We can use this analogy to determine whether a transistor is of the PNP type or NPN type by testing its Resistance between the three different leads, Emitter, Base and Collector. We saw in the first tutorial of this transistors section, that transistors are basically made up of two Diodes connected together back-to-back. This allows the amplifier to drive the required power through the load loudspeaker in both directions at the stated nominal impedance and power resulting in an output current which is likely to be in the order of several amps shared evenly between the two complementary transistors. The NPN transistor conducts for only the positive half of the signal while the PNP transistor conducts for negative half of the signal. They both have a DC current gain, Beta, ( Ic/Ib ) matched to within 10% and high Collector current of about 15A making them ideal for general motor control or robotic applications.Īlso, class B amplifiers use complementary NPN and PNP in their power output stage design. Well, having two different types of transistors “PNP” and “NPN”, can be a great advantage when designing power amplifier circuits such as the Class B Amplifier.Ĭlass-B amplifiers uses “Complementary” or “Matched Pair” (that is one PNP and one NPN connected together) transistors in its output stage or in reversible H-Bridge motor control circuits where we want to control the flow of current evenly through the motor in both directions at different times for forward and reverse motion.Ī pair of corresponding NPN and PNP transistors with near identical characteristics to each other are called Complementary Transistors for example, a TIP3055 (NPN transistor) and the TIP2955 (PNP transistor) are good examples of complementary or matched pair silicon power transistors. You may think what is the point of having a PNP Transistor, when there are plenty of NPN Transistors available that can be used as an amplifier or solid-state switch?. To cause the Base current to flow in a PNP transistor the Base needs to be more negative than the Emitter (current must leave the base) by approx 0.7 volts for a silicon device or 0.3 volts for a germanium device with the formulas used to calculate the Base resistor, Base current or Collector current are the same as those used for an equivalent NPN transistor and is given as. The Base voltage V B which is biased negative with respect to the Emitter and is connected to the Base resistor R B, which again is used to limit the maximum Base current. This time the Emitter is connected to the supply voltage V CC with the load resistor, RL which limits the maximum current flowing through the device connected to the Collector terminal. The voltage sources are connected to a PNP transistor are as shown. So for a PNP transistor to conduct the Emitter is always more positive with respect to both the Base and the Collector. The voltage between the Base and Emitter ( V BE ), is now negative at the Base and positive at the Emitter because for a PNP transistor, the Base terminal is always biased negative with respect to the Emitter.Īlso the Emitter supply voltage is positive with respect to the Collector ( V CE ). The construction of a “PNP transistor” consists of two P-type semiconductor materials either side of an N-type material as shown below. In other words for a PNP transistor, the Emitter is more positive with respect to the Base and also with respect to the Collector. Then, PNP transistors use a small base current and a negative base voltage to control a much larger emitter-collector current. The main difference between the two types of transistors is that holes are the more important carriers for PNP transistors, whereas electrons are the important carriers for NPN transistors. This produces a Positive- Negative- Positive type of configuration, with the arrow which also defines the Emitter terminal pointing inwards in the PNP transistor symbol.Īlso, all the polarities for a PNP transistor are reversed which means that it “sinks” current into its Base as opposed to the NPN transistor which “sources” current through its Base. Basically, in this type of PNP transistor construction, the two interconnected diodes are reversed with respect to the previous NPN transistor.
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