Alciatore D.G., Histand M.B. Introduction to Mechatronics and Measurement Systems

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> 0

–

base (B)

collector (C)

emitter (E)

Figure 3.15 npn bipolar junction transistor.

Figure 3.16 pnp bipolar junction transistor.

–V

< 0

–V

< 0

–

base (B)

collector (C)

emitter (E)

3.4 Bipolar Junction Transistor 91

The most common type is the npn BJT, which we discuss in detail and use in our

examples. As shown in Figure 3.15 , it consists of a thin region or layer of p-type

silicon sandwiched between two regions or layers of n-type silicon. Three leads are

connected to the three regions, and they are called the collector, base, and emitter.

As denoted by the bold n in Figure 3.15 , the n-type silicon in the emitter is more

heavily doped than the collector, so the collector and emitter are not interchangeable.

The corresponding circuit schematic symbol is also shown in the figure with currents

and voltages defined and labeled. The construction, schematic, and notation for the

pnp BJT is shown in Figure 3.16 . The remainder of this section focuses on the npn

bipolar junction in Figure 3.15 .

is the voltage between the collector and emitter, and V

is the voltage

between the base and emitter. The relationships involving the transistor currents and

voltages follow:

(3.10)

–=

(3.11)

–=

(3.12)

For the transistor to be on, the base-to-emitter junction must be forward biased

( V

 0.7V, so V

 V

 0.7V). When this is the case, a large collector current can

flow (I

> 0) with a small base current ( I

<< I

), and there will be a small voltage

drop ( V

) in the collector-to-emitter circuit ( V

 V

 V

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92 CHAPTER 3 Semiconductor Electronics

T o understand how the npn BJT functions, we begin by considering the base-

to-emitter junction. Because this junction is forward biased ( V

> V

), electrons

diffuse from the emitter n-type region to the base p-type region. Because the base-

to-collector junction is reverse biased ( V

> V

), there is a depletion region that

would ordinarily prevent the flow of electrons from the base region into the col-

lector region. However, because the base region is manufactured to be very thin

and the emitter n-type region is more heavily doped than the base, most of the

electrons from the emitter accelerate through the base region with enough momen-

tum to cross the depletion region into the collector region without recombining with

holes in the base region. Remembering that conventional current is in the opposite

direction of electron motion, the result is that a small base current I

flows from the

base to the emitter and a larger current I

flows from the collector to the emitter.

The small base current controls a larger collector current, and therefore the BJT

functions as a current amplifier. This characteristic can be approximated with the

following equation:

= βI

(3.13)

which states that the collector current is proportional to the base current with an

amplification factor known as the beta (  ) for the transistor. Manufacturers often use

the symbol h

instead of  . For typical BJTs, beta is on the order of 100, but it can

vary significantly among transistors. Beta is also temperature and voltage dependent;

therefore, a precise relationship should not be assumed when designing specific tran-

sistor circuits.

Because of the BJT’s base-collector current characteristics, it can be used to

amplify current or to simply switch current on and off. This on-off switching is the

basis for most digital computers because it allows easy implementation of a two-

state binary representation. We focus on switch design and not amplifier design in

our mechatronic applications. Amplifier design requires a more in-depth study of

BJTs and is covered thoroughly in electrical engineering microelectronics textbooks.

3.4.2 Common Emitter Transistor Circuit

If a BJT’s emitter is grounded and an input voltage is applied to the base, the result

is the common emitter circuit shown in Figure 3.17 . As the base current is gradu-

ally increased, the base-to-emitter diode of the transistor begins to conduct when

is about 0.6 V. At this point I

begins to flow and is roughly proportional to

( I

  I

). As I

is further increased, V

slowly increases only to about 0.7 V

while I

rises exponentially. As I

rises, the voltage drop across R

increases and V

drops toward ground. The collector cannot drop completely to ground; otherwise,

the base-to-collector pn junction would also be forward biased. When V

reaches its

minimum, the transistor is said to go into saturation. In this mode, the collector cur-

rent is determined by R

, and the linear relation between I

and I

no longer holds.

The characteristics of the common emitter transistor circuit can be sum-

marized by plotting the collector current I

versus the collector-emitter voltage

for different values of base current I

. The resulting family of curves (see

Figure 3.18 ) describes the common emitter characteristics for the transistor.

