A secant from the origin and a tangent at a point answer different resistance questions.
🧠 Memory hook: Static is a secant; dynamic is a tangent.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- R_static = V/I — origin-to-operating-point secant resistance
- r_d approximately Delta V/Delta I — local small-signal resistance from nearby graph points
- (1/2)mv_f² = (1/2)mv_i² - qV_b — carrier kinetic-energy change across an opposing junction barrier
How to approach it
- 1Identify forward, reverse, or barrier-energy context
- 2Read axis units before forming a ratio
- 3Use local graph increments for dynamic resistance
Common slip-ups that cost marks
- •Using V/I when dynamic resistance is requested
- •Reading milliampere and microampere axes as equal
- •Ignoring the sign of carrier energy change across a barrier
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
In a meter bridge, resistance 2 ohm is in the left gap and balance occurs at 40 cm from the left end. Find the right-gap resistance.
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