Basic Electronics - University of Ottawarhabash/BIECh14.pdf · 1 14 Basic Electronics In this chapter, we lead you through a study of the basics of electronics. After completing the

1

14 Basic Electronics

In this chapter, we lead you through a study of the basics of electronics. After

completing the chapter, you should be able to

Understand the physical structure of semiconductors. Understand the essence of the diode function. Understand the operation of diodes. Realize the applications of diodes and their use in the design of rectifiers. Understand the physical operation of bipolar junction transistors. Realize the applications of bipolar junction transistors. Understand the physical operation of field-effect transistors. Realize the application of field-effect transistors. Perform rapid analysis of transistor circuits.

REFERENCES

1. Giorgio Rizzoni, Principles and Applications of Electrical Engineering, McGraw Hill, 2003.

2. J. R. Cogdel, Foundations of Electronics, Prentice Hall, 1999. 3. Donald A., Neaman, Electronic Circuit Analysis and Design, McGraw Hill,

2001. 4. Sedra/Smith, Microelectronic Circuits, Oxford, 1998.

Basic Electronics 2

14.1 INTRODUCTION

Electronics is one of the most important fields in existence today. It has greatly influenced everything since early 1900s. Everyone nowadays realize the impact of electronics on our daily life. Table 14-1 shows many important areas with tremendous impact of electronics.

Table 14-1 Various Application Areas of Electronics

Area Examples of Applications

Automotives Electronic ignition system, antiskid braking system, automatic suspension adjustment, performance optimization.

Aerospace Airplane controls, spacecrafts, space missiles. Telecommunications Radio, television, telephones, mobile and cellular

communications, satellite communications, military communications.

Computers Personal computers, mainframe computers, supercomputers, calculators, microprocessors.

Instrumentation Measurement equipment such as meters and oscilloscopes, medical equipment such as MRI, X-ray machines, etc.

Microelectronics Microelectronic circuits, microelectromechanical systems.

Power electronics Converters, Radar Air traffic control, security systems, military

systems, police traffic radars. According to one dictionary, electronics means the physics of electrons and

their utilization. In a broader means, it may be defined as the field of manipulating electrical currents and voltages using passive and active devices that are connected together to create electronic circuits. These circuits may contain simple resistors, capacitors, conductors (that are generally called components), diodes, transistors and more complicated devices that contain millions of transistors.

In electronics, voltage and current become electrical signals. Signals contain information about a variety of activities in the physical world such as weather, temperature, pressure, speed, etc.

This chapter explains the function of semiconductor diodes and transistors. These devices find applications in many practical circuits used in electronic

Basic Electronics 3

systems and electric power systems. The emphasis in this chapter is on the basic techniques for the analysis of electronic circuits. 14.2 HISTORY OF ELECTRONICS

The field of electronics has long history starting early 1990s. This field is responsible for the development of many important areas such as telecommunication, computers, instrumentation, aviation, medicine, services, etc. It is impossible to imagine what our lives would be like without access electronics. Technologies associated with electronics have made our lives easier. Modern society is indeed unworkable without the existence of electronic devices and appliances. For example, emerging telecommunication services have greatly enhanced the ability of individuals and groups to communicate with each other and have facilitated the speed of information to persons and machines in both urban and rural environments. Table 14-2 includes the history of several important areas of electronics.

14.3 SEMICONDUCTORS

A semiconductor is material that is intermediate between a conductor and an insulator. Semiconductors are present in our everyday life. The computer that we use at has more than one electronic chip. Cars, TV’s, coffee machines, washing machines, etc are all equipped with them. 14.3.1 History

Although the semiconductor was late in reaching its present development, its history began long before the electron tube. Historically, we may go as far back as 1883 when Michael Faraday discovered that silver sulfide, a semiconductor, which has a negative temperature coefficient. The term negative temperature coefficient is just another way of saying its resistance to electrical current flow decreases as temperature increases. The opposite is true of the conductor. It has a positive temperature coefficient.

Another ancestor of semiconductor devices was the crystal detector, used in early wireless radios. This device (patented by a German scientist, Ferdinand Braun, in 1899) was made of a single metal wire touching against a semiconductor crystal. The result was a rectifying diode (so called because it has two terminals), which lets current through easily one way, but hinders flow the other way. By 1930, though, vacuum-tube diodes had all but replaced the smaller

Basic Electronics 4

but much quirkier crystal detector. The crystals were left to languish as a kids’ toy in the form of “crystal radios.” Table 14-2 Brief History of Electronics

Invention Year: Inventor Application 1904 USA: Sir Ambrose Fleming

The first rectifier, the diode, Fleming Valve (Edison effect).

1906 USA: Lee de Forest Invention of the audion tube, a three-element vacuum tube in which the grid controlled the current, which made modern radio possible.

1992 UK Regular radio broadcasting started in London.

1885-1889 Germany: Heinrich Rudolf Hertz

The first to broadcast and receive radio waves in the laboratory.

Vacuum Tube and Radio

1909 UK: Guglielmo Marconi

First wireless signal across the Atlantic Ocean. A3000-km distance between St. John’s (Newfoundland) and Poldhu (Cornwall), on the Southwest tip of England was bridged.

1923 UK: Valdimir Kosma Zworykyn

Iconoscope: This was used as a TV screen in the all electronic system developed by EMI.

1926 UK: John Logie Baird

Demonstrated the first television.

1934 USA: Philo T. Farnsworth

First electrical scanning TV camera.

1936 UK: BBC Regular broadcasting TV programs at Alexandra Palace, London.

Television

1982: Sony First flat screen TV. 1935 Scotland: Robert Alexander Watson-Watt

He received his eleventh radio-location patent, a device for detecting and locating an approaching aircraft.

Radar 1940 USA: Alfred Lee

Loomis He had already built a working low-power CW radar for aircraft detection when the British brought the magnetron to the U.S.

1947 USA: Bardeen, Brattain and Shockley

Transistor was invented in Bell Laboratories.

1954: Texas Instruments First transistor radio. 1967: Sony First radio using integrated circuits. Mid-1980s The capability of including one million

transistors on silicon chip.

Transistor

Mid-1990s The capability of including ten million transistors on silicon chip.

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1946 USA: Brainard, Mauchly, Eckert, Sharpless

One of the first electronic digital computers.

