How to Connect a PNP Transistor in a Circuit

PNP Transistor

This article will show how you can connect a PNP Transistor in a circuit for switching or for amplification.

A PNP transistor is just like an NPN, only the voltages applied to the transistor are opposite polarities. We will go over the reasons for each voltage and why each is its polarity.

PNP Circuit

Below is the circuit for connecting a PNP transistor, which can function as a switch or an amplifier.

PNP transistor circuit

Notice that the voltage which is supplied to the emitter of the transistor is negative voltage. And positive voltage must be supplied to the base of the transistor. The collector is connected to ground. This setup allows the PNP transistor to turn on and function effectively as a switch or for amplification. Current flows from the collector of the transistor to the emitter and out from the emitter. Why these voltages and their respective polarities are needed in order for the circuit to work is explained in detail below. In order to explain it, we need to go over the internal construction of a PNP transistor.

How a PNP Transistor Works

A PNP transistor is a transistor which is made up of 3 regions, the base, emitter, and collector. The collector and emitter regions are made up of p-type material, which means they are predominately composed of holes. The base region is made up of n-type material. It is predominately composed of electrons.

Below is a visual diagram of the construction and composition of a PNP transistor.

PNP Transistor Composition

In order for current to flow from the emitter to the collector, the transistor must be biased in the following way, shown below:

PNP voltages

The negative voltage that is given to the collector region of a PNP transistor is reverse biased voltage. It is necessary in order to let current flow from the emitter to collector region. Note that positive and negative charges are attracted to each other. So the collector region, which is made of mostly holes, are attracted to the negative charge of the negative voltage and flow toward it. Without this negative voltage to the collector, current flow in a PNP transistor could not occur. This is why negative voltage must always be added to the collector of an PNP transistor.

Positive voltage is applied to the base of the transistor. When this occurs, the electrons of the PNP transistor are attracted to the positive voltage and flow to it and out of the base region. This depletes the base region of electrons, and the base region gets smaller and smaller, until the holes in the emitter region have enough escape power to power through the middle barrier into the collector region and then out of the transistor.

This is how a PNP works and why the circuit above allows it to function.

Related Resources

How to Connect a Transistor as a Switch in a Circuit
How to Connect a (NPN) Transistor in a Circuit

Types of Transistors

Bipolar Junction Transistors (BJTs)

Junction Field Effect Transistors (JFETs)

Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)

Unijunction Transistors (UJTs)

What is Transistor Biasing?

How to Test a Transistor