Transistors

• Example of TransistorsThe transistor is one of the most important component used in electronics, even integrated circuits have many thousands of them within it. Many electronic products and their circuits rely on sensors in order to function, most sensors provide only a very small output current or voltage, it is necessary to amplify this current or voltage in order to drive an output device.

Transistors

A Transistor is an semiconductor which is a fundamental component in almost all electronic devices. Transistors are often said to be the most significant invention of the 20th Century. Transistors have many uses including switching, voltage/current regulation, and amplification - all of which are useful in renewable energy applications.

Function of Transistors

• Transistors amplify current, for example they can be used to amplify the small output current from a logic chip so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.

• A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).

• The amount of current amplification is called the current gain, symbol hFE

Types of Transistors

• Circuit Symbols

There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors. The leads are labeled base (B), collector (C) and emitter (E).

Transistors

• Transistors are manufactured in different shapes but they have three leads (legs). The BASE - which is the lead responsible for activating the transistor.The COLLECTOR - which is the positive lead.The EMITTER - which is the negative lead.

• These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels! They are not always set out as shown in the diagrams to the left and right, although the ‘tab’ on the type shown to the left is usually next to the ‘emitter’

How a transistor works

• A transistor controls a large electrical output signal with changes to a small input signal. This is analogous to the small amount of effort required to open a tap (faucet) to release a large flow of water. Since a large amount of current can be controlled by a small amount of current, a transistor acts as an amplifier.

• A transistor acts as a switch which can open and close many times per second.

• Try the following exercises using breadboards. Take photos and make notes of the results.

Simple Transistor Circuit

• Pictured below is a very simple circuit which demonstrates the use of transistors. When a finger is placed in the circuit where shown, a tiny current of around 0.1mA flows (assuming a finger resistance of 50,000 Ohms). This is nowhere near enough to light the LED which needs at least 10mA. However the tiny current is applied to the Base of the transistor where it is boosted by a factor (gain) of around 100 times and the LED lights!

Transistor Currents

• The diagram shows the two current paths through a transistor. You can build this circuit with two standard 5mm red LEDs and any general purpose low power NPN transistor (BC108, BC182 or BC548 for example). The small base current controls the larger collector current.

Transistor Currents

• When the switch is closed a small current flows into the base (B) of the transistor. It is just enough to make LED B glow dimly. The transistor amplifies this small current to allow a larger current to flow through from its collector (C) to its emitter (E). This collector current is large enough to make LED C light brightly.

• When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs are off.

• A transistor amplifies current and can be used as a switch. • This arrangement where the emitter (E) is in the controlling

circuit (base current) and in the controlled circuit (collector current) is called common emitter mode. It is the most widely used arrangement for transistors so it is the one to learn first.

Building Circuits with Transistors

Build the following circuits using prototype breadboards and take a photograph of each circuit you build with your name tag

•Transistor Currents

•Using a transistor as a switch

•Using a transistor switch with sensorsCorrectly draw a circuit diagram for each for further reference to these circuits go to the following site:

http://www.kpsec.freeuk.com/trancirc.htm#types

A Darlington Pair is two transistors connected together to give a very high current gain. In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page.

Who Invented the Transistor?

• In the mid 1940’s a team of scientists working for Bell Telephone Labs in Murray Hill, New Jersey, were working to discover a device to replace the then present vacuum tube technology. Vacuum tubes were the only technology available at the time to amplify signals or serve as switching devices in electronics. The problem was that they were expensive, consumed a lot of power, gave off too much heat, and were unreliable, causing a great deal of maintenance.

The Transistor

• The scientists that were responsible for the 1947 invention of the transistor were: John Bardeen, Walter Brattain, and William Shockley. Bardeen, with a Ph.D. in mathematics and physics from Princeton University, was a specialist in the electron conducting properties of semiconductors. Brattain, Ph.D., was an expert in the nature of the atomic structure of solids at their surface level and solid-state physics. Shockley, Ph.D., was the director of transistor research for Bell Labs.

Their original patent name for the transistor was: “Semiconductor amplifier; Three-electrode circuit element utilizing semiconductive materials.” In 1956, the group was awarded the Noble Prize in Physics for their invention of the transistor. In 1977, John Bardeen was awarded the Presidential Medal of Freedom.