![]() Make sure you adjust accordingly.Ī history note. In the PNP version, a logic LOW turns on the transistor. ![]() In the NPN configuration, a logic HIGH turns on the transistor. The catch is, when you change sexes, you also change logic. If your application is for logic switching, all you need to do is exchange the emitter and collector in your design so that the forward/reverse bias rule is maintained, as shown in the two bottom circuits. (Electron and hole mobility are not equal, especially at higher frequencies.) Please note, I said most - not all - amplifiers will work with this exchange. ![]() And that means if you have mixed sexes, each and every transistor has to have a sex change. Bottom line, most small-signal amplifiers will work equally well if you replace an NPN with a PNP and reverse the power supply polarity. Substituting a PNP in the circuit reverses the current flow through the base-emitter diode and the voltage on the collector. The amount of influence is called the gain of the transistor, or hFE. The current through the collector-emitter path is controlled by the current flowing through the base-emitter junction. The breakover voltage of this diode is the VCE parameter listed on the spec sheet, and varies from one transistor type to another. The collector (C), on the other hand, goes to +V - in effect, reverse biasing that diode. This forward-biases the base-emitter diode, which exhibits the characteristic 0.7 volts voltage drop. The base (B) goes to +V via the Rb resistor. In the NPN configuration, the emitter (the lead that looks like the arrow of a diode, labeled (E) goes to negative (ground). ![]() The polarity of the power supply is reversed. Notice that both circuits are identical - except for one thing. On the left is an NPN (negative-positive-negative) transistor and on the right is a PNP (positive-negative-positive) transistor. Let’s take the common-emitter amplifier as shown below, for example. For a transistor to work, one diode is forward biased and the other is reverse biased. NPN and PNP transistors are interchangeable if you remember one simple rule: A bipolar transistor is essentially two back-to-back diodes with the base being the common connection. Both types can be used as amplifiers, switches, or in other electronic circuit configurations, depending on the specific requirements of the circuit design.Can an NPN transistor be wired as a PNP transistor? Is it a matter of reversing connections? It’s important to note that while the basic principles and behavior of NPN and PNP transistors are different, their applications and usage can be similar. In a PNP transistor, the base current is also an input current, but the polarities of the collector and emitter currents are reversed compared to an NPN transistor. In an NPN transistor, the base current is typically an input current, and both the collector and emitter currents are output currents. Current Polarities: The currents in NPN and PNP transistors have opposite polarities. In contrast, a PNP transistor is in an “on” state when a negative voltage is applied to the base-emitter junction, enabling current flow from the emitter to the collector.Ĥ. In an NPN transistor, it is in an “on” state when a positive voltage is applied to the base-emitter junction, allowing current to flow from the collector to the emitter. Transistor Operation: NPN and PNP transistors have opposite modes of operation. In a PNP transistor, the base-emitter junction is forward-biased, and the base-collector junction is reverse-biased.ģ. In an NPN transistor, the base-emitter junction is forward-biased (positive voltage applied to the base with respect to the emitter), while the base-collector junction is reverse-biased (negative voltage applied to the base with respect to the collector). Biasing: NPN and PNP transistors require different biasing configurations. The majority charge carriers (electrons for NPN, holes for PNP) determine the direction of current flow.Ģ. Polarity of Current Flow: In an NPN transistor, the current flows from the collector (C) to the emitter (E), and in a PNP transistor, the current flows from the emitter (E) to the collector (C).
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