In this article, we will talk about a possible option for "powering up" the cascade on a bipolar transistor by connecting two or more transistors of the same type in parallel. The purpose of this inclusion is to increase the value of the maximum permissible operating current of the stage.
Simple "parallelization" of transistors does not, as a rule, give the expected positive result, since transistors even of the same type and from one batch of release, nevertheless, they have a certain spread of parameters, within the limits of production tolerances.
As a result, in practice it is not possible to achieve a uniform distribution of currents between each transistor. Moreover, this difference will increase with increasing current through the transistors and their heating. In the end, most of the load current will flow through one of the transistors, which will cause its failure, or incorrect operation of the entire stage as a whole.
Figure 1 shows a variant of switching on two (or more) transistors, which allows you to avoid such a mismatch.
For the best thermal stabilization both transistors should be installed on the same heat sink. The equalization of the operating currents is carried out by including resistors in the emitter circuits, the resistance of which is selected from the calculation of the voltage drop on each in the region of 0.7... 1 volt within the maximum value of the operating current cascade.
Power these resistors also depends on the amount of current flowing through them. For example, in audio frequency power amplifiers, the resistance values of such resistors are in the range of 0.1... 0.8 Ohm.
Moreover, the maximum collector current (total) two transistors will be allowed not two, but only one and a half times more than for one transistor. The diagram shows an example of turning on transistors of the p-n-p structure, of course, in the same way you can turn on transistors of a reverse structure. The gain of the stage according to such a scheme will correspond to the same value as for one transistor, since the base currents will be evenly distributed.
You can increase the efficiency of the cascade by turning on the transistors according to the circuit shown in Fig. 2. Such a circuit is similar to a composite transistor and contains a resistor that must be selected experimentally for each type of transistor used.
When picking this resistance it is necessary to achieve equal values of the collector currents with the maximum possible value of the gain of the entire stage. As a rule, this is achieved with a resistance R ranging from tenths to units of Ohm (for different types of transistors).
Magnification gain is achieved here due to the fact that the signal is successively amplified by the first and then the second transistor.
The same scheme can be applied in the case of using transistors of different conductivity.
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