9.3 Transistor.doc

SEK. MEN SAINS MUZAFFAR SYAH

197

A transistor has three leads connected to the emitter, base andcollector.

The emitter emits or sends charge carriers through the thin base layerto be collected by the collector.

There is two-type of transistor: npn transistor and pnp transistor. In an npn transistor the emitter sends negative electrons to the

collector. In an pnp transistor, the p-type emitter sends positive holes to the

collector. In both cases, the arrow on the emitter shows the direction of current

flow. The output current, of a transistor flows between the emitter and the

collector. The current in the collector lead is called collector current, IC.

The base current, IB is used to control the collector current throughthe transistor. The base current can be used to switch the collectorcurrent on or off.

1. Base current is too small compared to the collectorcurrent. The unit of base current is A while theunit for the collector current is mA.

( Ic >>>> IB )Current = Collector currentAmplication base current

2. Emitter current, IE is equal the sum of base currentand collector current’

IE

= IB

+ IC

Ie

> Ic

> IB

Transistor as acurrent amplifier

3. A small change in the base current, results in a big change in thecollector current, Ic >>>>Ib

Transistor as anautomatic switch

4. If there is no current flow in the base circuit, then there is also nocurrent flow in the collector circuit.IB = 0 then IC = 0 transistor is switch offIB flows then IC flows transistor is switch on

Describe a transistor in terms of its terminal

9.3


198

Activity 1: To find the relationship between the base current (IB) and collector current, (IC) in atransistor

Hypothesis: _________________________________________________________________________

Manipulated variable : _________________________ Responding variable : ___________________

Controlled variable : ____________________________

Apparatus : transistor, resistors of 2.2 kΩ, 3.9 kΩ, 4.7 kΩ, 6.8 kΩ, 8.2 kΩ and 10.0 kΩ, connectingwires, dry cells, milliammeters with range 0 – 1 mA and 0 – 100 mA

1. Switch S1 is open and switch S2 is closed.The readings of the ammeters are recorded.

2. Both switches S1 and S2 are closed. Thereadings of the ammeters are recorded.Repeat the experiment by replacing the 10.0kΩ in the base circuit with 8.2 kΩ, 6.8 kΩ,4.7 kΩ, 3.9 kΩ and 2.2 kΩ.

R / kΩ 2.2 3.9 4.7 6.8 8.2 10.0IB / mA 0.60 0.45 0.32 0.20 0.13 0.08Ic / mA 94.0 70.0 46.0 36.0 24.0 10.0

Plot graph of IC against IB.

Calculate the gradient of the graph.

Discussion

1. Which milliammeters measureBase current:Collector current :

2. When S1 opened, S2 closed :IB = ______ IC = _____

3. How would the readings of themilliammeter A1 and A2 compare?

4. What does the gradient of the graphrepresent?


199

Conclusion:A small increase in the ______ current results in a big increase in the ____________ current.

Potential divider circuit

Choose a suitable resistor R1 and a variableresistor R2. The voltage at base terminal can beadjusted to switch the transistor on or off.

If the variable resistor = 0, base voltage = 0 andthe transistor remains off.

If the variable resistor is increased, the basevoltage increases.

When the base voltage reaches certain minimumvalue, the base current switches the transistoron.

The large collector current flows through the transistor causing the bulb to light up.

If the variable resistor in the transistor is replaced by a device such as light dependent resistor(LDR), a thermistor or a microphone, the transistor can be used as an automatic switch controlledby light, heat or sound

Activity 2: A light controlled switch

1. The LDR has a very ___________ resistance in darkness and a _________ resistance inlight. R is a fixed _____________

2. The LDR and R form a potential divider in the circuit.

Circuit switches on the light at daytime andswitches off the bulb at night automatically

Circuit switches on the bulb at night andswitches off the bulb at day time automatically

Draw a circuit diagram

In daylight, the LDR has a very _______resistance as compared to R.

Therefore the base voltage is ___________enough to switch the transistor on and tolight ________ the bulb.

In darkness, the LDR has a very _________resistance and therefore the base voltage istoo ________ to switch the transistor________. The bulb light _____

In daylight, the LDR has a very _______resistance as compared to R.

Therefore the base voltage is too___________ to switch the transistor on

In darkness, the LDR has a very _________resistance and the base voltage is ________to switch the transistor ________ to light______ the bulb.

Describe how a transistor can be used as an automatic switch.


200

Relay

Alarm

RB

R

Thermistor Diode

Activity 3: Design a temperature operated switch

The resistance of a thermistor varies with temperature. When the temperature increases, theresistance of the thermistor will _________

Figure shows a transistor-based circuit that

function as a heat controlled switch.

At room temperature, the thermistor has a

………. resistance compared to R. Therefore,

the base voltage of the transistor is too low

to switch on the transistor.

When the thermistor is heated, its ……………. drops considerablely compared to R. Therefore,

the ……………., VB is high enough to switch ……. the transistor. When the transistor is switch

on, the relay switch is activated and the relay is switched ………. The circuit can also be used

in a fire alarm system.

A transistor functions as a current amplifier by

allowing a small current to control a larger current.

