Discrete
BJT (Bipolar Junction Transistors)
Classic current-controlled transistors — still the right choice for analog and low-cost switching.
What is it?
A BJT is a three-terminal current-controlled switch / amplifier. Small base current controls much larger collector current. Available as NPN (more common) or PNP. Compared to MOSFETs: BJTs have predictable VBE drop (~0.7 V), good linear-region behaviour, and very low cost — still preferred for many analog and discrete-logic applications even where MOSFETs are technically faster.
When do you need it?
- Small-signal switching (driving LEDs, relays, opto-couplers).
- Linear regulators and current sources (BJT VBE is very predictable).
- Analog amplifier stages (audio preamps, op-amp output stages).
- Level-shifting between logic families.
- Low-cost discrete logic (one-shot, R-S flip-flops, etc.).
How to pick the right one
- Polarity
- NPN (more common, faster) or PNP (simpler high-side driving).
- VCEO (max C-E voltage)
- Maximum collector-to-emitter voltage. Typical: 30 V, 45 V, 80 V, 200 V.
- IC (max collector current)
- Continuous current rating. 100 mA / 500 mA / 1 A / 3 A common for general-purpose BJTs.
- hFE (current gain)
- Ratio of IC to IB. Typically 100-400 — spec'd at a given IC.
- fT (transition frequency)
- Where gain falls to 1. Determines maximum useful switching speed. >100 MHz for switching, >1 GHz for RF / video amplifiers.
- Package
- SOT-23 / SOT-323 / SOT-363 for small signal; SOT-89 / SOT-223 / TO-126 for medium power; TO-220 / TO-247 for high-power discrete.
What Magnias offers
Magnias BJT line covers all the classic small-signal numbers (BC817 / BC807 / BC847 / BC857 in SOT-23 and SOT-363 dual configurations), digital transistors with integrated base resistors, plus higher-power 1-3 A parts in SOT-89 / SOT-223.
Common questions
BJT or MOSFET for switching?
MOSFET wins above ~500 mA — lower conduction loss, no continuous base current. BJT wins below 100 mA where simplicity and low cost matter more. The boundary is fuzzy in the 100-500 mA range.
Why use a digital transistor (with built-in resistor)?
Saves one resistor on the BOM and gives controlled drive directly from a logic output. The trade-off is fixed input characteristic — fine for ON/OFF switching, not for analog.
What's beta-droop and when does it matter?
hFE drops at high collector current. A BJT that has hFE=200 at 10 mA might only have hFE=50 at 1 A. Always check the hFE-vs-IC curve at your actual operating point.