Diode Models and DC Circuit Analysis
Piecewise diode analysis assigns every diode an ON or OFF state, solves the resulting linear circuit, and verifies that the calculated diode voltage and current support each assumed state.
Why this shows up in the exam
Finding current in multi-diode networks · Selecting a safe series resistor · Comparing ideal and constant-drop answers
Learn the idea
Solve a diode circuit by assuming states, replacing each diode by its model, and checking consistency. An ideal conducting diode is a short and a blocking diode is an open switch. A practical constant-drop model instead subtracts the stated silicon or germanium forward voltage.
🧠 Memory hook: Assume, replace, solve, then check every diode.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- V_D = 0 and I_D >= 0 for an ideal ON diode — ideal forward-conducting diode condition
- I_D = 0 and V_anode <= V_cathode for an ideal OFF diode — ideal blocking diode consistency condition
- R_series >= (V_s - V_F) / I_max — minimum series resistance that limits forward current
How to approach it
- 1Write anode and cathode potentials beside every diode
- 2Try the most plausible ON/OFF combination
- 3Reject any solution that violates its assumed diode state
Common slip-ups that cost marks
- •Keeping a diode ON after the solved current becomes negative
- •Using 0.7 V when the problem explicitly says ideal diode
- •Ignoring internal or stated forward resistance
🌟 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.
A transistor has common-emitter current gain beta = 50. If base current is 20 microA, find collector current.
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