Transistors

1. A transistor is a double p-n junction semiconductor with three terminals, 

a. the emitter (e), 

b. the base (b) 

c. the collector (c).

2. Figure 1 below shows the illustration of a transistor. It looks like a combination of 2

p-n junction diodes.

(Figure 1)

3. In a transistor, the emitter emits charge carriers (free electrons or holes).

4. The charge carriers move towards the base.

5. Under certain condition, large amount of the charge carriers will pass through the

thin base layer and to be collected by the collector.

Types of the Transistors

1. There are 2 types of transistors:

a. npn transistor

b. pnp transistor

2. Figure 2 below shows the illustration of the npn and pnp transistor and Figure 3

below shows the symbol of both npn and pnp transistor.

3. For the symbol of the transistor, the arrow shows the direction of current. Take

note that, for the emitter and base, the current always flow from the positive

terminal to the negative terminal.

(Figure 2: Illustration of the npn and pnp transistor)

(Figure 3: Symbol of the npn and pnp transistor)

How a Transistor Work?

(Figure 4)

1. In the Figure 4 above, there are 2 circuits in the connection:

a. the base circuit

b. the collector circuit

2. The base circuit is forward bias whereas the collector circuit is reverse bias (This

will be discuss in "The Connection of a Transistor").

3. Table below shows the response of bulb 1 (B1) and bulb 2 (B2) when switch 1 (S1)

and switch 2 (S2) are closed.

S1 S2 B1 B2

Open Open  Does not light up  Does not light up

Close Open  Light up Does not light up 

Open Close  Does not light up  Does not light up

Close Close  Light up Light up 

3. From the table, we can see that, the collector circuit is controlled by the base

circuit.

4. Current will flow in collector circuit only when the base circuit is closed

Connection of Transistor

1. The terminals of a transistor must be connected to the terminals of a cell correctly

to avoid damaging the transistor.

2. Transistor should be connected in such a way that

a. the emitter-base circuit is forward bias

b. the collector-base circuit is reverse bias.

Example:

Emitter-Base: Forward Bias

Collector-Base: Reverse Bias

Connection: CORRECT

Example:

Emitter-Base: Forward Bias

Collector-Base: Reverse Bias

Connection: CORRECT

Example:

Emitter-Base: Forward Bias

Collector-Base: Forward Bias

Connection: INCORRECT

Example:

Emitter-Base: Forward Bias

Collector-Base: Forward Bias

Connection: INCORRECT

Current in a Transistor

1. The current flows in the base, emitter and collector is called the base current (IB),

the emitter current(IE) and the collector current(IC) respectively.

2. Figure below shows the direction of the current in an npn transistor.

3. In general, IE is related to IB and IC through the formula

IE = IB + IC

Another thing that you need to know about the 3 currents is

IE > IC > IB

Transistor as an Amplifier

1. The major application of a transistor is as a current amplifier.

2. A transistor can be used to amplify ('magnify') current changes because a small

change in base current produces a large change in collector current.

3. A simple transistor amplifier circuit is shown in Figure 1 below.

(Figure 1)

4. The graph in Figure 2 below shows the relationship between the base current and

the collector current. From the graph, we can conclude that, the collector current is

directly proportional to the base current.

(Figure 2: The collector current is directly proportional to the emitter

current)

5. Since the small change in the base current IB results in a big change in the

collector current, IC, the transistor therefore function as a current amplifier.

6. The ratio IC/IB is called the amplification factor.

Amplification Factor=ICIB

7. Figure 3 below shows another amplification circuit. In this case however, the base

current is varying because of the small alternating voltage produced by the

microphone.

(Figure 3)

8. The small changes in base current cause much larger changes in collector

current.

9. The collector circuit includes an earphone through which you would hear an

amplified version of the original sound.

10.The input capacitor passes on current changes from the microphone but blocks

the steady current which might otherwise flow through the microphone from the

potential divider. Such a current would upset the biasing effect of the potential

divider.

The Transistor as an Automatic Switch

1. Transistor can be used as automatic switches.

2. In the diagram above, the bulb is off when the collector current is off or very small.

It is switched on when the collector current become large.

3. We have learned that in a transistor, the collector current is controlled by the base

current, or the base voltage.

4. The greater the base voltage is, the greater the base current, and hence the

greater the collector current.

5. Therefore the bulb can be switched on and off by varying the voltage supplied to

the base.

6. The voltage across the base can be controlled a potential divider.

7. According to the potential divider rule, the voltages across the resistor R1 and

R2 are given by the following equations:

V1=R1R1+R2×V

and

V2=R2R1+R2×V

8. Therefore, by varying the resistance of R1 and R2, we can control the voltage

across the base V2, and hence switch the bulb on and off.

The LDR

1. A light-dependent resistor (LDR), or photoresistor, is a resistor sensitive to light.

2. In darkness, the LDR has a resistance about 1 million Ohm.

3. In bright light however, the resistance of the LDR falls to only a few hundred

Ohms.

Light Operating Switch

1. In a light operating switch, we connect an LDR to the potential divider.

2. As a result, the voltage across the base vary according to the presence or

absence of light.

3. Example 1 and 2 below shows how the resistance of the LDR, the base voltage,

the base current and the collector current change in different conditions.

Example 1

Bright Surrounding:Resistance of LDR: LowBase voltage: High

Dark SurroundingResistance of LDR: HighBase voltage: Low

Base current: High

Collector current: HighBulb: ON

Base current: Low

Collector current: LowBulb: OFF

Conclusion 

The bulb will be switched on when the surrounding is bright and switched off when

the surrounding is dark.

Example 2

Bright Surrounding:Resistance of LDR: LowBase voltage: LowBase current: Low

Collector current: LowBulb: OFF

Dark SurroundingResistance of LDR: HighBase voltage: HighBase current: Low

Collector current: HighBase current: ON

Conclusion 

The bulb will be switched on when the surrounding is dark and switched off when the

surrounding is bright.

Thermistor

1. In a heat operated switch, the LDR is replaced by a thermistor.

2. A thermistor is a resistor which its resistance changes as the temperature

changes.

3. There are 2 types of thermistor:

a. The positive temperature coefficient (PTC) thermistor

b. The negative temperature coefficient (NTC) thermistor

4. For the PTC thermistor, the resistance of the thermistor increases as the

temperature increases whereas for the NTC thermistor, the resistance of the

thermistor decreases as the temperature increases.

5. In SPM, we assume all the thermistor used is the NTC thermistor, unless it is

stated otherwise.

Heat Operated Switch

1. The circuit of a heat operated switch is similar to the light operated switch, except

that the LDR is replaced by an NTC thermistor.

2. If heat is applied to the thermistor, its resistance drops. As a result, the base

voltage will increase and the transistor is switched on and the bulb lights.

Sound Controlled Switch

1. Figure above shows the circuit design of a sound controlled switch.

2. The microphone is used to convert sound to electric current.

3. The variable resistor is adjusted as such that the transistor is switched on when

sound is detected by the microphone.

4. The function of the capacitor is to prevent the direct current from the cell to flow in

the base circuit.