Common-Emitter Transistor as Amplifier or Switch
In the active region of an npn common-emitter amplifier, the base-emitter junction is forward biased and collector-base junction reverse biased. Cutoff approximates an open switch and saturation approximates the ON state.
Why this shows up in the exam
Common-emitter signal amplification · Transistor switching · Checking base and collector junction bias
Learn the idea
Proper junction bias lets a common-emitter transistor amplify a signal or switch between cutoff and saturation. Forward bias at the base-emitter junction controls carrier injection, while reverse bias at the collector-base junction sweeps carriers into the collector. A changing base signal can therefore control a larger output change.
🧠 Memory hook: Active npn: base-emitter forward, collector-base reverse.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Delta I_C approximately beta Delta I_B — small-signal current amplification in the active region
- V_CE = V_CC - I_C R_C — collector-emitter voltage in a simple collector-resistor circuit
How to approach it
- 1Identify device type and terminal junctions
- 2Classify cutoff, active, or saturation
- 3Apply gain only after confirming active operation
Common slip-ups that cost marks
- •Forward biasing both junctions for linear amplification
- •Using beta after the transistor has saturated
- •Treating vacuum-triode OCR records as verified BJT evidence
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A transistor has common-emitter current gain beta = 50. If base current is 20 microA, find collector current.
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