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The basic characteristics of the bipolar transistor


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Disclosure of function of the transistor (from the English words TRANSfer resISTOR) is already indicated in its name - the transformation of resistance. Its discovery in 1947 created a revolution in electronics and all fields associated with it. Depending on which charge carriers participate in current conduction transistor distinguish transistors:

  1. bipolar, the essence of which is controlled by injection of minority carriers in the emitter base and subsequent extraction of carriers collector, wherein the current-conducting participate both charge carriers - electrons and holes.
  2. unipolar, whose operating principle is based on the interaction of the conductive channel between the emitter and the collector transverse electric field (Field Effect Transistor - FET), the current conduction in the channel is effected by one type of charge carrier (channel N - electrons, channel P - holes).
Bipolar transistor

The bipolar transistor is an electronic component formed by the three areas of semiconductors with different types of conductivity arrangements NPN or PNP, which form a pair of PN junctions. The transistor has three electrodes, and a base (used to control the transistor), the collector and emitter. The base is compared to other much thinner electrodes (up to 100 times). The emitter is compared to the collector, which has the same conductivity type doped more. The transistor is used as a signal amplifier or as a switch. It is also part of many other devices, such as logic circuits. Predominates the using of unipolar transistors in complex integrated circuits.

The transistor effect

The ability to amplify the power transistor is called so transistor effect, which is an essential characteristic of the transistor. After applying a positive voltage to a negative collector and the emitter electrode of the transistor only passes a very small current in order nanoampér (This current increases with temperature). To more current through transistor must be based on through resistor bring positive voltage. This will open a transition base - emitter (There must be at least a voltage of 0.7 V). For NPN transistor base current flows into the holes. Since the base is very thin and rapidly fills holes from the emitter to the electrons start flowing. Some progress to the positive pole of the source through the base, but the majority is tightened positive voltage at the collector of the transistor and thus will pass through the collector current, which is many times larger than the base current.

princip NPN tranzistoru

The higher is the base current, the higher is the collector current. This can be explained by the increasing base current increases as the number of free charge carriers and thus decreasing the resistance of the transistor, resulting in increase of the collector current while the voltage drop between the collector and emitter. If you drop the voltage to the level of a few tenths of a volt, we say that the transistor is saturated. In this state, it passes a relatively large current, with negligible power dissipation. Saturation state is used when the transistor operates as a switch. If we are at a constant base current to increase the voltage between the collector and emitter, collector current will initially grow rapidly and eventually stabilize at a nearly constant value, and the transistor begins to behave as a constant current source (see output characteristic curve).


Graphical representation of the characteristics

  1. The so-called output characteristics are plotted in the first quadrant of the graph characteristics. The size of the output collector current increases with the growth of the base-flow.
  2. Directive of so-called transfer (current) characteristic in the second quadrant shows the current gain coefficient β.
  3. The so-called input characteristics is shown in the third quadrant. Dependence of the input characteristics on voltage UCE is minimal, therefore frequently the input characteristic determined for a single value UCE.
  4. Graphic dependence of the input voltage on the output is shown in the fourth quadrant in the form of so-called reverse voltage transfer characteristic for different values of the base-current.
v3echny charakteristiky tranzistoru


Characteristics of the bipolar transistor

Input characteristic
vstupní charakteristika

Input characteristics show dependence on the base-current voltage between base and emitter. This is indicate to some tension between the collector and emitter. Their shape corresponds to the current-voltage characteristics of diodes, as measured voltage and current in a PN junction.

From these characteristics can be determined h11 parameter, that is the input resistance

h11 =   UB 

Output (collector) characteristic
vystupní charakteristika

The output characteristics of the dependency of the collector current on the voltage between collector and emitter, while a constant base current. With increasing current base increases the maximum achievable collector current as the base current resistance affects the entire transistor.

From these characteristics can be determined parameter h22, that is the output conductivity.

h22 =   IC 


The transfer (current) characteristic
přenosová charakteristika

The transfer characteristic specify the dependence the collector current to base current. It is reported to some tension between the collector and emitter. Using this feature you can define a parameter h21 (also  β), which is the current gain coefficient..

The current gain coefficient

The current amplification factor h21 (or β) is defined as the ratio of change of the collector current to a change in the base-current of output short, ie when zero voltage change UCE.

h21 ≡ β = (  ΔIC  ) UCE - konst. (1)

Reverse characteristics

These characteristics indicate the dependence of the voltage between the base emitter voltage between the collector and emitter, while a particular base current. Therefore describe backhaul voltage from output to input. From these characteristics can be determined hybrid parameter h12, it is reverse voltage transfer.

h12 =   UB 


Reverse characteristic is generally not measured, but constructed on the basis of the input characteristic.


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