Why is a transistor so useful


Currents and voltages in bipolar transistors¶

In the following figure, the currents and voltages relevant for a transistor are explicitly for the more common transistor variant, namely one -Transistor drawn in: [4]

For the tension between base and collector and the base current Basically the same relationship applies as between voltage and current across a diode. From a certain limit value (with normal transistors around ) the base current increases with increasing tension very quickly. In order to be able to control a transistor, the base current has to be deliberately limited. [5]

Currents at one -Transistor

For the relationship between the currents and a constant gain factor is sufficient for many applications to accept; this value can usually be taken from the data sheet of the transistor. A gain factor of means, for example, that a base current of with a strength of a collector-emitter current has the consequence. What happens now, however, if the connection of the collector is suddenly cut with an otherwise unchanged circuit? It can then no longer flow into the collector.

You can see that in this case the current through the base also changes: The base-emitter path and the collector-emitter path form a common diode, but with a fixed current divider. If one side of the current divider is not supplied with power, all of the current must flow through the other side. If the circuit is otherwise unchanged, there will be no more flow through the base, but suddenly . The gain factor rather describes the emitter current that occurs, which is usually divided up in such a way that only is drawn from the base, and the rest of the electricity comes from the collector (as long as electricity is available there).

Tensions in one -Transistor

The value of tension along the collector-emitter path depends on how the transistor is used.

As the above example shows, a bipolar transistor should be understood less as a variable resistor and more as a controllable current source: whereas with a potentiometer the resistor (so the ratio can be regulated from the applied voltage and the resulting current intensity, with a bipolar transistor only the current intensity can be regulated actively regulated; the corresponding tension along the -The transistor adjusts the distance automatically.

Component fluctuations

In the case of a bipolar transistor, as described in the last section, a load current is generated by means of a control current controlled. The gain factor , which indicates the ratio of these two currents, shows, however, considerable differences from component to component even with the same transistor types.

The gain factor can be adjusted using (mostly cheaper) multimeters of a transistor, as they have a built-in function. In practice, this function is not often used, for example because of the gain factor is strongly frequency-dependent (transistors usually amplify better at low frequencies than at higher frequencies). A single numerical value is therefore not sufficient to characterize a transistor. Since such a measuring circuit is quite simple, a gain measuring function (often also referred to as) can be marketed well as an additional feature.

The BC547 transistor, which is often used as a standard in Europe, is available in three amplifier classes, for example: , and . The following value ranges for the gain factor for the BC547 transistor can be obtained from a data sheet remove:

  • :
  • :
  • :

Because the specific value the current amplification factor can vary, transistor circuits should as far as possible be designed in such a way that they are insensitive to deviations in this parameter.

Characteristic fields of transistors

In order to be able to plan the behavior of a transistor in a circuit, one should have a rough value for the current gain factor , the maximum permitted collector current , the maximum collector-emitter voltage as well as the maximum power loss know.

However, if you don't want to operate a transistor as a switch but as an amplifier, individual values ​​are often not sufficient to characterize a transistor. Much more useful are so-called "characteristic fields" in which the collector current as a function of the collector voltage is specified. This relationship depends on the current strength through the base of the transistor, so that there is not just one, but several characteristics in a family of characteristics.

Characteristic field of a transistor (source: Datasheetcatalog): collector-emitter current as a function of .

All characteristics have (regardless of the base current ) common that the electricity equals zero when there is no voltage between the collector and the emitter. The greater the tension becomes, the greater the current will be through the transistor. The value of However, there is an upper limit, as saturation eventually occurs - this is synonymous with the transistor switching fully through. [6]

A second important characteristic is the collector-emitter current depending on the base-emitter voltage at. This is done for mostly a logarithmic scaling is chosen. The straight line that results in such a diagram corresponds to a (logarithmically scaled) diode characteristic.

Collector-emitter current as a function of the base voltage with a transistor (source: Datasheetcatalog)

Such a characteristic can be used to estimate which -Current occurs at a certain voltage applied to the base. You can also use this diagram to roughly estimate the size of the base current at a certain base voltage is by getting the collector current divided by the current gain factor (also estimated) of the transistor.

Really exact values ​​should not be expected if one is guided by the characteristics of a transistor; this would also hardly make sense, since the individual specimens of a transistor type, as described in the last section, can show considerable fluctuations.