PNP Transistor

In this article of our series we will see PNP Transistors. This time, the arrow that identifies the emitter terminal points inward in the transistor symbol. In addition, all polarity of a PNP transistor is reversed. This means that unlike the NPN transistor, which "originates" the current from the base, it "sinks" the current into its Base. The main difference between the two types of transistors is that the holes are more important carriers for PNP transistors. Electrons, on the other hand, are important carriers for NPN transistors.

Next, PNP transistors use a small base current and a negative base voltage to control a much larger emitter-collector current. In other words, for a PNP transistor, emitter is more positive than base, as well as collector.

The structure of a "PNP transistor" consists of two p-type semiconductor materials on both sides of an N-type material, as shown below.

PNP transistor configuration

PNP Transistor

(Note: Defines emitter and conventional current flow as "in" for a PNP transistor.)

The structure and terminal voltages for an NPN transistor are shown above. The PNP transistor has very similar characteristics to its NPN bipolar cousins, but the polarity of current and voltage directions is reversed for any of the three possible configurations examined in the first tutorial. Common base, common emitter and common collector.

PNP Transistor
PNP transistor connection

The voltage between the base and emitter (VBE) is negative in the base and positive in emitter. Because for a PNP transistor, the base terminal is always negative biased relative to emitter.

In addition, emitter supply voltage is positive according to collector (VCE). Therefore, emitter transmission of a PNP transistor is always more positive than both the base and the collector.

Voltage sources connected to a PNP transistor are as shown. This time emitter, the load resistance RL, which limits the maximum current flowing from the device connected to the collector terminal, connects the supply voltage to the VCC. Base voltage, etc., which is negatively biased according to emitter and is also dependent on base resistance RB used to limit maximum base current.

To cause the base current to flow in a PNP transistor, the base must be more negative than emitter (the current must be separated from the base) with approximately 0.7 volts for a silicon device or 0.3 volts for a germanum device. The formulas used to calculate base resistance are the same as those used for an equivalent NPN transistor of base current or collector current.

PNP Transistor

We can see that the main differences between an NPN transistor and a PNP transistor are the correct biased of transistor junctions, as current directions and voltage polarities are always opposite. So for the circuit above: IC = Ie – IB must be separated from the base as current.

In general, the PNP transistor can replace NPN transistors in most electronic circuits. The only difference is the polarity of the voltages and the current flow directions. PNP transistors can also be used as switching devices. An example of a PNP transistor switch is shown below.

A PNP Transistor Circuit

The output properties for a PNP transistor are very similar to those for an equivalent NPN transistor, except that they are rotated by 180 degrees to take into account curves, reverse polarity voltages and currents (that is, the electron current for a PNP transistor flows out of the collector and towards the battery). The same dynamic load line can be drawn on I-V curves to find the working points of PNP transistors.

PNP Transistor
Complementary Transistors

When there are many NPN transistors that can be used as an amplifier or solid state switch, what do you think it means to have a PNP transistor?… Having two different transistor types "PNP" and "NPN" can be a great advantage when designing power amplifier circuits such as class B amplifiers.

It uses class amplifiers (if this is a connected PNP and PNP). B "Complementary" or "Compatible Pair" is used to control the current flow through the engine equally in both directions at different times to transmit to the output stage and reversible H-Bridge motor control circuits where we want to move back.

A pair of corresponding NPN and PNP transistors with almost identical characteristics are called complementary Transistors, for example, a TIP3055 (NPN transistor) and TIP2955 (PNP transistor) are good examples of complementary or matching dual silicon power transistors. Both have a DC current gain of 10%, Beta (Ic/Ib) and a high collector current of about 15a, making them ideal for general motor control or Robotic applications.

In addition, class B amplifiers use complementary NPN and PNP in the power output stage design. The NPN transistor transmits only the positive half of the signal, while the PNP transistor transmits the negative half of the signal. This allows the amplifier to maintain the necessary power from the load speaker in both directions with the specified nominal impedance and power, resulting in an output current that is likely to be in several rows of amps shared evenly between the two complementary transistors.

Identification of PNP Transistor

In the first tutorial of this section of transistors, we found that transistors consist mainly of two Diodes connected to each other in a row.

We can use this analogy by testing the resistance of a transistor between three different cables, emitters, bases and collectors to determine whether it is a PNP type or an NPN type. Testing each pair of transistors with a multimeter in both directions will lead to a total of six tests with the expected resistance values in the Ohms given below.

  1. Emitter-base terminals – must behave like a normal diode from emitter to base and follow only one path.
  2. Collector-base terminals – collector base connection should behave like a normal diode and follow only one path.
  3. Emitter-collector terminals – from emitter to collector, should not move in both directions.

Terminal Resistance Values for PNP and NPN Transistors

PNP Transistor
Terminal Resistance Values for PNP and NPN Transistors

We can then define a PNP Transistor as normally "off". However, according to emitterine, a small output current (B) in the base and negative voltage will make it "on" by allowing a much larger emitter-collector current to flow. It works when PNP transistors are much larger than And Vc.

In other words, the bipolar PNP transistor will only work if both the base and collector terminals are negative according to emitter

In the next tutorial on bipolar transistors, instead of using the transistor as an amplifier, we will look at how the transistor works in saturation and cutting zones when used as a solid state switch. Bipolar transistor switches are used in many applications to change a DC current from LEDs requiring only a few milliamper switching currents at low DC voltages, or from motors and relays that may require higher currents at higher voltages to "on" or "off".