Bipolar transistor

Bipolar transistor – three-ended (three electrodes), semiconductor electronic component having the ability to amplify the signals of direct current and alternative current, so every transistor is an amplifier. 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 UT = 0.6 – 0.7V), less common are germanium Ge (UT = 0.2 – 0.3V). Transistors are used everywhere: from amplifiers, generators, power switching systems to computers and more advanced systems.

Construction of bipolar transistor:

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 and the construction of its junctions

PNP bipolar transistor symbol and the construction of its junctions

Diode replacement model of NPN transistor

Diode replacement model of PNP transistor

Distribution of currents in NPN transistor

Operating principle of bipolar transistor:

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

• Cut-off – Base-Emitter junction is not biased at all or it is reverse-biased. The collector’s current values are very small,
• Forward active – 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, inverted – Base-Emitter junction is reverse biased and Base-Collector is forward biased. Current amplification is small,
• Saturation – Collector-Emitter Voltage drops to the small amount. Base current is so large that the collector circuit cannot amplify it β times more.

Basic mathematical formulas describing bipolar transistor:

Bipolar transistor formulas

Due to the small amount of zero current collector I_CE0 (about nA and pA), this parameter is ignored in the calculations.

α– current gain parameter,

β [hFE]– current gain parameter – the ratio of amount of carrier injected into the collector to the amount of carriers in the base,

gm [S – Siemens] – transconductance of the bipolar transistor representing its amplifying properties.

The characteristics of the bipolar transistor:

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

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.

Limit parameters of the transistor:

• U_{EB0max –}the maximum permissible Base-Emitter reverse bias,
• U_CB0max– the maximum permissible Base-Collector reverse bias,
• U_CE0max– the maximum permissible Base-Emitter forward bias,
• I_Cmax– the maximums collector current,
• I_Bmax – the maximum base current.

Bipolar transistor operating systems:

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

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

The scheme of OC common collector

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.

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.

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.

The polarization system of transistors:

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

The system with potentiometric base power

The system with forced base current

The system with coupling collector

The system with potentiometric base power and coupling emitter

Computing tasks:

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