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–

Figure 3.17 Common emitter circuit.

Figure 3.18 Common emitter characteristics for a transistor.

saturation

region

active

region

= 0

cutoff region

3.4 Bipolar Junction Transistor 93

The transistor has a cutoff region (where no collector current flows), an active

region (where collector current is proportional to base current), and a satura-

tion region (where collector current is strictly controlled by the collector circuit,

assuming sufficient base current). When designing a transistor switch, we need

to guarantee that the transistor is fully saturated when it is on. In full saturation,

is at its minimum, which is about 0.2 V for a BJT. So in saturation, the base-

to-emitter junction is forward biased ( V

 0.7 V), there is a small drop from

the collector to the emitter ( V

 0.2 V), and the voltages at the leads of the

transistor are related as:

 V

 0.7 V (3.14)

 V

 0.2 V (3.15)

Example 3.4 shows how to determine how much base current and input voltage are

required to saturate a transistor. The power dissipated by the transistor ( I

) is

smallest, for a given collector current, when it is fully saturated. If the transistor is

not fully saturated, it gets hot faster and can fail.

Probably the best way to understand the function of the BJT is to perform volt-

age and current measurements on actual circuits and plot the results. We’ve done

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94 CHAPTER 3 Semiconductor Electronics

The 2N3904 is a small-signal transistor manufactured by many companies as a general pur-

pose amplifier and switch. If you examine the specifications online or in a discrete transistor

handbook, you can find a complete list of ratings and electrical characteristics. Here is some

of the information provided:

■ Maximum collector current (continuous)  200 mA

■ V

(sat)  0.2 V

■ h

   100 (depending on collector current and many other things)

In the following circuit, what minimum input voltage V

is necessary to saturate the

transistor?

10 k

1 k

10 V

0 V

0.2 V

0.7 V

Because V

(sat) for the 2N3904 is 0.2 V, when the transistor is fully saturated the collector

current is

10 V 0.2 V–

()

1k⁄Ω9.8 mA==

Because the DC current gain h

is about 100, I

must be at least I

/100 or 0.098 mA.

Because V

 0.7 V, the base current can be related to the input voltage with

0.098 mA V

0.7 V–()10 k⁄Ω==

Therefore, the minimum required input voltage for saturation is

= 0.98 V + 0.7 V = 1.68 VV

min

Normally you would use a voltage larger than this (e.g., 2 to 5 times larger) to ensure that the

transistor is fully saturated, even with variances in parameters.

Guaranteeing That a Transistor Is in Saturation

EXAMPLE 3.4

just that for the two circuits shown in Figure 3.19 , using a 2N3904 small-signal

transistor. Lab Exercise 5 also includes some experiences to help develop an under-

standing of how transistor circuits function. The first circuit ( Figure 3.19a ) is the

common emitter configuration that has been the topic of this section. The second

circuit ( Figure 3.19b ) has an additional resistor in the emitter circuit that results in

what is called emitter degeneration.

Figure 3.20 shows the results for the common emitter circuit ( Figure 3.19a ).

In Figure 3.20a , notice how the base-to-emitter forward bias voltage V

and the

Lab Exercise

Lab 5Transistors

and photoelectric

circuits

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V = 5 V

(a) common emitter

(b) emitter degeneration

V = 5

Figure 3.19 Transistor experiments .

Figure 3.20 Common emitter experimental results .

active

region

saturation

region

Ib (mA)

Ic (mA)

(b) currents

Vin (V)

Vbe (V)

Vce (V)

active

region

saturation

region

(a) voltages

3.4 Bipolar Junction Transistor 95

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Figure 3.21 Emitter degeneration experimental results .

Vin (V)

Vbe (V)

Vce (V)

active

region

saturation

region

(a) voltages

(b) currents

Ib (mA)

Ic (mA)

active

region

saturation

region

emitter

degeneration

96 CHAPTER 3 Semiconductor Electronics

collector-to-emitter voltage drop V

do not change much after the transistor is satu-

rated, even when input voltage V

is increased well above the minimum required for

saturation. In Figure 3.20b , notice how the collector current I

does not increase in

the saturation region as the base current I

is increased above what is required for

saturation. As demonstrated in Example 3.4, it is important to fully saturate a transis-

tor by ensuring enough base current flows (by choosing an appropriate base resistor

for the available input voltage). However, it is also clear that increasing the input

voltage and base current significantly above the minimum required for saturation

does not provide any meaningful increase in collector current and will only result in

extra heat and energy loss in the base-to-emitter circuit.