1968: Robert Noyce and Gordon Grove

Founded Intel (INTegrated Electronics) Corporation.

1970 First commercial microprocessor chip, the Intel 4004 based on small 4-bit digital words.

Computers

Later Succeeding models expanded the capability to 8-bit, then 16 bit, and finally 32-bit words.

In June 1948, a significant breakthrough took place in semiconductor

development. This was the discovery of point-contact transistor. Here at last was a semiconductor that could amplify. This discovery brought the semiconductor back into competition with the electron tube. A year later, junction diodes and transistors were developed. The junction transistor was found superior to the point-contact type in many respects. By comparison, the junction transistor was more reliable, generated less noise, and had higher power-handling ability than its point-contact brother. So, during the war, much effort was put into improving the semiconductors, mostly silicon and germanium, used in crystal detectors. 14.3.2 Conduction in Semiconductors

This section briefly introduces the mechanism of conduction in a class of materials called semiconductors. Semiconductors are crystals that, in their pure state, are resistive (this means, their electrical properties lie between those of conductors and insulators). When the proper impurities are added (a process often called doping) in trace amounts (often measured in parts per billion), semiconductors display interesting and useful properties.

For the sake of comparison, consider the conductivity of through common material. Copper, a good conductor with a very high concentration of free electrons, has a conductivity of 0.59×106 S/cm; glass, an insulator, may range between 10-16 and 10-13 S/cm; and a semiconductor, has a conductivity that varies from 10-8 to 10-1 S/cm.

We have said in Chapter 10 that a conducting material is characterized by a large number of conduction band electrons, which have a very weak bond with the basic structure of the material. Therefore, an electric field easily imparts energy to the outer electrons in a conductor and enables the flow of electric current. In a semiconductor, however, we need to consider the lattice structure of

Basic Electronics 6

the material, which is characterized by covalent bonding. Figure 14-1 illustrates the lattice structure of silicon, a very common semiconductor.

Thermal energy can cause the atoms in the lattice structure to vibrate; when sufficient kinetic energy is present, some of the valence electrons break their bonds with the structure and become conduction electrons. These free electrons enable current flow in the semiconductor. Importantly, is that the number of charge carriers depends on the amount of thermal energy present in the structure. Accordingly, semiconductor properties are a function of temperature.

Light can also give this energy boost and create what is called electron-hole pair: a free electron and a free hole: this phenomenon is called absorption. Photoconductivity is the increase of current in a semiconductor due to the absorption of photons. Light has a dual nature: it behaves as a wave and as a particle. The particle associated with light is called a photon. Photons can have different energies. When light illuminates a semiconductor: the photons with the right energy are absorbed by the material the electrons from the valence band have enough energy to jump to the conduction band the conductivity increases due to the higher number of electrons in the conduction band. The electron needs a minimum energy to jump to the conduction band. This minimum energy is the energy gap between the valence band and the conduction band. Photons with energies higher than the energy bandgap of the material will be absorbed.

Figure 14-1 Lattice structure of silicon.

The free valence electrons are not the only mechanism of conduction in a semiconductor. Whenever a free electron leaves the lattice structure, it creates a corresponding positive charge within the structure. The vacancy caused by the departure of a free electron is called a hole. Remember, whenever a hole is

Si

Si

Si

Si

Si

Si

Basic Electronics 7

present, we have, in effect, a positive charge. The positive charges also contribute to the conduction process, in the sense that if valence band electron jumps to fill a hole, thereby neutralizing a positive charge, it accordingly create a new hole at a different location. The phenomenon becomes relevant when an external electrical field is applied to the material.

It is important to point out that the mobility differs greatly for the two types of carriers. Holes move more slowly around the lattice than free electrons. To understand this, consider the fact that a free electron has already broken the covalent bond, whereas for a hole to travel through the structure, an electron must overcome the covalent bond each time the hole jumps to a new position. Also, when electrons move in one direction, holes move in the opposite direction.

According to the above view of semiconductors, we may anticipate a semiconductor as having two types of charge carriers: free electrons and holes. They move in opposite directions when an external field is applied across the semiconductor, giving rise to a net flow of current in the direction of electric field.

Sometimes, while a free electron moves in the immediate neighborhood of a hole it will recombine with a hole to form a covalent bond. Two charge carriers will be lost whenever this process takes place. This process is called recombination. In spite of recombination, the net balance is such that a number of free electrons always exist at a given temperature. These electrons are therefore available for conduction. The number of free electrons available for a given material is called the intrinsic concentration ni. For example, at room temperature, silicon has 316 melectrons/ 101.5 ×=in . Remember that there must be an equivalent number of holes present as well. 14.3.3 Doping

Semiconductor technology usually uses pure or intrinsic semiconductors with added impurities. The process of adding impurities to the crystalline structure of the semiconductor is called doping. The amount of these impurities is controlled, and the impurities can be of two types. The elements providing the impurities are called donors since they donate an additional free electron to their lattice structure.

A similar situation arises when additional holes are added by an element, which is called an acceptor, since it accepts a free electron from the structure and generates a hole in doing so.

The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium creates dramatic changes in their electrical properties, producing n-type and p-type semiconductors. The n-type elements are

Basic Electronics 8

semiconductors doped with donor elements conduct current predominantly by means of free electrons. When an acceptor element is used as the dopant, holes constitute the most common carriers and the resulting semiconductor is called p-type.

An example, a crystal of pure silicon can be turned into a reasonably good electrical conductor by adding an impurity such as arsenic or phosphorus (for an n-type semiconductor) or aluminum or gallium (for a p-type semiconductor). In general, a single type of semiconductor material is not very useful. Good applications happen only when a single semiconductor crystal contains both p-type and n-type regions. 14.4 THE pn JUNCTION AND DIODES 14.4.1 The pn Junction

A semiconductor material does not in and of itself possess properties that make it useful for the construction of electronic circuits. However, when a section of n-type material and a section of p-type material is brought in contact to form a pn junction, a number of useful characteristics arise. The pn junction forms the basis of the semiconductor diode, a widely used element in electric circuits.

Figure 14-2 illustrates an idealized pn junction, where on the p side we see a dominance of positive charge carriers, or holes, and on the n side, the free electrons dominate. Now, around the junction, in a small section called the depletion region, free electrons and holes come into contact with each other and recombine, thus leaving virtually no charge carriers at the junction.

