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How does Bipolar Transistor operate? – Definition and Tutorial

Bipolar Transistor

bipolar transistor

Bipolar transistor – three-ended (three electrodes), current-controlled semiconductor electronic component, which has the ability to amplify the signals of direct current and alternating current, so every transistor belongs to the amplifiers family. The amplifier is a device, which can control more power with usage of less power.

There are two types of bipolar transistors: N-P-N transistors and P-N-P transistors. Electrodes of the bipolar transistor have the following names: – C – collector, B – base, E – emitter. Silicon Si transistors are most often used (Threshold Voltage VT = 0.6 – 0.7V), less common are germanium Ge (VT = 0.2 – 0.3V). Transistors are used almost everywhere: from amplifiers, generators, power switching systems to computers and more advanced systems.


Bipolar Transistor – Tasks for students

If you are a student or simply want to learn how to solve bipolar transistor tasks, please visit this section of our website where you can find a wide variety of electronic tasks.


Bipolar Transistor – Construction

Bipolar transistor consists of three semiconductor regions with different types of conductivity: N-P-N or P-N-P. In this example, two p-n junction forms (diodes): Base-Emitter (BE) and Base-Collector (BC).

npn bipolar transistor symbol

Fig. 1. NPN bipolar transistor symbol and the construction of its junctions

pnp bipolar transistor symbol

Fig. 2. PNP bipolar transistor symbol and the construction of its junctions

Diode replacement models of transistors

Fig. 3. Diode replacement model of NPN transistor

pnp Diode replacement models of transistors

Fig. 4. Diode replacement model of PNP transistor

Distribution of currents in NPN transistor

Fig. 5. Distribution of currents in NPN transistor

Bipolar Transistor – Principle of operation 

Main feature of bipolar transistors is the possibility to control a high current with usage of small one. Depending on its operation point, transistor might be in four modes of operation:

  • Cut-off mode– Base-Emitter junction is not biased at all or it is reverse-biased. The collector’s current values are very small,
  • Forward-active mode (most often called active mode) – Base-Emitter junction is forward-biased and the Base-Collector junction is reverse-biased. Here, it is worth noting to not exceed the voltage of the junction (silicon or germanium diodes), which could result in the flow of large base current and possible damage to the transistor. Collector current takes a value of β times increased of the base current value. Base-Emitter voltage inject majority carriers from the emitter through the junction to the base – (in N-P-N electrons and in P-N-P holes). Carriers injected from the emitter into the base region (float) (the phenomenon of diffusion) into the region of Base-Collector junction region where their concentration is lower). Here, under the impact of the electric field in the depletion region they are attracted to the collector. As a result of these operations, a small current shall flow between the base and emitter allowing greater current to flow between the collector and emitter electrodes.
  • Reverse-active mode (inverted mode) – Base-Emitter junction is reverse biased and Base-Collector is forward biased. Current amplification is small,
  • Saturation mode – Collector-Emitter Voltage drops to the small amount. Base current is so large that the collector circuit cannot amplify it β times more.

Bipolar Transistor – Current-voltage characteristics

bipolar transistor characteristics

Fig. 6. Family of current-voltage characteristics of the bipolar transistor (OE)

bipolar transistor characteristics ob

Fig. 7. Family of current-voltage characteristics of the bipolar transistor (OB)

These regions of the transistor are commonly used according to need for example:

  • Transistor as an amplifier – transistor operating in a forward-active region might be used to construct the system, which will amplify electrical current.
  • As a switch (valve) – here the transition between saturation region (on) and cutoff (off) is used. It is used in digital and pulse circuits.

Bipolar Transistor – Limiting parameters 

  • VEB0maxthe maximum permissible Base-Emitter reverse bias,
  • VCB0max– the maximum permissible Base-Collector reverse bias,
  • VCE0max– the maximum permissible Base-Emitter forward bias,
  • ICmax– the maximums collector current,
  • IBmax – the maximum base current.

Bipolar Transistor – Operation systems

The system of common collector

Amplified voltage of the input signal is put between the base and collector of the transistor, whereas the signal after amplification is received between collector and emitter. Voltage amplification of this circuit is close to unity, so the output of the amplifier receives “repeated” voltage from the input hence the second commonly used name of this amplifiers – emitter.

amplifier system with common emitter

Fig. 8. The scheme of alternative current voltage of amplifier system with common emitter (OE)

The system of common base

Amplified voltage of the input signal is put between the base and emitter of the transistor, whereas the signal is received between the base and collector after amplification.

amplifier system with common base

Fig. 9. The scheme of alternative current voltage of amplifier system with common base (OB)

The system of common collector

Amplified voltage of the input signal is put between the base and emitter of the transistor, whereas the signal after amplification is received between the collector and emitter. Emitter electrode is therefore quite “common” for input and output signals – hence the name of the system.

OC common collector

Fig. 10. The scheme of OC common collector

Bipolar transistor as a switch

The bipolar transistor is suited to be operating as a switch. The principle of its operation is based on two operation states of the transistor: cut-off and saturation. Under the influence of the signal (voltage), the transistor is activated and goes from the cut-off state, through the active state, to saturation. When there is no more control voltage, the transistor returns to the cut-off state. While in cut-off state, transistor has a very high resistance, so it won’t pass any signal (it can be considered as a break in the circuit). However, when the transistor is saturated it has low resistance and the situation is opposite.

The ideal transistor switch should change states almost immediately and have a very steep (vertical) transient characteristic and the switching time should equal zero.

There are ways to significantly speed up the operation of the transistor switching process:

  • Decrease the value of the transistor’s base resistor,
  • Incorporate parallel capacitance to the transistor’s base resistor. It eliminates the integration effect and shortens the time of switching on the transistor,
  • Connect the base and collector of the transistor through a germanium diode (such switch becomes quasi-saturated), which is characterized by a higher saturation value and shorter switching times. Disadvantage of this system is the higher voltage value in the low state, because the transistor does not saturate.
  • Connecting the power supply to the base of the transistor.

Bipolar Transistor – Polarization systems

The most frequently encountered bias systems of the transistors are presented below:

system with potentiometric base power

Fig. 11. The system with potentiometric base power

system with forced base current

Fig. 12. The system with forced base current

system with coupling collector

Fig. 13. The system with coupling collector

system with potentiometric base power and coupling emitter

Fig. 14. The system with potentiometric base power and coupling emitter

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