If another P-type material or N-type material is added to a single PN junction, another junction will be formed. Emitter Follower & Darlington AmplifierĪfter knowing the details about a single PN junction, or simply a diode, let us try to go for the two PN junction connection.Transformer Coupled Class A Power Amplifier.The only difference is in their biasing and the polarity of the power supply for each type. The principle of operation of NPNs and PNPs is exactly the same. The basic principle of any BJT is to control the current of a third terminal with the voltage between the other two terminals. Here is a list of some classic general-purpose BJTs:
Here is a summary of the operation modes depending on their voltages: For example, to put an NPN into saturation mode, VB should be higher than VC and VE. PNPs and NPNs work pretty much alike, but their modes are different because of the current polarities. NPN transistors require a power supply with positive polarity with respect to common terminals, but PNP transistors require a negative power supply. It is clear that the current directions and voltage polarities in PNPs and NPNs are always opposite to each other. The NPN transistor always has an arrow pointing out. The arrow shows the direction of the current and how it is always on the emitter.ĥ. In a PNP transistor, however, the current flows from the emitter to the collector ( Fig. In an NPN transistor, the current flows from the collector (C) to the Emitter (E) ( Fig. In a PNP transistor, a positive voltage is given to the emitter terminal to produce current flow from the emitter to collector. In an NPN transistor, a positive voltage is given to the collector terminal to produce a current flow from the collector to the emitter. Reverse Active: Like active mode, the current is proportional to the base current, but flows in reverse.The transistor appears as a near short circuit between the collector and emitter terminals. Saturation: BJT operates in this zone in switching operations.Active: BJT operates in this zone for amplifier circuits because the transistor can act as a fairly linear amplifier.Cutoff: BJT operates in this zone in switching operations.An NPN transistor has a layer of P-doped semiconductor between two N-doped layers (Courtesy of Wikibooks) A PNP transistor has a layer of N-doped semiconductor between two layers of P-doped material (Courtesy of Wikibooks)ģ. BJTs can operate in different modes depending on the junction bias:Ģ. The junctions between N and P regions are similar to the junctions in diodes and they can be forward-biased or reverse-biased as well. 2) while the NPN transistor has one P region between two N regions ( Fig. The PNP transistor has one N region between two P regions ( Fig. The semiconductor physics of BJTs will not be discussed here, but it is worth mentioning that a BJT is fabricated with three separately doped regions with two junctions. The transistor symbol indicates the three terminals.The Base is responsible for controlling the transistor while the Collector is the positive lead, and Emitter is the negative lead. A transistor is an active device with three terminals, and these three terminals are known as the Emitter (E), the Base (B), and the Collector (C) ( Fig. BJTs are made of doped materials and can be configured as NPN and PNP. There are two main types of transistor: bipolar junction transistors (BJTs) and field effect transistors (FETs).
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