Practically, it is not possible to join two such crystals together. Therefore, doping different impurities into different parts of a single crystal may only create a practical pn junction.

The extra electrons in the n region will lose energy by filling the holes in the p region. This leaves an empty zone, or depletion region, around the junction as shown to the right. This action also leaves a small electrical imbalance inside the crystal. The n region is missing some electrons so it has a positive charge. Those electrons have moved to fill holes in the p region, which therefore has a negative charge. This electrical imbalance amounts to about 0.3 V in a germanium crystal, and about 0.65 to 0.7 V in a silicon crystal. This will vary somewhat depending on the concentration of the impurities on either side of the pn junction.

Basic Electronics 9

Figure 14-2 A pn junction.

Assume that a voltage is applied to the outside ends of a pn crystal. First, we consider a positive voltage is applied to the n-type material. In response, the positive voltage applied to the n-type material attracts any free electrons towards the end of the crystal and away from the junction, while the negative voltage applied to the p-type end attracts holes away from the junction on this end. The result is that all available current carriers are attracted away from the junction, and the depletion region grows accordingly larger. There is no current flow through the crystal because all available current carriers are attracted away from the junction, and cannot cross. We are here considering an ideal crystal-in real life, the crystal cannot be perfect, and some leakage current does flow. This is known as reverse bias applied to the semiconductor crystal.

Figure 14-3 Reversed-biased pn junction.

The p-type depletion region

The n-type depletion region

n

p

Holes Free electrons Depletion layer

Basic Electronics 10

Figure 14-4 Forward-biased pn junction.

Now, we assume the applied voltage polarities have been reversed. The negative voltage applied to the n-type end pushes electrons towards the junction, while the positive voltage at the p-type end pushes holes towards the junction. This has the effect of shrinking the depletion region. As the applied voltage exceeds the internal electrical imbalance, current carriers of both types can cross the junction into the opposite ends of the crystal. Now, electrons in the p-type end are attracted to the positive applied voltage, while holes in the n-type end are attracted to the negative applied voltage. This is the condition of forward bias. Because of this behavior, an electrical current can flow through the junction in the forward direction, but not in the reverse direction. This is the basic nature of an ordinary semiconductor diode. 14.4.2 The Diode

The simplest nonlinear circuit element is the diode. Just like a resistor, the diode has two terminals; but unlike the resister, which is a linear element (has a linear relationship between the current flowing through it and the voltage across it), the diode has a nonlinear current-voltage relationship.

Diodes are components that allow current to flow in only one direction due in part to the fact that they only have a single p-n junction. Like many components, diodes have a positive side or leg (anode), and a negative side (cathode). When the voltage on the anode is higher than on the cathode then current flows through the diode (the resistance is very low). When the voltage is lower on the anode than on the cathode then the current does not flow (the resistance is very high).

An easy way to remember this is to look at the symbol for a diode. The arrow in the diode symbol points the direction in which it allows current (hole flow) to flow. The cathode of a diode is generally marked with a line next to it (on the diode body). Diodes are also some times marked with an identifying color code

n

p


(similar, but not identical, to that used for resistors). Figure 14-5 shows the circuit symbol for the diode along with a sketch showing the two terminals, anode and a cathode.

Figure 14-5 (a) Diode symbol. (b) The cathode of a diode is generally marked with a line.

Figure 14-6 summarizes the behavior of the diode by means of its i-v characteristics. Note that a third region appears in the diode i-v curve that has not been discussed in the previous section. The reverse breakdown region to the far left of the curve represents the behavior of the diode when a sufficiently high reverse bias is applied. Under such a large reverse bias (greater than the voltage VZ, the diode conducts current again, this time in the reverse direction. To explain the mechanism of reverse conduction, we need to visualize the phenomenon of avalanche breakdown. When a very large negative bias is applied to the pn junction, sufficient energy is imparted to charge carriers that reverse current can flow. Because of the large electric field, electrons are energized to such levels that if they collide with other charge carriers at a lower energy level, some of their energy is transferred to the carriers with lower energy, and these may contribute to the reverse conduction process as well. The process is called impact ionization. Now, these new carriers may also have enough energy to energize other low-energy electrons by impact ionization, so that once a sufficiently high reverse bias is provided, this process of conduction is very much like an avalanche: a single electron can ionize several others.

(a)

(b)


Figure 14-6 The i-v characteristics of the diode. 14.5 DIODE APPLICATIONS 14.5.1 Diode Switch

As a simple application, the diode can be considered as on/off device. The behavior of the diode in terms of current justifies this application. As long as the current is positive in the direction of the arrow of the circuit symbol, the diode acts as a short circuit and the current flows without hindrance. The diode is ON. When the voltage is positive in the direction opposite to the diode arrow, however, the diode acts as an open circuit and no current can flow. The diode is OFF.

Figure 14-7 (a) Closed loop circuit containing diode. (b) Ideal diode as short circuit. (c) Ideal diode as an open circuit.

Current

Voltage

Forward current

Leakage current

Reverse voltage

Avalanche current

Breakdown voltage

R R R

+ vD - + vD - + vD -

iD iD iD

vD vD vD


Consider the circuit in Figure 14-7 (a), which contains a battery, an ideal diode and a resistor. Assume that the diode is conducting. The diode can be substituted by a short circuit as shown in Figure 14-7(b). This is consistent with the assumption (i.e., diode ON).

Now, assume the diode is OFF. In this case, an open circuit and no current flows through the circuit as shown in Figure 14-7 (c) would represent the diode. 14.5.2 Rectification

AC is universally used for the generation, distribution and use of electric power. For this reason, most electronic equipment contains a power supply circuits. The function of the power supply is to convert the AC into DC. A major part of the power supply is a rectifier circuit. Half-Wave Rectifier

Figure 14-8 shows a typical half-wave rectifier circuit. The diode is said to rectify the AC signal. The circuit couples only the positive half of the input voltage to the load. The AC signal is represented as a sine wave. The load is represented as a resistor, although in practice the load would be an electronic circuit with various elements.

When the input voltage is positive, the diode turns ON and current flows with a sinusoidal shape. When the input voltage is negative, the diode turns OFF and no current flows. The resulting current flows in spurts, and the voltage across the load is that portion of the sinusoidal input that is positive. The diode blocks the negative part of the AC waveform.

