---
title: "DC load line in a diode"
subject: "Electronics Devices and Circuit"
chapter: 1
key: "semester_3/electronics_devices_and_circuit:dc-load-line-diode"
source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/dc-load-line-diode
site: BCT Engineering
---

# DC load line in a diode

Electronics Devices and Circuit · Chapter 1: Diodes

- Asked in: 2080 Bhadra, 3 marks
- Video: [Load Line Analysis of Diode](https://www.youtube.com/watch?v=TaTGnbxIMdY) — Neso Academy
- 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/dc-load-line-diode
- Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-1

## Question

What is dc load line in diode? Write its significance.

## Short answer

The dc load line is the straight line ID=(VDD−VD)/R, from (0, VDD/R) to (VDD,0) with slope −1/R. Its intersection with the diode characteristic defines the Q-point, enabling graphical bias analysis.

## Answer

The **dc load line** of a diode is the straight line, plotted on the diode's current–voltage axes, that represents the external circuit's constraint on the diode in dc conditions. For a series circuit with supply VDD and resistor R, KVL gives

```text
ID = (VDD - VD)/R
```

It is a straight line of slope -1/R joining the points (VD = 0, ID = VDD/R) and (VD = VDD, ID = 0).

#### Significance
- Its intersection with the diode characteristic is the **operating point (Q-point)**, i.e. the actual dc current and voltage of the diode in the circuit.
- It lets us find the Q-point graphically without solving the non-linear diode equation.
- The two ends show the limits: VDD/R is the maximum possible current (diode short) and VDD is the maximum voltage (diode open).
- Changing R changes the slope, and changing VDD shifts the line, so the effect of component changes on the bias point can be seen at once.
- It guides the choice of a stable bias point (and of the swing available) in rectifier and amplifier design.

## Check yourself

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

1. What is the dc load line?
   A. The diode's reverse characteristic
   B. A line of constant power
   C. The ac load line
   D. A line showing the circuit's dc constraint on the diode, ID = (VDD - VD)/R
   Answer: D. It comes from KVL and is plotted on the diode's i-v axes. (Key point: Definition)

2. What is the significance of the intersection of the load line and diode curve?
   A. It gives the Q-point, the actual dc current and voltage
   B. It gives the ac gain
   C. It is the breakdown point
   D. It gives the cut-in voltage
   Answer: A. The intersection gives the dc operating point. (Key point: Q-point)

3. What happens to the load line when R is changed?
   A. Nothing
   B. It shifts parallel
   C. It disappears
   D. Its slope (-1/R) changes
   Answer: D. R sets the slope -1/R, while changing VDD shifts the line. (Key point: Effect of changing R and VDD)

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