--- title: "Large signal model of pn diode" subject: "Fundamental of Electrical and Electronics Engineering" chapter: 4 key: "semester_1/fundamental_of_electrical_and_electronics_engineering:diode-large-signal-model" source: https://www.bctengineer.com/q/semester_1/fundamental_of_electrical_and_electronics_engineering/diode-large-signal-model site: BCT Engineering --- # Large signal model of pn diode Fundamental of Electrical and Electronics Engineering · Chapter 4: Diodes - Asked in: 2081 Chaitra (new course), 4 marks - 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_1/fundamental_of_electrical_and_electronics_engineering/diode-large-signal-model - Whole chapter: https://www.bctengineer.com/md/semester_1/fundamental_of_electrical_and_electronics_engineering/chapter-4 ## Question Explain the large signal model of a pn junction diode. ## Short answer The large-signal model treats the diode as a switch. Ideal model: short when forward, open when reverse. Constant voltage drop model: a 0.7 V battery in series with ideal diode. Piecewise-linear model adds a forward resistance rf. ## Answer The **large signal model** of a pn junction diode is an approximate equivalent circuit used when the voltage and current swings are large (as in rectifiers, clippers and switches), so the diode is treated as switching between ON and OFF states rather than working at a small region around a Q-point. The real diode follows the exponential law I = Is (e^(V/ηV_T) − 1). This is replaced by straight-line (piecewise-linear) pieces. Three common levels are used: #### 1. Ideal diode model - Forward bias: short circuit (V_D = 0, any current). - Reverse bias: open circuit (I = 0). - Used when supply voltages are much larger than 0.7 V. #### 2. Constant voltage drop (simplified) model - ON when V_D ≥ V_γ: a battery V_γ (0.7 V for Si, 0.3 V for Ge) in series with an ideal diode. - OFF when V_D < V_γ: open circuit. - Most often used in hand analysis. #### 3. Piecewise-linear model - ON: battery V_γ in series with the forward resistance r_f (slope of the straight line, typically a few ohms to tens of ohms) and an ideal diode. - OFF: open circuit (or a very large reverse resistance). - I_D = (V_D − V_γ)/r_f for V_D > V_γ. ```text Model Equivalent (ON) I-V shape Ideal --o-- vertical at 0 Const. drop --|+ -|-- (0.7 V) vertical at 0.7 V Piecewise --|+ -|--/\/\-- slope 1/r_f V_γ r_f from 0.7 V I | / piecewise linear | / | / slope = 1/r_f |___/____________ V 0 V_γ ``` The diode is first assumed ON or OFF, the circuit is solved, and the assumption is checked (I_D > 0 for ON, V_D < V_γ for OFF). The large signal model is different from the small signal model, which uses only the dynamic resistance r_d = ηV_T/I_D around a Q-point. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. When is the large signal model of a diode used? A. For tiny signals around a Q-point B. Only for Zener diodes C. When swings are large, as in rectifiers and clippers, treating the diode as ON/OFF D. Only at high frequency Answer: C. Large swings drive the diode between ON and OFF states; small-signal uses r_d. (Key point: When large-signal model is used) 2. In the constant voltage drop model, the diode is ON when: A. V_D > 0, modelled as a short B. V_D < 0.7 V C. V_D equals zero D. V_D >= V_gamma (0.7 V Si), modelled as battery in series with ideal diode Answer: D. The diode conducts at V_gamma and is then a battery V_gamma in series with an ideal diode. (Key point: Constant voltage drop model) 3. In the piecewise-linear model, the ON current is: A. I_D = (V_D - V_gamma)/r_f B. I_D = V_D/r_f C. I_D = V_gamma/r_f D. I_D = Is e^(V/V_T) Answer: A. The model adds forward resistance r_f in series with V_gamma. (Key point: Piecewise-linear current) 4. How is a circuit with a diode solved? A. Always assume ON and do not check B. Assume ON or OFF, solve, and check the assumption C. Use r_d = eta V_T/I_D in all cases D. Replace the diode with an open circuit Answer: B. Check I_D > 0 for ON or V_D < V_gamma for OFF. (Key point: Solving procedure) ## Related in other subjects - [Large signal models of PN junction diode](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Alarge-signal-models-diode) — Electronics Devices and Circuit, chapter 1, asked 4× - [Large signal / piecewise linear diode models](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Apn-diode-large-signal-model) — Basic Electronics Engineering, chapter 2, asked 4× - [PN junction diode I-V characteristics](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Apn-diode-iv-characteristics) — Basic Electronics Engineering, chapter 2, asked 5× - [Small signal model, AC resistance of diode](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Apn-diode-small-signal-ac-resistance) — Basic Electronics Engineering, chapter 2, asked 2×