The output of the half-wave rectifier is not pure DC power. The output, however, contains a DC component. The DC portion of the output as the time average is

( ) ( )

L

P

T

T/

T

L

PT

LDC

RV

dt dt wt RV

T dt ti

T I

π=

∫+∫=∫=2

2/

000sin11

(14.1)

where ω = 2π/T. The DC component of the load voltage is


π

V R I V PLDCDC ==

(14.2)

Figure 14-8 (a) Power supply circuit. (b) Half-wave rectifier circuit. (c) Half-wave rectifier waveforms.

Power supply

circuit AC signal DC signal

RL

Diode rectifier iL(t)

(a)

(b)

(c)

t t

vS

vS

vL

vL


Example 14-1: Draw a model circuit that results in the V-I characteristics given in Figure 14-9. Figurec14-9 V-I characteristics. Solution: According to the graph, no current flows for negative voltages. This means that an ideal diode should be included in this circuit. For the positive voltages, the ideal diode would offer zero resistance. A series circuit consisting of an ideal diode and a resistor will exhibit the characteristics of Figure 14-9. This circuit is shown in Figure 14-10. The slope of the V-I characteristics is

mA/V 3 2VmA 6 Slope ==

∆∆

=VI

The slope is equivalent to a resistor of

Ω=∆∆

= 333.33 IVR

Figure 14-10 A circuit to generate the relationship in Figure 14-9.

I

V

6 mA

2 V

R

I

VDD


Example 14-2: Find the current ID and the diode voltage VD for the circuit in Figure 14-10 with VDD = 10 V and R = 1 kΩ. Solution: We assume that VD is 0.7 V. By writing KVL around the closed loop circuit, we obtain

mA 9.3 10.7 - 10 ==

=R - VV I DDD

Example 14-3: A 12-V (RMS) AC source is connected to a 10-Ω resistor with a diode in a half-wave rectifier circuit. Find the peak and the average current in the load. Solution: Figure 14-8b shows the circuit. The peak current would be the peak positive voltage divided by the resistance since the diode is ON for positive source voltage

A70110

212 . RV

IL

pp ===

The average current, which contains the DC component, is given by Equation

(14.1)

A 0.54 701====

π.

πIP

πRV I

L

PDC

Full-Wave Rectifier

The full-wave rectifier utilizes both halves of the input sinusoid. Figure 14-11 (a) shows a full-wave rectifier circuit that uses a transformer and two diodes. The transformer secondary is center-tapped to supply identical but opposite voltages to the two diodes. When the center tap is grounded, the voltages at the opposite ends of the secondary windings are 180o out of phase with each other. The


connections to the diodes are arranged so that the diodes conduct on alternate half cycles. Each diode acts as a half-wave rectifier: D1 supplies the positive part of the DC signal and D2 supplies the negative part of the DC signal (Figure 14-11 (b).

The average value of current and voltage at the output of the full-wave rectifier is twice as great as the average current or voltage at the output of the half-wave rectifier. Figure 14-11 (a) Full-wave rectifier using a center-tapped transformer. (b) Waveform for the full-wave rectifier. Example 14-4: A bridge amplifier, similar to that of Figure 14-11, is used to provide a 25-V, 5-A DC supply. What is the resistance of the load that will draw exactly 5 A? What is the required RMS source voltage to achieve the desired DC voltage? Solution: The load resistance can be calculated by using Ohm’s law

vL AC input

D1

R

D2

vL

t

(a)

(b)

D1 ON D2 ON


Ω=== 5

525

IV R

L

LL

Full-Wave Bridge Rectifier

The bridge amplifier employs four diodes in a bridge configuration as shown in Figure 14-12. The operation of the rectifier is similar to that of two half-cycles of the AC waveform separately. Diodes D1 and D2 conduct during the positive half-cycle, while diodes D3 and D4 conduct during the negative half-cycle. Due to the structure of the bridge, the flow of current through the load is in the same direction during both halves of the cycle, therefore, the full-wave rectification of the waveform.

One advantage of a bridge rectifier over a conventional full-wave rectifier is that with a given transformer, the bridge rectifier produces a voltage output that is nearly twice that of the conventional full-wave circuit. Another advantage of the bridge rectifier circuit over the full-wave rectifier utilizing the center-tapped transformer is that only about half as many turns are required for the secondary winding of the transformer. Figure 14-12 Bridge amplifier. 14.5.3 DC Power Supply

The main application of rectifier circuits is in the conversion of AC to DC power. The circuit that accomplishes this conversion is usually called a DC power supply. Figure 14-13 shows a typical DC power supply. DC power supply is useful for electronic devices such as TVs, computers, radios, etc., since these devices require DC to operate.

D1

D2 D3

D4

R AC input

vS

vL


Transformer

It is clear from Figure 14-13 that a transformer is employed to obtain an AC voltage that is reasonably close in magnitude to the desired DC supplying voltage. For example, one may wish to step a 110-V RMS line voltage down to a lower DC voltage by means of a transformer prior to rectification and filtering, to eventually obtain 12-V DC power. It consists of two separate coils wound around an iron core that magnetically couples the two windings. The primary winding having N1 turns, is connected to the circuit of the power supply; and the secondary winding, having N2 turns, is connected to the circuit of the DC power supply. By selecting an appropriate turns ratio (N1/N2) for the transformer, the designer can step the line voltage down to the value required to yield the particular DC voltage output.

In addition to providing the suitable voltage for the DC power supply, the transformer provides electrical isolation between the electronic equipment and the power-line circuit. This isolation minimizes the risk of electric shock to the equipment and users.

Following the step-down transformer is a bridge rectifier, a filter capacitor, a voltage regulator, and finally the load. The rectifier has been already discussed in previous section. Filter Capacitor

The rectifier circuit produces a DC component in its output. The output may be described as a desired DC component plus an undesired ripple. A filter, as in Figure 14-13, is a circuit used to remove an undesirable signal in a circuit. In such case, we need a filter to eliminate, or at least reduce, the ripple component from the output of the amplifier. The filter introduced in Figure 7-11 is a simple one, a capacitor across the load.

