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Transistors Introduction • Transistors are : – Electronic switches – Amplifiers of electrical current • Many sensors (e.g. LDR, Thermistor, etc) only provide a very small output current or voltage. • To provide a practical working circuit, it may be necessary for a designer to amplify (e.g. make bigger) this current or voltage in order to drive an output device (e.g. Bulb, LED, Buzzer, etc). There are two types • Bipolar Transistors (NPN or PNP) • Field Effect Transistors (FET) Note … We have only got to know about Bipolar NPN Transistors. Transistors are semiconductor devices. An NPN transistor is made from three layers of n-type and p-type semiconductor material. Case Appearance Transistors come in a variety of cases: Transistor Leads • A transistor has three leads which are called: i. The Base (b) (responsible for activating the transistor) ii. The Collector (c) (The positive lead) iii. The Emitter (e) (The negative lead) • It is important that each of the leads is correctly identified E B C E B C Transistor circuit symbol How does a Transistor work? • For a transistor to work it has to receive a small voltage (e.g. 0.6 - 0.7 Volts) at its base. • This small voltage will result in a small electrical current to flow into the base and out of the emitter. • The resistance between the collector and emitter will fall. • A larger current will then flow from the collector to the emitter. Amplification of current • The amplification of a transistor is known as the GAIN. • Two current paths flow through a transistor: – The base current which is small – The collector current which is large • The gain of a transistor is found by dividing the collector current by the base current and is represented by the symbol hFE. Transistor Calculations Example 1 Calculate the gain of a transistor if the : collector current is 100mA when the base current is 0.5 mA Example 2 The hFE gain of a transistor is 40 when the collector current is 150 mA. Calculate the base and the emitter current A Transistor Circuit E G A F D B C Questions on the transistor circuit 1. Which of the components is the INPUT? 2. Which components are the output? 3. Which two components make up the potential divider? 4. What does component C do? 5. What is component D and how does it work? 6. What does component E do? Single transistor circuit Two transistor circuit • By adding a second transistor, the performance of the circuit is improved. • The second transistor benefits from the gain of the first transistor and the circuit becomes more sensitive to changes at the input. Darlington Pair • A common method of connecting two transistors is called a DARLINGTON PAIR or DARLINGTON DRIVER. The total gain is the gain of the first transistor multiplied by the gain of the second transistor. Two transistor circuit (Darlington Pair)