The magnitude of the …………………., IC is primarily

determined by the ………………….., IB. A ………..

change in the base current, IB will cause a ……..

change in the collector current, IC. The current

amplification can be calculated as follows:

Transistor circuit II (need one cell)Rx and RY : Voltage divider

VRx = Rx x V

( Rx + Ry)

VRY = Ry x V

( Rx + Ry)

high

Current AmplificationB

C

I

I

Transistor as a Current Amplifier

IC

IE

R1

R2

R IB

mA

A


201

TUTORIAL 9.3

OBJECTIVE QUESTIONS1. Diagram 1 shows a symbol for npn

transistor. Electrodes P, Q and R refer to

Diagram 1

P Q RA Base Emitter CollectorB Base Collector EmitterC Collector Base EmitterD Emitter Base Collector

2. The diagram shows the symbol for atransistor.

Which of the following shows thecorrect name of the electrode P and thetype of the transistor? (2003)

Electrode P Type of transistorA Emitter pnpB Collector pnpC Emitter npnD Collector npn

3. Which symbol represents the pnptransistor?

4. What are the electrical components thatmust be placed at P and Q in the circuitas shown in Diagram 2

Diagram 2

P QA Bulb BulbB Battery BulbC Resistor BatteryD Bulb resistor

5. An npn transistor is connected to a dcpower supply as shown in Diagram 3.

Diagram 3

Which of the following is correct?A Ie > Ic >IbB Ie > Ib >IcC Ic > Ie >IbD Ic > Ib >Ie

6. In which circuit does the bulb light up?(2004)


202

7. The function of transistor in the circuitshown in Diagram 5 is ___

Diagram 5A. RectifierB. AmplifierC. Automatic switchD. Voltage divider

8. The function of the transistor in thecircuit shown in Diagram 6 is

Diagram 6A. ReceiverB. A switchC. TransmitterD. Heater

10. Which of the following shows the correctcircuit to light up the bulb during daylightbut light off the bulb at night.

11. In which circuit will the light-emittingdiode (LED) light up when the switch ison? (2005)

12. Which of the following circuits canfunction? (2006)

13. Diagram shows an automatic switchcircuit to light up a bulb at night.

What changes should be done to lightup the bulb during the day time? (2007)A. Reverse the therminals of the batteryB. Interchange R1 and R2

C. Replace the npn transistor with a pnptransistor

D. Replace resistor R3 with a resistor oflower resistance


203

STRUCTURE QUESTIONS1. Diagram 7 shows a light dependent

resistor (LDR), resistors R and S, a lightemmiting diode (LED), a transistor and abattery that will be connected to form acircuit. The LED emits lights when it is ina bright surroundings.

Diagram 7

(a) (i) State one function of a transistor.

________________________________[1]

(ii) Complete the circuit in Figure 25.1 sothat the LED emits light in a brightsurroundings.

[1]

(iii) Give one reason why LED emits lightin a bright surroundings.

_________________________________[1]

(b) What modification is required to thecircuit so that the LED will emits lightwhen the surroundings becomedark?

________________________________[1]

(c) An alarm is needed which emitssound when there is a fire. Twomodifications have to be made to thecircuit in (a)(ii) by replacingelectronic components.

(i) State one electronic componentwhich needs to be replaced. Givea reason for your answer.

_____________________________

_____________________________[2]

(ii) Name two electronic componentsthat are needed to replace theunsuitable components.

_____________________________

_____________________________[2]

E. In the space below, draw a circuitdiagram to show the new circuit.[2]

2. Diagram 7 shows a circuit that acts as aswitch to light up bulb M when thesurrounding at P is dark. P is a lightdependent resistor LDR.

Diagram 7

(a) What is the characteristic of LDR?

___________________________________


204

(b)(i) Name component Q.

__________________________________

(ii) What is the function of component Q?

_________________________________

(c) What is the function of resistor R?

___________________________________

(d) (i) What is the change to the voltage Vpwhen the surrounding P is dark?

__________________________________

(ii) How does the change of the voltagelight up the bulb M?

__________________________________

(e) Explain why the bulb must not beconnected directly to the circuit switch.

___________________________________________

___________________________________________

SPM 2006 Section A # 7(c)3. (c) The door of a lift is fitted with a light

transmitter and a detector which is a lightdependent resistor. If the lightdependent resistor detects light, therelay switch is activated and the lift doorwill close. Diagram 8 shows an electroniccircuit for the control system of the liftdoor.

Diagram 8

(i) State the relationship between theresistance and the intensity of lightreceived by the light dependentresistor.

________________________________

________________________________

(ii) Complete the circuit in Diagram 8 bydrawing the resistor and the lightdependent resistor using thesymbols given below.

(iii) Explain how the circuit functions.

_______________________________________

_______________________________________

_______________________________________

_______________________________________[3 m]


205

SPM 2007 Section B 10(d)4. Diagram 9 shows a circuit with a

transistor that acts as an automaticswitch.

Diagram 9

The transistor in the circuit causes thelight diode (LED) to light up when it isdark. A technician wants a fan labeled240 V, 100 W in a room to beautomatically switched on when theroom is hot.Suggest modifications that can be madeto the circuit in Diagram 4 so that the fancan be automatically switched on whenthe room is hot.State and explain the modificationsbased on the following aspects:- the electrical components that are

needed to replace the LED and the lightdependent resistor (LDR) in the circuit.

- The positions of the electricalcomponents in the circuit.

[10 m]

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9.3 Transistor.doc