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9.3 Transistor

Introduction

A transistor consists of a crystal of one type of doped semiconductor sandwiched between two crystals of the opposite type.

A transistor is a semiconductor device capable of amplification in addition to rectification.

It is the basic unit radio, television and computer.

Types of transistor

There are two types of transistor

(1) npn transistor

(2) pnp transistor

Structure ,schematic diagram and symbol of transistor

A transistor has tree leads; the leads are labelled

(1) Base (B) – Control the flow of charge carriers from E to C

(2) Collector (C) – Receive charge carriers from E

(3) Emitter (E) – Supply charge carriers to C

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Transistor circuit

A transistor cannot be working without combining with others electronic devices such as resistance and power supply (cell). The combination of three transistor , resistor and cell produced transistor circuit.

There are two types of the transistor circuits:

(1) Transistor circuit I (need two cells)

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BE: Base circuit (input)

CE: Collector circuitl(output)

Ib: base current

Ic: collector current

R1: to limit the base current

R2: to limit the collector current

E1: to supply energy to the base circuit

E2: to supply energy to the collector circuit

(2) Transistor circuit II (need one cell)

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Rx and RY : Voltage divider

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Example 1

The figure shows a transistor circuit. Resistor P has a resistance of 10 kW. In order to light the bulb , the potential difference across resistor P must be at least 2V.

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What is the maximum value of resistor Q when the bulb lights?

Solution

Working principle of a transistor

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(1) The base current is very small (in μA) when it compare with the collector current (in mA).

( Ib <<< D Ic )

Current amplification :

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(2) A small change in base current, DIb will cause a big change in the collector current, Ic

( ID b <<< D Ic)

(3) Ie = D Ib + Ic

From the working principles above , we conclude that a transistor functions as a current amplifier by allowing a small current to control a larger current.

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(4) When R1 = 0 , the base voltage VR1 = 0. The base current does not flow and the collector current als does not flow.

Ib = 0 and IC = 0

(5) When the resistance of R1 is increased, the base voltage will increase until the base voltage exceeds a certain minimum value, the base current flows and cause a large collector current flows.

From the working principles above , we conclude that a transistor functions as an automatic switch,so that the transistor turned ON or OFF.

(6) When there is no Ic flowing in the collector circuit , Ib still flows in the collector circuit.

(Ic = 0 hence Ib ¹ 0 )

(7) A transistor has not its own energy. The energy in a transistor is supplied by the power supply , such as cell.

Applications of transistors

(1) The transistor as an amplifier

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When a person speaks into a microphone, sound waves are converted into an alternating current .

The small changes in the base circuits cause the base current flows.

A small change in base current, will cause a big change in the collector current.

The earphone thus receives a large alternating current from the collector circuit and converts it into a loud sound.

The capacitor blocks a steady current (direct current) from flowing into the transistor and microphone.

(2) The transistor as a light controlled switch

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In bright light, the light-dependent resistor(LDR) has a very low resistance. Therefore the potential difference across LDR is low and hence the potential difference across resistor R is high. The base current flows

and cause a large collector current flows. The bulb lights up

In darkness , the light-dependent resistor(LDR) has a very high resistance. Therefore the potential difference across LDR is high and hence the potential difference across resistor R is low. The base current does not flow and cause the collector current does not flow. The bulb not lights up.

If the positions of the LDR and R are interchanged, the bulb is switched on in the dark and off in the bright light.

(3) The transistor as a temperature controlled switch

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When the thermistor is cold, it has a larger resistance than R. Therefore the potential difference across thermistor is high and hence the potential difference across resistor R is low. The base current does not flow and cause the collector current does not flow.

The bulb not lights up.

When the temperature rises,the resistance of thermistor falls and the bulb lights up.

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