BJT Basics
Bipolar junction transistor fundamentals — NPN / PNP, regions, configurations, and small-signal model.
Reference
Types
- NPN
- Current flows C→E when V_BE ≈ 0.7 V. Turn on by pulling B above E.
- PNP
- Current flows E→C when V_BE ≈ −0.7 V. Turn on by pulling B below E.
Operating regions
| Region | V_BE | V_CE | Use |
|---|---|---|---|
| Cutoff | < 0.6 V | any | Transistor off |
| Active | ~0.7 V | > V_CE(sat) | Linear amplifier |
| Saturation | ~0.7 V | < ~0.2 V | Switch fully on |
| Reverse active | — | — | Rare; poor performance |
Key equations
- β (hFE)
- Current gain. I_C ≈ β · I_B.
- I_E
- = I_B + I_C ≈ I_C (since β is large)
- V_BE(on)
- ≈ 0.6–0.7 V (Si), ~0.3 V (Ge)
- g_m
- = I_C / V_T (V_T ≈ 25 mV at room temp)
- r_π
- = β / g_m
- V_CE(sat)
- 0.1–0.3 V at rated I_C
Common configurations
| Config | V gain | I gain | Input Z | Output Z | Use |
|---|---|---|---|---|---|
| Common emitter | High (inverting) | High | Medium | Medium | General amplifier |
| Common collector (emitter follower) | ~1 | High | High | Low | Buffer / impedance match |
| Common base | High | ~1 | Low | High | RF stages, cascode |
Notes
- For switching a load, use R_B = (V_drive − 0.7) / (I_C / β) and add margin (β_forced = β/10 to guarantee saturation).
- Thermal runaway: I_C rises with temperature — use emitter degeneration or a thermal ballast.
Last updated: