---
title: "Large signal / piecewise linear diode models"
subject: "Basic Electronics Engineering"
chapter: 2
key: "semester_2/basic_electronics_engineering:pn-diode-large-signal-model"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/pn-diode-large-signal-model
site: BCT Engineering
---

# Large signal / piecewise linear diode models

Basic Electronics Engineering · Chapter 2: Diodes

- Asked in: 2075 Baisakh, 4 marks; 2071 Bhadra, 4 marks; 2068 Magh, 6 marks; 2072 Magh, 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_2/basic_electronics_engineering/pn-diode-large-signal-model
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-2

## Question

Explain the large signal (piece-wise linear) models of a PN junction diode.

## Short answer

Large-signal piecewise linear models approximate the diode as a switch. Ideal model: ON = short, OFF = open. Constant-drop model adds a 0.7 V battery in series. The linear model with forward resistance rf gives ON voltage Vγ + rf·ID, where rf models the slope of the I-V curve.

## Answer

When the signal applied to a diode is large compared with its knee voltage (about 0.7 V for silicon), the exponential I-V curve is replaced by straight-line approximations. These are the **large signal (piece-wise linear) models**. In each model the diode is a switch plus extra elements: ON when forward biased beyond the cut-in voltage, OFF otherwise.

#### 1. Ideal diode model
- Forward bias: short circuit (VD = 0, no resistance).
- Reverse bias: open circuit (I = 0).
- Used when the supply voltage is much larger than 0.7 V.

#### 2. Constant voltage drop (battery) model
- Diode ON when VD ≥ Vγ (Vγ = 0.7 V for Si, 0.3 V for Ge). It is replaced by an ideal diode in series with a battery Vγ.
- OFF when VD < Vγ: open circuit.

#### 3. Piece-wise linear model with forward resistance
- The forward curve is approximated by a line of slope 1/rf through Vγ: VD = Vγ + rf·ID. The diode is an ideal diode in series with a battery Vγ and a resistance rf (typically a few ohms to tens of ohms, rf = ΔV/ΔI of the linear portion).
- Reverse: open circuit (or a very large reverse resistance).

```text
 I                       Ideal diode: ON  = wire
 |      /  slope 1/rf    Const. drop: ON  = --|+ -|--
 |     /                 PWL        : ON  = --|+ -|--/\/\--
 |    /                              Vg         rf
 +---+----------- V      OFF (all models) = open circuit
    Vg
```

| Model | ON state | Accuracy |
|---|---|---|
| Ideal | short | lowest |
| Constant drop | Vγ battery | medium |
| Piece-wise linear | Vγ + rf | highest of the three |

**Example:** Vs = 5 V, R = 1 kΩ, Si diode. Ideal: I = 5 mA. Constant drop: I = (5 − 0.7)/1 k = 4.3 mA. With rf = 20 Ω: I = 4.3/1.02 = 4.22 mA.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why are piece-wise linear models used?
   A. To replace the exponential I-V curve by straight-line approximations for large signals
   B. To model very small signals
   C. To avoid using a battery
   D. To increase diode speed
   Answer: A. Large signals make the exponential curve replaceable by straight segments. (Key point: Purpose)

2. In the constant voltage drop model, the ON diode is
   A. A short circuit
   B. An ideal diode in series with a battery Vγ
   C. An open circuit
   D. A resistor only
   Answer: B. The ON state is an ideal diode with a battery Vγ (0.7 V Si, 0.3 V Ge). (Key point: Constant voltage drop model)

3. What does the third model add?
   A. A capacitor
   B. A forward resistance rf in series, VD = Vγ + rf·ID
   C. A parallel resistor
   D. A second battery
   Answer: B. The sloped line adds rf in series with Vγ. (Key point: PWL with resistance)

4. Vs = 5 V, R = 1 kΩ, Si diode, constant drop model. What is I?
   A. 5 mA
   B. 4.3 mA
   C. 3.5 mA
   D. 4.22 mA
   Answer: B. I = (5 − 0.7)/1 kΩ = 4.3 mA. (Key point: Example)

## Related in Basic Electronics Engineering

- [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×

## 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 model of pn diode](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Adiode-large-signal-model) — Fundamental of Electrical and Electronics Engineering, chapter 4