Bipolar Junction Transistors (BJTs)

Structure and operating regions

A bipolar junction transistor has emitter, base and collector terminals. NPN and PNP devices have opposite voltage and current polarities. For an NPN in forward-active operation the base-emitter junction is forward biased and the base-collector junction reverse biased. This region is used for linear amplification. Cutoff has negligible conduction in the simplified model. Saturation forward biases both junctions and is used in many switching applications; the active-region relation IC = βIB should not be imposed after saturation is reached. Reverse-active operation has poorer and different gain characteristics.

Active-region relations

IE = IC + IB, β = IC/IB and α = IC/IE = β/(β + 1). A simplified exponential model is IC ≈ IS exp(VBE/VT), ignoring Early effect and other nonidealities. VBE is not a universal fixed 0.7 V; it depends on current, temperature and device. At a bias current IC, small-signal transconductance gm = IC/VT and rπ = β/gm. With emitter AC grounded and output resistance neglected, common-emitter gain is approximately −gm(RC parallel RL). The approximation requires small signals around a valid active-region bias.

Worked example

An NPN stage has VCC = 10 V, RC = 2 kΩ, emitter grounded and base current 20 μA. Assume β = 100 in forward-active operation. Candidate IC = 100 × 20 μA = 2 mA. VC = VCC − ICRC = 10 − 0.002 × 2000 = 6 V, so VCE = 6 V. With the base near its ordinary forward-biased voltage, the collector-base junction is reverse biased, consistent with the assumed region. At VT = 25 mV, gm = 0.002/0.025 = 0.08 S and rπ = 100/0.08 = 1.25 kΩ. With no external load and neglecting ro, the small-signal gain is −0.08 × 2000 = −160. Large input swings invalidate this linear estimate.

Switching and design limits

Base drive, collector load and supply determine whether saturation occurs. Check power dissipation, safe operating area, temperature and switching time using device data. Stored charge in saturation can slow turn-off. Do not use a nominal β as a guaranteed switching design value.

Quick check

1. What is β when α = 0.99? β = α/(1 − α) = 99.
2. Which junctions are forward biased in saturation? Both base-emitter and base-collector.
3. Does a common-emitter small-signal voltage gain invert phase? Yes, in the midband model.