Transistors

media type="custom" key="2690097" toc The transistor will likely go down as one of the greatest in modern history… A transistor is a tiny electronic device to control the flow of electricity. It has two key abilities: the first to amplify an electric signal, and the other is to switch on or off (1 or 0), letting current trough or blocking it as necessary.
 * 1.5 Process components**
 * Introducing Transistors**


 * Transistors** are ** current amplifiers **, and Op Amps are voltage amplifiers.

There are two types of standard transistors, **n-p-n** and **p-n-p**, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Within this course we shall be using n-p-n **silicon transistors**. Below are some examples of common transistors The three leads of the transistor are called the **collector** **(C)** which is the positive lead, **base (B)** which is the lead responsible for activating the transistor and **emitter (E)** which is the negative lead. The leads are connected inside the metal body of the transistor to a small piece of silicon made, rather like a sandwich, from a single crystal with diffused layers of **//n-type//** silicon and **//p-type//** silicon. Transistors must be connected the correct way around in a circuit or they will be damaged. in order to establish which lead of the transistor is the collector or the base or the emitter, an underside view of the transistor is used. The circuit diagram symbol is used together with the underside view to determine the leads, for example as shown below: Not all transistor have the same underside with regards to the positioning of the emitter, collector and base leads and vary from one transistor to another, therefore it is important to check the manufacturers details.

Transistors have three leads which __must__ be connected the correct way round. Please take care with this because a wrongly connected transistor may be damaged instantly when you switch on.

= The transistor as a switch = Set up the following circuit:
 * Task 1**
 * 1) Be careful to connect the three terminals of the transistor correctly. Does the bulb L1 glow? Does the bulb L2 glow?
 * 2) Unscrew bulb L2. What happens to the bulb L1?

When the circuit is initially set up, bulb L1 glows quite brightly showing there is a good flow of current through the transistor and the bulb L1. But bulb L2 does not glow. There are two possible reasons for this: If there is no current flowing through the bulb L2 then unscrewing it and taking it out of its holder ought not to effect the circuit, but this does not happen - when bulb L2 is unscrewed bulb L1 goes out. This is because when a very small current flows through the base circuit of the transistor a much larger current can flow through //**between**// the collector and the emitter. But when there is no current flowing in the base circuit there can be no current flowing between the collector and the emitter. Therefore we can say that the base current is //**switching**// the collector-emitter current on and off.
 * 1) there is no electricity flowing in this part of the circuit. Or
 * 2) there is current flowing in this part of the circuit, but it is too small to make the bulb glow.

= The transistor as an amplifier = Using the same circuit as before but replacing the bulb L2 for a 47k variable resistor. When the value of the variable resistor is altered, this effects the size of the current flowing in the base circuit of the transistor, which then effects the size of the much larger current in the collector-emitter circuit. This means that the transistor is amplifying the changes in the base current. = Switching by light = Set up the following circuit: When the circuit is first set up, light is reaching the LDR and therfore its resistance is low. there is almost no current flowing in the base circuit of the transistor - consequently the collector-emitter circuit is switched off and the bulb L1 does not glow. When the LDR is covered up, no light reaches it and therefore the resistance is high, because there is now current in the base circuit, the collector-circuit is switched on and bulb L1 glows.
 * Task 2﻿**
 * 1) Very slowly alter the variable resistor. What happens to the brightness of bulb L1?
 * Task 3**
 * 1) Cover the LDR and observe what happens to bulb L1

This kind of circuit could be used as an automatic parking light. During the day the daylight switches the transistor off, but at night the lack of light switches the transistor on and the bulb glows. Swap the position of the 10k resistor and the LDR.
 * 1) Cover the LDR. What happens?
 * 2) Uncover the LDR. What happens?
 * 3) What might the circuit be used for?

= Switching by temperature = Set up the following circuit: Heat the thermistor (using a container of hot water) Alter the circuit as shown below: Heat the thermistor
 * Task 4 **
 * 1) What happens to bulb L1?
 * 2) Now try to explain how this circuit works and how it could be used as a fire alarm.
 * 1) What happens to the bulb L1
 * 2) What could this circuit be used for?

Set up the following circuit:
 * Task 5**
 * Investigate the effect of putting a capacitor in the base circuit**

Close the switch. Discharge the capacitor by closing the switch for 30 seconds. When the capacitor is fully discharged - close the switch again and use your stop watch to time how long the bulb L1 glows for. Now explain in detail what happens when the capacitor is being charged up and what effect this has.
 * 1) What happens to the bulb L1?
 * 1) Repeat the above with a 220µF, 470µF, 1000µF, 2200µF capacitor
 * 2) Draw a table to chart your results
 * Capacitor ||  || Resistor ||   || Time ||
 * 1000µF ||  ||   ||   ||   ||
 * 220µF ||  ||   ||   ||   ||
 * 470µF ||  ||   ||   ||   ||
 * 2200µF ||  ||   ||   ||   ||

Complete the following questions:
 * Task 6**
 * 1) A junction transistor has three terminals or legs. These are called the, the _, and the _.
 * 2) If no current flows into the __, the transistor is switched__.
 * 3) If current flows into the _, the transistor is switched __.__
 * 4) __What are the two main uses of a transistor.__
 * 5) __Name two types of transistors__ and __.

_ = More about transistors as amplifyers = = = The function of transistors is to **amplify current**, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, warning alarm or motor. 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** You can ignore this rating in low voltage circuits. ||
 * **IC max.** || Maximum collector current. ||
 * **VCE max.** || Maximum voltage across the collector-emitter junction.
 * **hFE** || This is the **current gain** (strictly the DC current gain). The guaranteed minimum value is given because the actual value varies from transistor to transistor - even for those of the same type! Note that current gain is just a number so it has no units. ||

= Resources = Here is a link to the Open University [] This site has some calculators for using the above equations along with some explanations, just ignor the title of the blog [] More calculators: []

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Conduct an investigation through the following excellent site and lean all about transistors, how they are used, the science, the history, the inventors and its future in electronics. There are many good sites which give information about transistors try some of these links:

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

= References = []