2.

0 INTRODUCTION

Before the discovery of sem-conductor devices, especially the transistor, vacuum tubes (valves) were
used in all electronic circuits. Vacuum tubes have a number of drawbacks. They are bulky, consume too
much power and require high voltage to operate. They also require time to heat up before they can start
operating. Television sets made using vacuum tubes take some time when switched on before they
operate. This is different from the transistorised TV sets which start operating immediately power is
turned on.

Transistors have many advantages over the vacuum tubes. They are small in size, less expensive,
consume less power, may open at low voltages and operate immediately power is turned on. Recent
development in semi-conductor technology has come up with high power transistors which can handle
hundreds of amperes with case. These transistors are used in equipment such as high current power
supplies and transmitters. Another area of this development is in the manufacture of modern electronic
equipment such as computers, digital watches and telephones.

This chapter discusses the characteristics and applications of transistors and thyristors. These devices
inchide, bipolar transistors, (BJT), field effect transistors (FET), unijunction or bijunction transistors (UT),
silicon controlled rectifiers (SCR) and triacs. To illustrate typical applications of these devices some
projects are described at the end of the chapter.

2.1 BIPOLAR TRANSISTORS

A transistor is a three laver semi-conductor device used for switching and amplifying electric current.
Transistors for general purpose application are manufactured from silicon but a few special purpose
transistors are made from germanium. The hipolar transistor also known as the bijunction transistor
(BJT) is one of the most commonly used transistors. It is referred to as bipolar because, both holes and
electrons are charge carriers

The two types of hipolar transistors are the n-p-n and the p-n-p transistors. In the n-p-n type, a very thin
layer of p-type material is sandwitched between two thick n-type materials. In the p-n-p type, a very
thin layer of n-type material is sandwitched between two thick p-type materials. Figure 2.1 shows the
construction and the circuit symbols of n-p-n and p-n-p transistors.
Fig. 2.1: Bipolar transistor

The two outer regions formed in the bipolar transistor are the emitter (E) and the collector (C) while the
inner region is the base (B). In the symbols the emitter is always indicated by an arrow head which
points in the direction of conventional current flow as is the caase of p-n junction diode. The junction
formed between the base and the emitter is known as the base-emitter (B-E) junction while that formed
between the base and collector region is known as the base collector (B-C) junction.

The base-emitter and base-collector junctions in bipolar transistor are similar to the p-n junction in
diodes. The n-p-n transistor is like two p-n junction diodes with their anodes connected together to form
the base region. This is illustrated in figure 2.1(a) (iii). The two cathodes form the emitter and the
collector regions. The p-n-p transistor on the other hand is like two p-n junction diodes with their
cathodes connected together to form the base region. This is illustrated in figure 2.1(b) (iii). The two
anodes form the emitter and the collector regions.

Transistors and Thyristors

49

Basing the Bipolar Transistor

For a mister to function properly in a circuit, the base-emitter junction must be forward based and the
base-collector junction reverse biased. This is illustrated in figure 2.2 for popmansistors. The principle of
operation for n-p-n and p-n-p transistors is the np - n the only difference being in the polarity of biasing
voltage N

0.02 * 1/2

V_{c}
V VE

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(b) hang transistor

v cc

I_{B}

I_{E}

V EE

(c) Biasing n-p-n transistor

v CC
V EE

v cC

(d) Biasing p-n-p transistor

Fig. 2.2: Biaxing bipolar transistors

alway How t collecte

the pa anods The handi region lectur

Consider the n-p-n transistor shown in figure 2.2(c). Provided VEE is greater than barrier potential (0.6V
for silicon and 0.2V for germanium) the base-emitter junction is forward haund. Electrons in the emitter
region l_{F} flow across junction from the emitter into the base repon. This is the normal forward for a
forward biased p-n junction. As soon as the electrons cross into the base region, they are attracted by
the positive potential of the collector.

Since the base material is very thin, 98 percent or more of the emitter electrons cross mo the collector
region and form the collector current 1. Only 2 percent or less of the chitter electrons form the base
current 1 w

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50
For the p-n-p transistor, the polarity of the d.c. biasing voltage must be as shown in figure 2.2 (b) and
(d). In this case, the transistor current is a movement of holes from emiter to collector region.

