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Lesson 13 – Transistors

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Lesson 13 – Transistors

As I introduce new terms, I have included a link to Wikipedia. Read ahead a little, and if you still need help you can click on any orange word below for more information than you probably want. 🙂


Basic Transistor Theory

A Transistor is a semiconductor device, that allows a small input signal to control a larger output signal. Because of that characteristic, a transistor is capable of acting as an amplifier. They are also used as switches. Many different types of transistors have been developed over the years. The most common types are the Bipolar Junction Transistors (BJT), and the Metal Oxide Semiconductor Field Effect Transistor (MOSFET), sometimes just called a Field Effect Transistor (FET).

The BJT is similar to a diode, but a diode has a single PN junction whereas a BJT transistor has two. The terms P and N refer to the type of materials used to make semiconductors. Most of them are made of silicon and some are made of germanium. The P-type and the N-type of materials that are used in manufacturing are “doped” with a small amout of various other materials to give it the desired properties.

There are two types of BJTs, an NPN transistor and a PNP transistor. They are basically the same but have opposite polarities depending on the “doping” used. Some transistors are discrete components, shown below in picture at the top left, but many more are found in integrated circuits (ICs), covered in the next lesson.

There are so many different types of transistors, I am showing some of the schematic symbols for them in the upper right drawing. Q1 is a PNP Bipolar Junction Transistor (BJT), Q2 is an NPN BJT, Q3 is an N-Chan Junction Field Effect Transistor (JFET), Q4 is a P-Chan JFET.

Q5 is an N-Chan Enhanced Metal Oxide Semiconductor Field Effect Transistor (MOSFET), Q6 is a P-Chan Enhanced MOSFET, Q7 is an N-Chan Depleted MOSFET, Q8 is a P-Chan Depleted MOSFET, Q9 is an N-Chan Unijunction Transistor (UJT), Q10 is a P-Chan UJT, Q11 is an NPN Darlington Transistor, and Q12 is Q12 is a PNP Darlington Transistor.

Wow, that was a mouthful! Since this course is written as a beginners course, It is not my intent to go over all the different types in detail, but rather just to give you a basic understanding of a simple Bipolar Junction Transistor (BJT). I will probably be writing a follow on course after this one that will cover Transistors, ICs, Microcontrollers and Microprocessors in more detail.

In a previous lesson, we learned that a diode is made up of a single PN junction. A transistor however, is made up of two PN junctions. Some transistors have a P-type material sandwiched between two N-types, and is therefore called an NPN transistor. The other type is a PNP type which we will discuss after we finish with the NPN.

Both types have 3 leads. In a Bipolar Junction Transistor, or BJT, they are abbreviated E, B, and, C which stands for Emitter, Base, and Collector. In the diagram above, the Emitter is shown on the bottom with the arrow pointing out. The Collector is at the top and the straight line in the middle is the Base.

The diagram above shows an NPN Bipolar Junction Transistor in what is called a Common Emitter configuration. It is named that because the emitter is common to both the base and the collector circuits. Look at how the two batteries are tied together at the emitter. There are also Common Base and a Common Collector configurations.

The voltage between the Emitter and the Base, called VBE, causes Base Current (IB) and Emitter Current (IE) to flow. That current, along with the Collector to Emitter Voltage (VCE), controls the amount of Collector Current (IC). The Emitter Current (IE) is equal to both the Base Current (IB), and the Collector Current (IC) because in the configuration both currents flow thru the Emitter.

A small change in Base Current results in a large change in Collector current. That is how a transistor can be used as an amplifier. Also the transistor acts like an insulator between the collector and the emitter unless the proper voltage is applied at the base. When the correct voltage is applied to the base, then the circuit between the emitter and collector becomes a conductor. That is how a transistor works as a switch.

The diagram below shows a PNP transistor. It’s the exact same circuit as the NPN transistor shown before but the materials used in the junctions consists of an N-type material sandwiched between two P-types. Because of that, the PNP transistor has the exact opposite polarity of the NPN transistor. Notice the arrow on the emitter points in, toward the base, and also both batteries are installed the opposite way, and the arrows indicating the direction of conventional current flow are reversed from the diagram above.

The two pictures below show what are called “heat sinks”. The one shown in top photo mounts right onto a transistor as shown, while those on the bottom has the transistor mounted onto the heat sink. They are used on transistors that carry more current to help keep them cool by removing some of the heat by transferring it to the air that surrounds it due to the “fins”.


Test your knowledge if you feel like it with a little test. No cheating now!

Q1: True or False….A PNP and an NPN transistor are opposites of each other in polarity.

In a Bilpolar Junction Transistor, wired in a Common Emitter Configuration, the Emitter Current is equal to ________________.

Q3: Transistors that draw more current may need to have a _________________ installed.


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