The capacitor stabilizes the voltage across the load resistor through the process of charge, holding the charge, and discharging. Current flows through the capacitor and charges it to the peak value of the input AC voltage. After its peak, the input voltage drops rapidly. If the voltage of the capacitor were to follow this voltage, a rapid discharge would have to occur. However, all diodes prevent discharge back through the input source and turn OFF when the input drops below the voltage on the capacitor, because at that moment all diodes become reverse biased. Therefore, the filter capacitor and the load become disconnected from the source, which continues with the negative part of its waveform. The capacitor discharge through the load. To realize that, we need to know the time constant, the initial value, and the final value. The time constant is RLC, the initial value is VP, and the final value is zero if the discharge continues forever.


Figure 14.13 Block diagram of a DC power supply. Voltage Regulator

For most applications, it is desirable that the DC supply be as steady and ripple free as possible. To ensure that the DC voltage generated by a DC supply is constant, DC power supply should contain voltage regulator. The regulator is an electronic circuit that can hold a DC load voltage relatively constant in spite of possible fluctuations in the DC supply.

The most common device employed in voltage regulation schemes is the Zener diode. This diode function on the basis of the reverse portion of the i-v characteristic (Figure 14-6). The operation of Zener diode is related to avalanche breakdown. It is usually achieved by means of heavily doped regions in the neighborhood of the metal-semiconductor junction. The high density of charge carriers provides the means for a substantial reverse breakdown current to be sustained, at a nearly constant reverse bias, the Zener voltage VZ.

14.6 TRANSISTORS

The transistor is a three-terminal semiconductor device that can perform two important functions that are fundamental for the design and operation of electronic and communication circuits: amplification and switching. Amplification means magnifying a signal by transferring energy to it from an external source, whereas a transistor switch is a device for controlling a relatively large current between or voltage across into terminals by means of a small control current or voltage applied at a third terminal.

The transistor has input and an output, suggesting that the cause-effect relationship go from left to right (Figure 14-14). The input affects the output; however, the output has little effect on the input. We may say that the transistor is a “one-way path” to the electrical signal. This one-directional property of the transistor isolates the output from the input and allows electrical engineers to build complex circuits. This complexity can be altered because circuits can be designed (or analyzed) one part at a time, unlike the two- or three-loop circuits in

Load

Voltage regulator

Filter

Diode

rectifier AC input VL


circuit analysis, which must be analyzed together. There are two major families for transistors: bipolar junction transistor (BJT)

and field effect transistors (FET).

Figure 14-14 Cause-effect relationship in a transistor circuit. 14.7 BIPOLAR JUNCTION TRANSISTOR (BJT) 14.7.1 Types of BJT

Joining three sections of semiconductor material, each with a different doping concentration, forms a bipolar junction transistor (BJT). The three sections can be either a thin n region sandwiched between two-p region, and a p region between two n regions. The resulting BJTs are called pnp and npn transistors, respectively. Figure 14-15 illustrates the approximate construction, and symbol for the two types of BJTs.

The three regions of the BJT are called the emitter, base, and collector. Each of the three regions connects to a wire, which allows electrical connections to be made. The base region is very narrow compared to the widths of the emitter and collector regions. 14.7.2 Operation of BJT

One may understand a BJT as two diodes placed back to back. In normal operation, the emitter-base junction is forward biased and the base-collector junction is reversing biased. The operation of the npn BJT may be explained by considering the two back-to-back pn junctions. The base-emitter (BE) junction acts very much as a diode when it is forward-biased; thus, one can picture the corresponding flow of hole and electron currents from base to emitter when the

Input

Output

Transistor


collector is open and the BE junction is forward-biased as shown in Figure 14-16 (a).

Figure 14-15 Bipolar junction transistors.

Figure 14-16 (a) Current flow in a npn BJT. (b) Flow of emitter electrons into the coleector in a npn BJT.

Now, consider reverse-biasing the base-collector (BC) junction. In this case, the electrons emitted by the emitter with the BE junction forward-biased reach the very narrow region, and after a few are lost to recombination in the base, most of these electrons are collected by the collector as shown in Figure 14-16 (b).

n

p

n

p

n

p

Collector

Emitter

Base

Collector

Emitter

Base

C C

Electrons Holes

n

p

n

B B

E

V

V1

V2

E

IE

IC

IE

IB IB

(a) (b)


Since the base is narrow, there is a high probability that the electrons will have gathered enough momentum from the electric field to cross the reverse-biased collector-base junction and make it into the collector. The result is that there is a net flow of current from collector to emitter (opposite in direction to the flow of electrons), in addition to the hole current from base to emitter. The electron current flowing into the collector through the base is substantially larger than that which flows into the base from the external source. Using KCL, we may have CBE I I I += (14.3)

The most important property of the BJT is that the small base current controls

the amount of the much larger collector current BC I I β= (14.4)

where β is a common emitter current gain. It is one of the key features of a

bipolar transistor, which depends on the physical properties of the transistor. Typical values of β range from 20 to 200.

The operation of a pnp transistor is completely analogous to that of npn device, with the roles of the charge carriers and signs of the current reversed. 14.7.3 Regions of Operation

According to the external biasing of the two junctions (base-emitter and base-collector) of the transistor, four possible regions of operation are provided (see Figure 14-17). The most important and used is the forward active mode of operation for which the transistor is optimized.

Of the other modes the reverse active mode must be described by the model as well. It is obtained by swapping the emitter and the collector so that the base-collector voltage is forward and the base-emitter voltage is reverse (or zero) biased. Due to the usually much lower reverse current gain, it is less useful than the forward active mode, however, used occasionally.

In the cut-off mode both junctions are reverse biased and hence no current can flow between emitter and collector. Therefore, the transistor is off and behaves like an open switch.

Conversely, in the saturation mode the transistor behaves like a closed switch as both junctions are forward biased which enables a large current to flow between emitter and collector.


Figure 14-17 Regions of BJT operation. 14.7.4 Transistor Input Characteristics

Consider the effect of placing a voltage at the BE junction, the input part of the transistor. It is known that the base-emitter junction forms a pn-junction diode. The input i-v characteristic is like that of a diode (Figure 14-6). The base current is very small until sufficient voltage exists across the junction to turn it ON. It is about 0.7 V for a silicon transistor. Once the junction is turned ON, the base current increases rapidly, with the BE voltage remaining constant at about 0.7 V. Accordingly, the BE characteristic can be modeled as either an open circuit (VBE < 0.7 V) or else a constant voltage of 0.7 V once the input voltage tries to go above that value.