Transistor as a Switch

Figure 2.3 shows a transistor being used to operate a lamp. It is switched on and off by controlling the
base-emitter junction bias. This is done by switch S

BB

Fig. 2.3: Transistor switched by D.C. voltage

When the switch is open the base-emitter is not biased and the transistor remains in the off state. No
current flows through the lamp. When the switch is closed, the base-emitter junction becomes forward
biased and the transistor conducts. A very small base current flows and causes a large collector current
to flow through the lamp. The lamp remains on as long as switch S, remains closed. S, and V may be
replaced by an a.c. signal source, see figure 2.4(a). In all the positive alternations of the a.c. signal
waveform, the transistor switches on and conducts when the base emitter junction is forward biased.
The on and off periods of the lamp are shown in figure 2.4(b).

AV

TIME

CC

(a) Schematic diagram

(b) Switching cycle of the lamp

Fig. 2.4: Silicon transistor switched by an asignal

Transistors and Thyristors

12 TRANSISTOR CIRCUIT CONFIGURATIONS

The transistor configuration is described in terms of the transistor terminal that is common bath these
and common-collector configurationit configurations are the common-cor common-base

Common-Emitter (CE) Configuration

In the common-emitter configuration, the input signal is fed between the base and emitter terminals,
while the output signal is taken between the collector and emitter terminals. This is illustrated in figure
2.5(a). In this mode of connection, the emitter is common to both the input and the output signals.
Majority of transistor circuits employ the common-emitter configuration because it is flexible and has
high voltage, high current and high power gains.

Common-Base (CB) Configuration

In this mode of connection, the base is made common to both the input and the output. Input signal is
fed between the emitter and the base and the output signal taken between the collector and the base
terminals. This mode of connection is illustrated in figure 2.5(b). Common-base amplifiers are used in
circuits where high frequency response and high voltage gain is required.

Common-Collector (CC) Configuration

In this configuration, the input signal is fed between the base and the collectur imisal while the output
signal is taken between the emitter and the collector terminals. The illustrated in figure 2.5(c). Common
collector amplifiers are used in circuits where m gain is required. This configuration is also known as the
emitter follower since the outpe taken at the emitter.

Transistors and Thyristors

53

Collector Characteristics

important characche circuit designer needs to know how it will belave chatcurrent to input current tc of
a transistor is its current amplificall behave de ratio of output current to input current for a given voltage
setting

The collector characteristics are curves showing how the output current varies with both tempat carent
and output voltage. This information is always available in the transistth mamatacner data sheet and it
makes the design work simpler.

An experimental set up for determining the collector output characteristics of sister is shown in figure
2.6. The curves are drawn by plotting dic. collector currents output currents) against d.c. collector-
emitter voltages V (output voltages) for chosen be curent I_{x} input currents).
5p overline 15V

kappa_{2}

(p, h)

i_{\mathfrak{H}}

v BE

I_{E}

R_{1}

Fig. 2.6: Circuit used to measure collector characteristics

In figure 2.6, R_{2}*j chosen to limit the maximum base current. The experiment procedure as follows:

1. Adjust R_{1} set base current I_{B} zero.

2. Vary collector-emitter voltage V from zero to say 16 volts in steps of 2 volts. Measure and record
collector current for each value of v ct

l_{c} 3. Repeat steps 1 and 2 for base currents of 10mu*A 20mu*A_{s} 40μA, 60μA and 80A
4. Plot the graphs of collector current 1 c against collector-emitter voltage v cz for each value of l_{n}
illustrated in figure 2.7.

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I_{R} = 136A_{0}

^ 2 H = M_{20}*K_{1}

10

I_{B} = s(p, h)

I_{B} - 4delta*mu*A_{i}

I_{B} = 25mu*A

I_{R} = Omega*mu*Lambda

Collectur voltage V CE (V)

Fig 27: Collector characteristics of a transistor

From figure 2.7, it can be seen that for a given value of base current I B^ * the collector oum rumains
almost constant over a wide range of collector-emitter voltage V The collecte current is thus dependent
on the base current and not the collector-emitter voltage l_{a} v_{w} For example, the collector current
1 c approximately 2mA when I_{y} = 20mu*A for all values collector-emitter voltage V

The ratio of this collector current of 2mA to the base current of gives a val 20mu*A of 100. This is
referred to as the current or current amplification factor transistor is general, rent gain of a wansistor
connected in the common-emitter configuration is the rame of collector current to huse current. It is
also referred to as the beta (8) of the transistor b_{w} That h ST = l c l 0

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