Bipolar Junction Transistor Currents
For a BJT, emitter current equals collector plus base current. In common-base notation alpha = Ic/Ie, while common-emitter current gain beta = Ic/Ib under the stated operating conditions.
Why this shows up in the exam
Finding transistor terminal currents · Converting alpha and beta · Explaining carrier transport through a thin base
Learn the idea
A transistor uses a small base current to control a larger collector current while conserving terminal current. In active operation, the emitter injects carriers through a thin base and most reach the collector. Only a small fraction recombines in the base, producing the base current.
🧠 Memory hook: Emitter current splits into collector plus base.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- I_E = I_C + I_B — transistor terminal-current relation
- alpha = I_C / I_E — common-base dc current gain
- beta = I_C / I_B = alpha / (1 - alpha) — common-emitter gain and its relation to alpha
How to approach it
- 1Write Ie = Ic + Ib first
- 2Identify which current-gain definition is given
- 3Check the stated transistor operating mode
Common slip-ups that cost marks
- •Writing Ic = Ie + Ib
- •Using alpha and beta as interchangeable
- •Applying active-region gain relations in saturation without checking
🌟 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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