--- title: "Short note: CB output characteristics" subject: "Basic Electronics Engineering" chapter: 3 key: "semester_2/basic_electronics_engineering:short-note-cb-output-characteristics" source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-cb-output-characteristics site: BCT Engineering --- # Short note: CB output characteristics Basic Electronics Engineering · Chapter 3: Transistor - Asked in: 2073 Bhadra, 3 marks - Video: [CB Transistor (Output Characteristics)](https://www.youtube.com/watch?v=KPMPyfqMP9c) — Neso Academy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-cb-output-characteristics - Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-3 ## Question Write a short note on output characteristics of common base configuration. ## Short answer CB output characteristics plot IC vs VCB for constant IE. In active region, IC ≈ αIE (α < 1), curves are nearly flat with high output resistance. Saturation occurs for small forward VCB; cutoff with IE = 0 yields ICBO. Input impedance is low, no current gain, but voltage gain exists with no phase shift. ## Answer In the **common base (CB) configuration**, the base is common to the input (emitter-base) and output (collector-base). The **output characteristics** are the plots of collector current IC versus collector-base voltage VCB for various fixed values of emitter current IE. ```text IC (mA) | ______ IE = 6 mA | |______ IE = 4 mA | |______ IE = 2 mA | |______ IE = 0 (ICBO) +--------+----------------- VCB saturation active (VCB < 0) ``` #### Regions - **Active region:** the emitter-base junction is forward biased and the collector-base junction is reverse biased. IC ≈ α IE, almost constant and independent of VCB (the curves are nearly flat; α = 0.95 to 0.99). A small slope exists due to the Early effect (base width modulation). - **Saturation region:** both junctions are forward biased (VCB negative by a few tenths of a volt). IC falls rapidly to zero as VCB decreases. - **Cut-off region:** IE = 0; both junctions are reverse biased. IC = ICBO, a very small leakage current. #### Key points - Current gain α = ΔIC/ΔIE (at constant VCB), a little below 1. - Very high output resistance (about 1 MΩ), low input resistance. - No current gain but voltage gain exists; there is no phase shift. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The CB output characteristics are plots of which quantities? A. IC versus VCB at fixed IE B. IE versus VEB at fixed VCB C. IC versus VCE at fixed IB D. IB versus VBE at fixed VCE Answer: A. For common base, output curves show collector current against collector-base voltage for different emitter currents. (Key point: What is plotted) 2. In the active region of CB configuration, how does IC behave? A. IC ≈ α IE, nearly constant with VCB B. IC = β IB C. IC rises steeply with VCB D. IC = 0 Answer: A. With the emitter-base junction forward and collector-base reverse biased, IC ≈ α IE and the curves are almost flat. (Key point: Active region) 3. What are the condition and current in the cut-off region? A. IE = 0, both junctions reverse biased, IC = ICBO B. IE large, both junctions forward biased C. IB = 0, IC = ICEO D. VCB = 0, IC = IE Answer: A. With IE = 0 only the small leakage ICBO flows in the collector. (Key point: Cut-off region) 4. Which junction condition defines saturation in the CB output characteristics? A. Both junctions reverse biased B. Both junctions forward biased, with VCB negative C. Only the collector junction forward biased D. Emitter junction reverse biased Answer: B. In saturation both junctions are forward biased, VCB goes slightly negative and IC drops rapidly. 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