Forward active mode Reverse active mode

Saturation mode Cutoff mode

Reverse bias

Forward bias

Forwardbias

Reverse bias

Forward bias

Forward bias

Reverse bias

Reverse bias

C

E

BB

C

E

C C

B

EE

B


14.7.5 Transistor Output Characteristics

Let us call the fraction of electrons that diffuse across the narrow base region as α and the fraction of electrons that recombine with holes in the base to create the base current as 1-α. The ratios of the base, collector, and emitter currents are therefore ( ) EBEC i-α ii α i 1 and == (14.5)

The value of α is fairly constant throughout the active region for a given

transistor. α Characterizes the current gain of the device. The current gain is usually described in terms of β of the transistor. This is defined as

-αα

ii

β B

C1

==

(14.6)

In terms of β, the ratio of currents become

( ) BCE

BC

i β iβ

β i

i i

11

+=+

=

= β

(14.7)

The above equations are valid only in the active region.

Example 14-5: A transistor has a β of 100. Find the collector and emitter currents if iB = 10 µA. Solution: We assume that the transistor is in the active region. Use Equation (14.7)

( ) ( ) mA011A 1011001

mA1A10100

. µ i β i

i i

CE

BC

=×+=+=

=×== µβ


Example 14-6: In the circuit of Figure 14-18, a 5 V battery in series with a 5-kΩ resistor is connected to the base circuit of a npn transistor, with the positive voltage connected to the base of the transistor. Find iB. Figure 14-18: The base-emitter circuit. Solution: Figure 14-18 shows the base emitter circuit. The pn junction between base and emitter will be ON, so the base-emitter voltages is approximately 0.7 V. Therefore, KVL around the base emitter loop is

mA 0.215 k 50.7-5

0 0.7 k 5 -5

=Ω

=

=+Ω×+

B

B

i

i

Example 14-7: Analyze the circuit shown in Figure 14-19 to determine the voltages at all nodes and the currents through all branches. Assume VCC = 12 V, RC = 2 kΩ, and RE = 0.5 kΩ. Solution: The base of the BJT is connected to the ground, which means the base voltage is zero. Since the base is at zero volts, the emitter-base junction cannot conduct and the emitter current is zero. Also, the collector-base junction cannot conduct. The collector current is zero since the n-type collector is connected through RC to the positive power supply while the p-type base is at zero volts. The base current and emitter currents are also at zero. Accordingly, the BJT is in the cutoff mode of operation.

5 V

5 kΩ

0.7 V

B

E


Figure 14-19 BJT circuit with the base grounded. 14.8 BJT APPLICATIONS 14.8.1 Switch

As we have discussed earlier that the real switch is either an open circuit (OFF) or a short circuit (ON). A real switch, like the wall switch for the lights, has a large resistance when OFF and a small resistance when ON. To function properly as a switch, the device must have a large OFF resistance compared with the resistance level of the load such that almost all the voltage appears across the switch and very little voltage appears across the load. Likewise, in the ON state, the switch must have a small resistance compared with the resistance level of the load so that almost all the voltage appears across the load with very little voltage across the switch.

The BJT can function as an electronic switch in the circuit shown in Figure 14-20 if its OFF resistance is much larger than its load resistance, RC, and its ON resistance is much smaller than its load resistor.

RC

RE

VCC

2 kΩ

0.5 kΩ

10 V

0 mA 10 V

0 mA

0 V

Cutoff

0 mA


Figure 14-20 A npn BJT amplifier-switch circuit. 14.8.2 Digital Logic

Consider the simple transistor inverter circuit shown in Figure 14-21. If the input is approximately zero volts, the transistor is in cutoff and the output is high and equal to VCC. However, if the input is high and equal to VCC, the transistor is driven into saturation, and the output is low and equal to VCE (sat). 14.8.3 Amplifier

BJTs can be used in amplifier circuits. For example, consider a radio receiver that receives a small signal from the antenna (e.g., 10 mV) and produces a high output voltage (e.g., 10 V). Such radio receiver requires a voltage gain of 1000. This gain may be accomplished in stages. Each transistor amplifier stage taking as its input the output of the previous stage.

Coupling Capacitors

Capacitors C1 and C2 in Figure 14-22 are coupling capacitors. Large capacitors block DC signal but pass AC signal. If the impedance of the capacitors is small relative to the impedance of the circuit, the time-varying signals will pass through the capacitors undiminished. Therefore, the DC circuit is isolated from the AC

VCC RB

0.7 V

B

E

RC

vin

vout

iC

iB +

-


circuit within one stage and stages are isolated from each other by the coupling capacitors at the input and the output.

Figure 14-21 A BJT inverter circuit.

Figure 14-22 A stage of an amplifier circuit.

vS

Input Coupling

Signal Source

DC Bias

Basic Amplifier

Output Coupling

Output Load

VCC

RC

RL

R1

R2

RS CS

CC

B

VI RB

RC

Vo

VCC = 5 V


Bias Circuit

The DC signal is supplied to the transistor base by the voltage divider circuit, R1 and R2. The coupling capacitor blocks the DC signal from entering the source and the load. Although there is only one power supply (VCC), we may replace the VCC symbol with two voltage sources. It is important to know that the power supply acts independently on the bias and collector circuits. Also, we will make use of the voltage divider between resistors R1 and R2. Example 14-8: Consider the small-signal amplifier shown in Figure 14-22. Assume VCC = 18 V, R1 = 160 kΩ, R2 = 10 kΩ, RC = 2 kΩ, RL = 2 kΩ, β = 100. Find the base and collector DC currents and the DC voltage from collector to emitter. Does the BJT operate in the active region? Solution: A DC current is supplied to the BJT base by the voltage divider R1 and R2. The coupling capacitors act as blocks to the DC current; therefore the equivalent circuit at DC is as shown in Figure 14-23 (a). Although, practically there is one power supply, the VCC symbol can be replaced with two voltage sources. The circuit in Figure 14-23 (a) can further be reduced to the circuit shown in Figure 14-23 (b) by converting the voltage divider to a Thevenin equivalent circuit (Thevenin theorem can be found in any circuit analysis book). The symbol VBB is used for the open-circuit voltage at the base of the BJT. Figure 14-23 (a) DC bias circuit. (b) Equivalent of the input portion.

