--- title: "DC load line and operating point of diode" subject: "Electronics Devices and Circuit" chapter: 1 key: "semester_3/electronics_devices_and_circuit:dc-load-line-operating-point" source: https://www.bctengineer.com/q/semester_3/electronics_devices_and_circuit/dc-load-line-operating-point site: BCT Engineering --- # DC load line and operating point of diode Electronics Devices and Circuit · Chapter 1: Diodes - Asked in: 2079 Bhadra, 1+4 marks; 2075 Chaitra, 5 marks; 2073 Shrawan, 4 marks; 2071 Shrawan, 5 marks; 2070 Ashad, 1+3 marks (asked 5 times) - 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-operating-point - Whole chapter: https://www.bctengineer.com/md/semester_3/electronics_devices_and_circuit/chapter-1 ## Question What is Dc load line? Find operating point for the diode circuit graphically using load line method. ## Short answer The DC load line is ID = (VDD – VD)/R. Graphically, draw a line from (0, VDD/R) to (VDD, 0) on the diode I–V plane; its intersection with the diode characteristic gives the Q-point (VDQ, IDQ). For a 5 V supply and 1 kΩ resistor, IDQ ≈ 4.25 mA, VDQ ≈ 0.75 V. ## Answer The **dc load line** is the straight line on the diode's i–v plane that shows every (VD, ID) pair allowed by the external circuit (source and resistor) under dc conditions. The **operating point (Q-point)** is where this line crosses the diode's actual characteristic curve. #### Circuit ```text VDD ---/\/\/---+---|>|---+ R | D | ID -> GND ``` Applying KVL to the loop: ```text VDD = ID·R + VD → ID = (VDD - VD)/R ``` This is a straight line in the VD–ID plane (the load line) with: - ID-intercept (VD = 0): ID = VDD/R - VD-intercept (ID = 0): VD = VDD - slope = -1/R #### Graphical procedure 1. Draw the diode characteristic ID versus VD (given or measured). 2. Mark the two points (0, VDD/R) and (VDD, 0), and join them with a straight line. 3. The intersection of the line and the curve is the Q-point (VDQ, IDQ). Read VDQ and IDQ from the axes. ```text ID VDD/R |\ | \ diode curve | \ _.- IDQ | \ _.Q | \'| | \| +------+----\---- VD VDQ VDD ``` #### Example VDD = 5 V, R = 1 kΩ, silicon diode: the intercepts are 5 mA and 5 V. The line cuts the curve at about VD ≈ 0.75 V, ID ≈ (5 - 0.75)/1 kΩ ≈ 4.25 mA. The Q-point is both on the diode curve and on the load line, so it satisfies the diode and the circuit equations at the same time. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the dc load line of a diode circuit represent? A. The diode's own i-v characteristic B. The reverse breakdown limit C. All (VD, ID) pairs allowed by the external source and resistor D. The ac signal swing Answer: C. It is the straight line from KVL that shows every voltage-current pair the circuit permits. (Key point: DC load line meaning) 2. For a series VDD, R and diode circuit, which equation gives the load line? A. ID = (VDD - VD)/R B. ID = VDD/(R + VD) C. ID = VDD x R - VD D. ID = VD/R Answer: A. KVL gives VDD = ID R + VD, so ID = (VDD - VD)/R with slope -1/R. (Key point: Load line equation) 3. What are the load line intercepts? A. ID = VDD/R on the current axis and VD = VDD on the voltage axis B. ID = 0 and VD = 0 C. ID = VDD and VD = VDD/R D. ID = VDD x R and VD = R Answer: A. Setting VD = 0 gives ID = VDD/R; setting ID = 0 gives VD = VDD. (Key point: Intercepts) 4. How is the Q-point found graphically? A. At the intercept with the VD axis B. At the midpoint of the load line C. Where the load line intersects the diode characteristic D. At the intercept with the ID axis Answer: C. The Q-point lies on both the diode curve and the load line, so it satisfies both the device and the circuit equations. 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