on video What are the differences between BJT and JFET transistor?


 In this lesson, I will explain you the main differences between BJT and JFET transistors used in switching and amplifying in electronic circuits. As a figure, eventhough these transistors are very similar to each other, The fields of their usage and working principles are different. Let's have a look at these differences.

Both transistor types have three pins. In the  BJT transistor, these are called Base, Collector and Emitter, while in the  JFET transistor, they are named Gate, Drain and Source. In both transistor types, the locations of the terminal names may vary depending on the model.

The symbols are shown as follow  for both transistor types, terminal  naming and abbreviations are shown as follow in the figure.

In the BJT transistor, the current between the Collector and Emitter is controlled by Base current, while in JFET transistor, the current between Drain and Source is controlled by  Gate voltage.  BJT transistors are two types, NPN and PNP, while JFET transistors are two types
For instance, when we look at the datasheet of the commonly used  BC546 / 547/548 BJT transistor, we can reach the information of NPN   type transistor and pin names.  Here we need to consider the surface of transistor where the names are written while determining the pins. We identify the pins according to the names corresponding to the numbers 1, 2 and 3. For example, in this BJT transistor, the pin on the left corresponds to the Collector, in the middle  is the Base and the pin on the right corresponds to Emitter.  We can also find the maximum operating parameters here and a lot of information about  electrical characteristics.

Likewise, in the datasheet of the commonly used  2N5638 JFET transistor
 If we list the main differences between the two transistors; While BJT provides current control with current, JFET provides current control with voltage. While the production cost of BJTs is low, the production cost of JFETs is higher. While BJTs are affected by temperature quickly, JFETs are less affected by temperature. As such, BJTs deteriorate quickly, while JFETs are more durable and long-lasting. Therefore, BJTs are generally preferred in low current hobby electronic applications, while JFETs are mostly preferred in low voltage commercial applications. In addition, while the switching speed of BJTs is low, JFET's switching speed is high.

In JFET, Drain current is found as shown in the formula : ID = "ID" SS. (1- "VGS" / "VP") 2.  Here, while the IDSS is the maximum Drain current that transistor can withstand, VP  is the critical voltage value. These values are also constant values. From this formula, we can see that the Drain current directly depends on the VGS voltage value. While the Collector current controlled in BJT directly depends on  Base current, the Drain current controlled in JFET directly depends on the voltage  that is connected between Gate and Source.  It is assumed that there is a Silicon diode between Base and Emitter in BJT. A voltage value of approximately 0.7V is required for the silicon diode to be transmitted. Therefore, this voltage is considered as 0.7V while in transmission.  In JFET, since the internal resistance at the Gate input is very high, Gate current is always considered zero. There is no current input from the gate pin. The working principle of transistors can basically be explained in this way.

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