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Emitter base voltage9/11/2023 ![]() ![]() ![]() A word of caution is in order at this time. In essence, we would have two junction diodes possessing a common base, thus eliminating any amplification and defeating the purpose of the transistor. If both junctions were forward biased, the electrons would have a tendency to flow from each end section of the N P N transistor (emitter and collector) to the center P section (base). The reverse-biased junction in an NPN transistor.Īt this point you may wonder why the second PN junction (base-to-collector) is not forward biased like the first PN junction (emitter-to-base). However, the minority current electrons (as you will see later) play the most important part in the operation of the NPN transistor. These minority carriers actually conduct the current for the reverse-biased junction when electrons from the P material enter the N material, and the holes from the N material enter the P material. The minority carriers for the reverse-biased PN junction are the electrons in the P material and the holes in the N material. As you recall, this current was produced by the electron-hole pairs. This current is called minority current, or reverse current. However, there is a very small current, mentioned earlier, that does pass through this junction. The second PN junction (base-to-collector), or reverse-biased junction as it is called (figure below), blocks the majority current carriers from crossing the junction. For each electron that fills a hole in the P material, another electron will leave the P material (creating a new hole) and enter the positive terminal of the battery. Since electrons are majority current carriers in the N material, they pass easily through the emitter, cross over the junction, and combine with holes in the P material (base). ![]() With the emitter-to-base junction in the figure biased in the forward direction, electrons leave the negative terminal of the battery and enter the N material (emitter). The forward-biased junction in an NPN transistor. For instance, notice the NPN transistor below: The letters of these elements indicate what polarity voltage to use for correct bias. A simple way to remember how to properly bias a transistor is to observe the NPN or PNP elements that make up the transistor. At the same time the second PN junction (base-collector junction) is biased in the reverse, or high-resistance, direction. For the transistor to function in this capacity, the first PN junction (emitter-base junction) is biased in the forward, or low-resistance, direction. To use the transistor as an amplifier, each of these junctions must be modified by some external bias voltage. The action at each junction between these sections is the same as that previously described for the diode that is, depletion regions develop and the junction barrier appears. Just as in the case of the PN junction diode, the N material comprising the two end sections of the NP N transistor contains a number of free electrons, while the center P section contains an excess number of holes. With this information fresh in your mind, let's proceed directly to the NPN transistor. ![]() This concept is the basic theory behind how the transistor amplifies. In this manner, a power gain would be obtained across the crystal. Thus, if a crystal were to contain two PN junctions (one forward-biased and the other reverse-biased), a low-power signal could be injected into the forward-biased junction and produce a high-power signal at the reverse-biased junction. By using the Ohm's law formula for power (P = I 2 R) and assuming current is held constant, you can conclude that the power developed across a high resistance is greater than that developed across a low resistance. In turn, a reverse-biased PN junction is comparable to a high-resistance circuit element. You should recall from an earlier discussion that a forward-biased PN junction is comparable to a low-resistance circuit element because it passes a high current for a given voltage. ![]()
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