VCC VCC

R1

R2

RC

IB

RC

IB RB

VBB

VCC

(a) (b)


The open-circuit base DC bias voltage is

V 1.0 160 10

10 18 =+

×=BBV

The DC output impedance of the bias network is

Ω=+×

== k 9.411 160 10160 10 160 10 BR

A silicon transistor requires a threshold voltage of VBE = 0.7 V to turn ON the

base-emitter junction, therefore

mA 3.1

A 31 k 9.411

0.7 - 1.0 0.7 -

B

B

==

=Ω

==

I IR

VI

C

B

BB

β

µ

Now consider the closed loop path in the output circuit of the amplifier and

apply KVL in order to find the collector-emitter voltage VCE.

V 11.8 k 2 mA 3.1 - 18 0 -=Ω×=

=++

CE

CECCCCV

VRIV

Therefore, the BJT is operating in the active region. If there were no input

signal, the BJT would have already a voltage of 11.4 V across it.

14.9 FIELD EFFECT TRANSISTOR The junction field-effect transistor (FET) was invented in the late 1940s but

was made practical by manufacturing a few years after the introduction of BJT. FET was the key device, which led to the solid state electronics revolution. Today, FET is virtually used in most digital electronic circuits.

Like the BJT, the FET comes in two types: The n-channel corresponds to the npn and the p-channel corresponds to the pnp. Unlike the BJT, FETs come in two varieties: the junction FET (JFET) and the metal-oxide semiconductor FET (MOSFET). The junction field effect transistor (JFET) and the metal-oxide-semiconductor FET (MOSFET) are voltage-controlled devices: that is a small


change in input voltage causes a large change in output current. FET operation involves an electric field, which controls the flow of a current through the device. In contrast, a bipolar transistor employs a small input current to control a large output current. The source (S), drain (D), and gate (G) terminal of the FET are analagous to the emitter, collector, and base of a BJT. The terms n-channel and p-channel refer to the material which the drain and source is connected. Figure 14-24 shows symbols of two MOSFET devices.

In application, the BJT has the disadvantage of low input impedance because the base of the transistor is the signal input and the base-emitter diode is forward biased. With the reverse biased input junction, FET has very high input impedance. Having high input impedance minimizes the interference with or “loading” of the signal source when a measurement is made. For example, if the device control depends on voltage only as in the case of FET, the device can be designed with very little current or even with zero current. So when we attach it to another device such as thermocouple, it will not disturb the input since it is drawing no power. Table 14-3 shows the control parameters of the BJT and FET.

Table 14-3 Control Parameters of the BJT and FET

BJT FET Base Collector Emitter Base current Collector current Collector-emitter voltage

Gate Drain Source Gate voltage Drain current Drain-source voltage

Figure 14-24 (a) Three-terminal symbol for the nMOSFET. (b) Three-terminal symbol for the pMOSFET.

S

G

D

G

S

D


Figure 14-25 Physical layout of an nMOS transistor. 14.9.1 The Field-Effect Transistors

The MOSFET is called a field-effect device because an electric field between the gate and the substrate, established by the applied gate voltage, controls the current flow between the source and the drain terminals. Figure 14-25 shows the physical structure and schematic symbol for the n-MOSFET. The p substrate is only lightly doped with acceptor atoms, whereas the source and drain n-type regions are doped with a higher concentration of donor atoms. The oxide layer forms an insulator that creates an extremely high resistance between the gate electrode and the other regions. The current flow consists of the majority carriers (electrons for n-type material). Since the gate junction is reverse biased and because there is no minority carrier contribution to the flow through the device, the input impedance is extremely high.

MOSFET is a special type of FET that works by electronically varying the channel length L along which charge carriers (electrons or holes) flow. This parameter is used to characterize the device. For example, when a MOSFET is referred to as a 0.4-µ device, this means that the channel is 0.4 µ in length. For microelectronic circuits that must include millions of devices on a single chip, the channel length is small, as the channel width W. Gates that minimize the product W × L also minimize parasitic capacitances and power dissipation. The wider the

Substrate

Drain

W

Source

Gate

n p

p

L

Oxide


channel, the better the device conducts. The charge carriers enter the channel at the source, and exit via the drain. The width of the channel is controlled by the voltage on an electrode called the gate, which is located physically between the source and the drain and is insulated from the channel by an extremely thin layer of metal oxide.

There are two ways in which a MOSFET can function. The first is known as depletion mode. When there is no voltage on the gate, the channel exhibits its maximum conductance. As the voltage on the gate increases (either positively or negatively, depending on whether the channel is made of p-type or n-type semiconductor material), the channel conductivity decreases. The second way in which a MOSFET can operate is called enhancement mode. When there is no voltage on the gate, there is in effect no channel, and the device does not conduct. A channel is produced by the application of a voltage to the gate. The greater the gate voltage, the better the device conducts.

The MOSFET has certain advantages over the conventional junction FET, or JFET. Because the gate is insulated electrically from the channel, no current flows between the gate and the channel, no matter what the gate voltage (as long as it does not become so great that it causes physical breakdown of the metallic oxide layer). Thus, the MOSFET has practically infinite impedance. This makes MOSFETs useful for power amplifiers. The devices are also well suited to high-speed switching applications. 14.9.2 MOSFET Operation

The three regions of operation of the MOSFET are the cutoff region, the triode region, and the active region. The MOSFET is in the cutoff region when there is no current flow between source and drain terminals. This will happen when the magnitude of the gate-to-source voltage is less than the threshold voltage. This voltage may be 1 V, but can be controlled in the manufacturing process.

The MOSFET is in the triode region when the current flow is a function of both the gate-to-source voltage and the drain-to-source voltage. The triode region may be called the linear region or the ohmic region.

When in the active region, the drain current is primarily a function of the gate-to-source voltage. The drain voltage has a small effect on current. The active region may be called sometimes the saturation region or the pinchoff region. 14.9.3 Input and Output Characteristics

Since the pn junction between gate and channel is reversed bias, very small current flows into the gate. Therefore, we may say that iG = 0 in Figure 14-26, and the drain current flows through the channel and out the source connection.


The gate controls the current in the channel through the electric field that affects the depletion region.

Figure 14-26 Circuit for determining FET characteristics.

