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.
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