--- title: "Large signal models of PN junction diode" subject: "Electronics Devices and Circuit" chapter: 1 key: "semester_3/electronics_devices_and_circuit:large-signal-models-diode" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/large-signal-models-diode site: BCT Engineering --- # Large signal models of PN junction diode Electronics Devices and Circuit · Chapter 1: Diodes - Asked in: 2083 Baisakh, 6 marks; 2081 Bhadra, 2+2+2 marks; 2068 Chaitra, 4 marks; 2070 Chaitra, 1+4 marks (asked 4 times) - Video: [Diode - Large Signal Models](https://www.youtube.com/watch?v=7iDSwixWESA) — techgurukula - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/large-signal-models-diode - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-1 ## Question Explain different large signal models of PN junction diodes. ## Short answer Four models: ideal diode (0 V forward, open reverse), constant voltage drop (CVD, 0.7 V for Si), piecewise-linear (cut-in voltage Vγ plus series resistance rD), and exponential Shockley model. The CVD model is most used for hand analysis; the exponential model is most accurate. ## Answer Large-signal models replace the non-linear diode with simple linear elements so that dc bias and large-voltage circuits can be solved by hand. Four models are used; they are listed here in order of increasing accuracy. #### 1. Ideal diode model The diode is a perfect switch: a short circuit (0 V drop) when forward biased and an open circuit when reverse biased. - Conducts if VD > 0; blocks if VD < 0. - Used when the supply voltages are much larger than 0.7 V. #### 2. Constant voltage drop (CVD) model A conducting diode is modelled as an ideal diode in series with a battery VD(on) (0.7 V for Si, 0.3 V for Ge). ```text A --->|--- (-|+) --- K ON : VD = 0.7 V ideal 0.7 V OFF : VD < 0.7 V, I = 0 ``` It is the model most commonly used in hand analysis. #### 3. Piecewise-linear model The curve is approximated by two straight-line segments: below the cut-in voltage Vγ the current is zero; above it the current rises linearly with slope 1/rD. ```text A --->|--- Vγ --- rD --- K ``` ```text ID = (VD - Vγ)/rD for VD ≥ Vγ, ID = 0 for VD < Vγ ``` It includes the diode's bulk (forward) resistance rD, so the voltage drop rises slightly with current. #### 4. Exponential (Shockley) model ```text ID = IS [exp(VD/(η VT)) - 1] ``` It is the most accurate model, but it needs iteration or a graphical solution. The CVD model approximates it well, because VD changes by only about 60 mV (η = 1) to 120 mV (η = 2) for every tenfold change in ID. #### Comparison | Model | Forward | Reverse | Accuracy | |---|---|---|---| | Ideal | short, 0 V | open | lowest | | CVD | 0.7 V battery | open | good | | Piecewise linear | Vγ + rD | open | better | | Exponential | Shockley eq. | -IS | best | All the models treat the diode as a one-way device; they differ only in how the forward drop is represented. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the ideal diode model, what does the diode do? A. Acts as a short (0 V) when forward biased and open when reverse biased B. Drops 0.7 V when forward biased C. Follows the exponential law D. Has a resistance rD in forward bias Answer: A. The ideal diode is a perfect switch with no forward drop. (Key point: Ideal diode model) 2. What does the constant voltage drop (CVD) model use for a conducting Si diode? A. A 0.3 V battery with resistance rD B. A resistor of 25 ohm C. A short circuit D. An ideal diode in series with a 0.7 V battery Answer: D. CVD is an ideal diode in series with VD(on) = 0.7 V (Si) or 0.3 V (Ge). (Key point: CVD model) 3. What distinguishes the piecewise-linear model from CVD? A. It ignores the cut-in voltage B. It includes cut-in voltage and a forward resistance rD, so current rises linearly above V_gamma C. It is exponential D. It models the junction capacitance Answer: B. Above V_gamma, ID = (VD - V_gamma)/rD, so the drop rises slightly with current. (Key point: Piecewise linear model) 4. Why is the CVD model a good approximation of the exponential model? A. VD changes only about 0.1 V when ID changes by a factor of ten B. Temperature does not matter C. IS is constant D. The diode has zero resistance Answer: A. Because of the exponential law, a tenfold current change shifts VD by only about 60 to 100 mV. 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