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Bipolar transistor is electronic components with three terminals and two PN junctions.

Bipolar transistor has three electrodes: emitter, base and collector. It has capability of amplifying both current and voltage signals. Bipolar transistor consists of three differently doped semiconductor regions. Base region is with opposite type of carriers from emitter and collector region. So if major carriers in base are electrons, bipolar transistor is of PNP type. If major carriers in base are holes, bipolar transistor is of NPN type.


In BJT symbol, small arrow symbolizes emitter. Direction of this small arrow represents direction of the emitter current, meaning that in NPN transistor emitter current flow out of the transistor through emitter, while in PNP transistor emitter current flow into transistor through emitter.

Bipolar transistor is constructed in such a way that base region is very narrow and it lay between emitter and collector region. Depending on polarization, bipolar transistor can operate in either:

  • Active mode (forward active)
  • Saturation
  • Cut-Off
  • Reverse-active

I-U characteristic of conducting bipolar transistor is given in figure bellow:


Active mode is when collector-base PN junction is reverse biased and base-emitter is forward biased. In this mode, current amplification is the highest and it is characterized with hFE (β) parameter, that is usually in range from one hundred to several hundreds times for signal transistors. In this mode collector current is hFE times higher than base current IC=hFE•IB. Electrical model of NPN BJT in active mode is given in Figure bellow. For PNP BJT electrical model is the same, but with opposite voltage polarity and opposite direction of collector current.


Saturation is when both PN junctions are forward biased. In this mode, collector current is only few times higher then base current, and if saturation is deep, base current can even be higher then collector current. This mode correspond to “ON” state of electric switch. Since voltage drop in collector branch is close to voltage supply, collector to emitter voltage is close to zero (0.2V). Electrical model of active mode is not valid for saturation. Electrical model of BJT in saturation is given in figure bellow:


Cut-Off is mode when both PN junctions are reverse polarized or not polarized (not sufficient voltage to forward bias base emitter PN junction). This mode corresponds to “OFF” state of electric switch, since all currents are negligible.

Reverse-active is when collector-base PN junction is forward biased and base emitter PN junction is reverse biased (just opposite to active mode). In this mode, hFE is with value from few hundreds to few tens of times lower then hFE in active mode (with value of typically from 1 to 50). This mode is used only in input stage TTL logic circuit and nowhere else.

Bipolar transistors are applied in electronic schemes as amplifiers, parts of digital logic circuits (TTL technology) and electronic switch.


Bipolar transistors are widely used in many schemes, such as:

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