Figure 3.21 shows the results for the emitter degeneration circuit ( Figure 3.19b) .

In Figure 3.21a , notice how, just as with the common-emitter circuit, the base-to-

emitter forward-bias voltage V

and the collector-to-emitter voltage drop V

not change much after the transistor is saturated, even when input voltage V

increased well above the minimum required for saturation. In Figure 3.21b , notice

how the collector current I

response is very different from the response in the

common-emitter circuit. As before, saturation and maximum collector current occur

at a fairly low base current. However, as the base current is increased, the collector

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Figure 3.22 Transistor switch circuit.

Figure 3.23 Models for transistor switch states.

(b) ON

out

= 0

> 0.7 V

= I

+ I

(a) OFF

< 0.7 V

out

= V

= 0

0.7 V

–

3.4 Bipolar Junction Transistor 97

current drops. This effect is called emitter degeneration. The reason for this is that

the base current adds to the collector current, causing an increase in the emitter cur-

rent ( I

 I

 I

), creating a larger voltage drop across the emitter resistor ( I

This reduces the voltage difference across the collector resistor, resulting in less col-

lector current. The collector current actually decreases to zero as the voltage at the

emitter ( V

 I

) approaches the collector supply voltage V

. With no voltage

difference across the collector resistor, no collector current will flow. For typical

mechatronics switching applications, the common-emitter configuration is more

appropriate because it is easy to saturate the transistor and ensure maximum collec-

tor current over a wide range of circuit parameters.

3.4.3 Bipolar Transistor Switch

Figure 3.22 illustrates a simple transistor switch circuit. When V

is less than 0.7 V,

the BE junction of the transistor is not forward biased ( V

< 0.7 V), and the tran-

sistor does not conduct ( I

 I

 0). You can therefore assume that the collector-

to-emitter circuit can be replaced by a very high impedance or, for all practical

purposes, an open circuit. This state, illustrated in Figure 3.23a , is referred to as the

cutoff or OFF state of the transistor. In cutoff, the output voltage V

out

is V

because

there is no current through or voltage drop across R

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98 CHAPTER 3 Semiconductor Electronics

When the BE junction is forward biased ( V

 0.7 V), the transistor conducts.

Current passes through the CE circuit, and V

out

is close to ground potential (0.2 V

for a saturated BJT). This state, modeled by the forward-biased diode illustrated in

Figure 3.23b , is referred to as the saturated or ON state of the transistor. We assume that

there is enough base current to saturate the transistor. The resistor R

(see Figure 3.22 )

is required in this circuit to limit the base current because the BE junction essentially

behaves like a diode. The relationship between the base current and R

is given by

 (V

i n

 V

) / R

(3.16)

When V

< 0.7, I

 0 and V

 V

The circuit in Figure 3.22 can serve as a semiconductor switch to turn on or off

an LED, electric motor, solenoid, electric light, or some other load (represented by

in the figure). These loads require large currents, ranging from milliamps to many

amps, to function properly. When the input voltage and current are increased enough

to saturate the transistor, a large collector current flows through the load R

. The

magnitude of the collector current is determined by the load resistance R

and the

collector voltage V

. When the base-to-emitter voltage is below 0.7 V, the transistor is

off, and no current flows through the load. The transistors used in power applications,

called power transistors, are designed to conduct large currents and dissipate more

heat. Power transistors are the basis for interfacing low-output current devices such

as integrated circuits and computer ports to other devices requiring large currents.

Relays, which mechanically make and break connections, are an alternative to

transistors. They cannot switch as fast as transistors and don’t last as long, but they are

very easy to use and can switch DC as well as AC power. For more information, see

Section 10.3. AC current can also be switched with a TRIAC (triode for alternating

current), which is a semiconductor device. For more information, see Internet Link 3.1.

Our objective is to turn a dashboard LED on or off with a digital device having an output

voltage of either 0 V or 5 V and a maximum output current of 5 mA. The LED requires

20–40 mA to provide a bright display and has a 2 V voltage drop when forward biased.

