--- title: "CE configuration input/output characteristics" subject: "Basic Electronics Engineering" chapter: 3 key: "semester_2/basic_electronics_engineering:ce-npn-characteristics" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/ce-npn-characteristics site: BCT Engineering --- # CE configuration input/output characteristics Basic Electronics Engineering · Chapter 3: Transistor - Asked in: 2071 Bhadra, 6 marks; 2067 Mangsir, 7 marks; 2069 Bhadra, 3 marks (asked 3 times) - Video: [BJT: Common Emitter Configuration (Input and Output Characteristics) Explained](https://www.youtube.com/watch?v=KynKHr2cXgk) — ALL ABOUT ELECTRONICS - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/ce-npn-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-3 ## Question Explain the common emitter configuration circuit of npn transistor with the help of input and output characteristics (draw the circuit diagram and I-V characteristics graph). ## Short answer In common-emitter configuration, input is applied between base and emitter, output taken between collector and emitter. Input characteristic shows negligible IB until VBE ≈ 0.7 V, then rises sharply; output shows saturation (VCE < 0.2 V), active region where IC ≈ βIB, and cut-off. β typically 50–300, output resistance ~40 kΩ. ## Answer In the **common emitter (CE) configuration**, the emitter terminal is common to both input and output. The input signal is applied between base and emitter and the output is taken between collector and emitter. It gives both current gain and voltage gain, with a 180° phase shift. #### Circuit ```text +VCC | [RC] | IC +--------o C (output VCE) |/ VBB (+)--[RB]---->--------| NPN IB B |\ v E | VBB (-), GND o--------------+--------o E (common) ``` VBB forward biases the base-emitter junction and VCC reverse biases the collector-base junction. Measure IB and VBE (input), and IC and VCE (output). #### Input characteristics Plot of IB versus VBE at constant VCE. - Similar to a forward-biased diode: IB is very small until VBE exceeds the cut-in voltage (about 0.7 V for Si), then it rises sharply. - Increasing VCE shifts the curve slightly to the right (a little less IB for the same VBE). - Input resistance: ri = ΔVBE/ΔIB at constant VCE (about 1 to 2 kΩ, higher than in CB). ```text IB | / VCE=0 | / / VCE>0 | / / +-------/-/------ VBE 0.7V ``` #### Output characteristics Plot of IC versus VCE at constant IB. ```text IC | ______ IB3 | /______ IB2 | /_______ IB1 | /________ IB=0 (ICEO) +----------------- VCE sat active breakdown ``` - **Saturation region:** VCE is small (below about 0.2 V); both junctions are forward biased; IC rises steeply. - **Active region:** the base-emitter junction is forward biased and the collector-base junction is reverse biased; IC ≈ β IB, almost independent of VCE. The transistor is used as an amplifier here. - **Cut-off region:** IB = 0, IC = ICEO (very small leakage); both junctions are reverse biased. - Output resistance: ro = ΔVCE/ΔIC at constant IB (about 40 kΩ). #### Current gain β = IC/IB (typically 50 to 300); ICEO = (1 + β) ICBO. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the CE configuration, which terminal is common? A. Collector B. Base C. Emitter D. None Answer: C. Emitter is common to input and output. (Key point: CE definition) 2. What does the input characteristic plot? A. IC vs VCE at constant IB B. IB vs VBE at constant VCE C. IE vs VCB D. VBE vs IC Answer: B. It resembles a forward diode with 0.7 V cut-in. (Key point: Input characteristic) 3. What are the junction biases in the active region? A. Both forward B. Both reverse C. BE reverse, CB forward D. BE forward, CB reverse Answer: D. Active: BE forward and CB reverse, IC ≈ βIB. (Key point: Active region) 4. What is IC at cut-off? A. ICEO, very small leakage B. β IB C. IB D. VCC/RC Answer: A. At IB = 0 both junctions are reverse biased. (Key point: Cut-off) 5. What marks saturation? A. VCE small (below about 0.2 V) with both junctions forward biased B. IB = 0 C. Large VCE D. Both reverse Answer: A. Both junctions forward and IC rises steeply. 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