NPN Transistor , which was discussed in the previous tutorial, has an exact opposite which is the PNP Transistor . The configuration of the two diodes in PNP transistor is in the reversed configuration compared to the NPN type. Hence, the Emitter terminal, which is identified by the arrow, is pointing inward in the transistor symbol. The polarities are also reversed which indicates that PNP transistors “sink” current while NPN transistors “source” current. Minimal amount of base current is used by PNP Transistors as well as negative base voltage to control the large amount of emitter-collector current. The structure of PNP transistor is composed of two P-type semiconductor materials which is placed on both sides of the N-type material as shown in the figure below.
A PNP Transistor Configuration
Note: Conventional current flow.
On the other hand, PNP Transistor has also some similarities to its counterpart NPN type aside from the polarities and biasing of the current as well as the voltage directions which are of opposite path as compared from the three possible configurations in the first tutorial which are the Common Base configuration, Common Emitter configuration and Common Collector configuration. Normally, negative voltage (-v e ) is needed at the collector terminal of the PNP transistors and as a result the flow of the current through the emitter-collector junction are determined by the Holes which is in contrary to the Electrons of the NPN type. That makes PNP transistors slower compared to its opposite type NPN in terms of its operation because the movement of holes across the depletion region tends to be slower than that of the electrons.
The Base terminal of a PNP transistor should be more negative than the Emitter terminal by approximately 0.3 volts for a germanium material and 0.7 volts for a silicon material to set off the Base current to flow. The formulas below best illustrates this operation which is also of the formulas used for the NPN transistor in finding the Base resistor, Base current or Collector current.
In reality, PNP transistor can be used as a replacement for the NPN transistor in electronic circuits though the difference would be on the polarities of the voltages and the direction of the current flow. PNP transistors can also act as a switching device as shown in the figure below.
A PNP Transistor Circuit
PNP and NPN transistors have similar appearance in terms of its Output Characteristics Curves . However the former are rotated by 180 o to take effect the reverse polarity of the voltages and currents. Take note that the currents flowing out of the Base and Collector terminal of a PNP transistor are negative.
Transistor Matching
What could be the possible advantage of having PNP Transistor wherein there are numerous NPN Transistors available in the market? As a matter of fact, having these two opposite types of transistors, PNP and NPN, can be of major advantage specially in designing amplifier circuits such as Class B Amplifiers which use a matched pair or complementary transistors or for reversible H-Bridge motor control circuits. Complementary Transistors are a pair of PNP and NPN transistors with almost the same characteristics such as matched pair silicon power transistors which specifically use a TIP2955 (PNP) and TIP3055 (NPN) pair. They are very suitable for a wide range of motor control or robotic applications since high Collector current of about 15A and Beta which is the DC current gain (I c / I b ) is matched to within 10%.
Identifying the PNP Transistor
Transistors are mainly fabricated by using two Diodes connected together one after the other as what have been presented in the first tutorial of the Transistors section. Resistance checking between the three different terminals, Emitter, Base and Collector can be done to determine whether the transistor is PNP or NPN type. By testing each pair of transistor terminals in both directions will result for a total of six tests with the expected resistance values in Ohm's given below .
1. Emitter-Base Terminals – The Emitter-Base junction should conduct in one way only just like a normal diode.
2. Collector-Base Terminals – The Collector- Base junction should conduct in one way only just like a normal diode.
3. Emitter-Collector Terminals – The Emitter-Collector junction should not conduct in either direction.