We use a transistor switch circuit employing a small-signal transistor (e.g., 2N3904

npn) to provide sufficient current to the LED. The required circuit follows.

5 V

10 k

LED

100

digital device

output

LED Switch

DESIGN

EXAMPLE 3.2

Internet Lin

3.1TRIAC - triode

for alternating

current

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3.4 Bipolar Junction Transistor 99

When the digital output is 0 V, the transistor is in cutoff, and the LED is OFF. When the

digital output is 5 V, the transistor is in saturation, and the base current is

5 V 0.7 V–()10

kΩ⁄ 0.43 mA=

which is within the specifications. The 100 Ω collector resistance limits the LED current to

a value within the desired range for the LED to be bright (20–40 mA):

(5 V − 2 V − 0.2 V) ⁄ 100 Ω = 28 mA

Lab Exercise 5 demonstrates how to wire and use various types of diode and

transistor circuits. Included are a basic transistor switch circuit, a motor driver circuit

with flyback protection (see Video Demo 3.2), and a photo-interrupter.

Let us summarize the guidelines for designing a transistor switch. The collec-

tor must be more positive than the base or emitter ( V

> V

). To be ON, the

base-to-emitter voltage ( V

) must be 0.7 V. The collector current I

is independent

of base current I

when the transistor is saturated, as long as there is enough base

current to ensure saturation. The minimum base current required can be estimated

by first determining the collector current I

and then applying I

min

≈ I

β.

For a

given input voltage, the input resistance must be chosen so that the base current

exceeds this value by a conservative margin (e.g., 5–10 times larger). The reasons

for this are that beta may vary among components, with temperature, and with

voltage; and the load resistance may change as current flows through it. It is also

important to calculate the maximum values of I

and I

to ensure that they fall

within the manufacturer’s specifications, and add or change series resistors if the

currents are too large.

3.4.4 Bipolar Transistor Packages

Transistor manufacturers offer their devices in a number of packages as illustrated

in Figure 3.24 . The small-signal transistor packages are often the TO-92, and the

power transistor packages are the TO-220. Surface mount technology is becom-

ing increasingly popular for use on production printed circuit boards, but such

devices are less useful for prototyping because of their small size. Figure 3.25 and

Video Demo 3.3 illustrate various common transistor packages. Included are BJTs,

metal-oxide-semiconductor field-effect transistors (MOSFETs, which are covered

in Section 3.5), and a photo-interrupter (covered in Section 3.4.6).

Lab Exercise

Lab 5Transistors

and photoelectric

circuits

Video Demo

3.2Turning a

motor on and off

with a transistor

Video Demo

3.3Transistors

CBE

BCE

TO-92

TO-220

SOT-23

(surface mount)

Figure 3.24 Bipolar transistor packages.

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Figure 3.25 Various common transistor packages.

BJTs MOSFETs photo-interrupter

Figure 3.26 Darlington pair.

100 CHAPTER 3 Semiconductor Electronics

3.4.5 Darlington Transistor

The schematic in Figure 3.26 represents a transistor known as a Darlington pair,

which usually comes in a single package. The advantage of this combination is that

the current gain is the product of the two individual transistor gains and can exceed

10,000. They may often be found in power circuits for mechatronic systems.

3.4.6 Phototransistor and Optoisolator

A special class of transistor is the phototransistor, whose junction between the base

and emitter acts as a photodiode (see Section 3.3.3). LEDs and phototransistors are

often found in pairs, where the LED is used to create the light, and this light in turn

biases the phototransistor. The pair can be used to detect the presence of an object

that may partially or completely interrupt the light beam between the LED and tran-

sistor (see Lab Exercise 5).

An optoisolator is composed of an LED and a phototransistor separated by a

small gap as illustrated in Figure 3.27 . The light emitted by the LED causes current

to flow in the phototransistor circuit. This output circuit can have a different ground

reference, and the supply voltage V

can be chosen to establish a desired output volt-

age range. With no common ground, the optoisolator creates a state of electrical

isolation between the input and output circuits by transmitting the signal optically

rather than through an electrical connection. A benefit of this isolation is that the

Lab Exercise

Lab 5Transistors

and photoelectric

circuits

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