As we knew earlier, the FET has several regions of operation which we may examine by fixing the input voltage vGS, and observing the output current iD as we vary the voltage vDS. Figure 14-26 shows the circuit for the experiment and Figure 14-27 shows the results. When vGS = 0 V, the current increases as of resistor for small values of vDS. This is the triode region. As vDS increases, the current begins to level off because the channel narrows at the drain end of the device. At the negative of pinchoff voltage –VT, the conducting channel reaches a minimum size and the current becomes constant, independent of further increase in vDS. This is the saturation region. This is equivalent to the active region of the transistor. For lower (negative) values of vGS, the FET changes from triode region to the saturation region at lower values of vDS. For vGS < VT, the FET is cutoff.

14.9.4 The Mathematical Model

In the triode region, the drain current is

( )

−=

2 -

2

2

2DS

DSTGST

DSSD

vvVv

VI

i for 0 <vDS <(vGS –VT), vGS > VT (14.8)

where IDSS is the saturation current for vGS = 0. Equation (14.8) describes a

parabola passing through the origin and tangent to the point of pincjoff. After pinchoff, the current is constant according to the following formula

D

G

S

iG = 0

iD

vDS

vGS vGG

vDD


Figure 14-27 Output characteristics for VT = -4 V and IDSS = 32 mA. ( )22 - TGS

T

DSSD Vv

VI

i = for vGS > VT, vDS > vGS -VT

(14.9)

Example 14-9: What value of vGS corresponds to iD = 10 mA and vDS = 5 V? Solution: This means that the FET is in the saturation region. Equation (14.9) may be applied ( )[ ]

V 18.1

4- - (-4)

30 15 22

−=

=

GS

GS

v

v

14.9.5 The MOSFET as an Amplifier

A simple amplifier model is shown in Figure 14-28. Resistors R1 and R2 are

used to create a no-signal or quiescent DC value of gate-to-source voltage VGSQ

0 2 4 6 8 10 vDS V

vGS = 0 V

vGS = -1 V

vGS = -2 V

vGS = -3 V

Triode region

10

20

30

Saturation region iD mA

IDSS

vGS = VP = -4 V


that will set the quiescent drain-to-source voltage VDSQ, in the active region. The active region extends from cutoff to the edge of pinchoff or the triode region. Cutoff occurs when ID = 0, leading to no voltage drop across RD and an output voltage level of VDS = VDD. Example 14-10: Consider the FET amplifier circuit in Figure 14-28. Find the gate and drain-source voltage and the drain current. Assume R1 and R2 = 1 MΩ, RD = 2 kΩ, RS = 5 kΩ, VDD = 12 V, VT = 1 V, IDSS = 30 mA. Assume large capacitors.

Figure 14-28 A MOSFET amplifier. Solution: The MOSFET is operating in the saturation region. The gate voltage is computed by applying the voltage divider technique between resistors R1 and R2 (no current flows in the FET).

V 6 12 21

21

2

=×=

+= DDG V

R RR v

--

CC

Signal Source

DC Bias

Basic Amplifier

Output Coupling

Output Load

VDD

Input Coupling

R1

R2

RL

RS

RD

vS

CC

CS


Since the FET is operating in the saturation region, we write DDSGSGGS ii - R v - v vv 5 - 6 ===

where the current is expressed in milliamperes. The drain current can be calculated from Equation (14.9)

( )( )[ ]22 4- -

4-mA -30 GSD vi =

We eliminate vGS between the above two equations, we obtain a quadratic

equation for the drain current. The two solutions are iD = 8.0 and 32 mA. The second value is unrealistic. With the drain current as 8.0 mA, the source voltage is 2.0 V and hence vGS = -2 V. The drain voltage can be obtained from KVL as ( ) V 4k 1mA 812 =Ω×−

Accordingly, the gate-drain voltage is –4V, and the transistor is operating in the saturation region.


SUMMARY • A pure semiconductor material is a rather poor conductor of current, with

conductivity falling between those of good conductor and good insulator. • In order to make a semiconductor useful in electronics, impurity atoms are

added to increase the conductivity of the material. This process is referred to as doping.

• If a voltage is applied to a doped semiconductor material, current is carried by the mechanism of drift, similar to conduction in a metal.

• A pn junction diode is formed when an n-doped region and a p-doped region are directly adjacent to each other. The current-voltage characteristcs of the diode are nonlinear. The current is an exponential function of voltage in the forward-bias condition, and is almost zero in the reverse-bias condition.

• The diode is a nonlinear device. An important aspect of the diode is its utilization in rectifier circuits. These circuits change an AC waveform that contains no DC value to a rectified waveform that contains a DC value.

• Rectifiers convert a sinusoidal (i.e. AC) to an approximate DC signal. A DC power supply utilizes rectifiers. A RC filter may be connected to the output of the rectifier circuit to reduce ripple effects.

• Bipolar junction transistor (BJT) is a three-terminal device that has three separately doped semiconductor regions and two pn junctions. The three terminals are called the base (B), emitter (E), and collector (C).

• The defining BJT action is that the voltage across two terminals (base to emitter) controls the current in the third terminal (collector). The BJT is in its active region when the base-emitter junction is forward biased and the base-collector junction is reverse biased.

• Major applications of the BJT include switching currents and voltages, amplifying time varying signals, and performing digital logic functions.

• The MOSFET is an important amplifying element especially for digital electronic circuits.

• The current in the MOSFET is controlled by an electric field. This electric field is a function of the gate voltage. In the nonactive region of operation, the drain current is a function of the drain voltage, whereas in the active region of operation, the drain current is independent of the drain voltage. The drain current is proportional to the width-to-length ratio of the transistor. This parameter becomes the primary design variable in MOSFET circuit design.


REVIEW QUESTIONS 1. What does the intrinsic semiconductor material mean? 2. How is a pn junction formed? 3. Define a load line in a simple diode circuit. 4. Describe a simple half-wave diode rectifier and sketch the output voltage as a

function of time. 5. Describe a simple full-wave diode rectifier and sketch the output voltage as a

function of time. 6. What is the advantage of connecting a RC filter to the output of the diode

rectifier circuit? 7. Describe how a BJT can be used to amplify a time-varying voltage. 8. What type of semiconductor material is the base region for a npn transistor? 9. Describe the basic operation of a MOSFET.

10. What is the major factor in determining the current flow between the drain and source terminals of the MOSFET.


PROBLEMS

14-1 Consider the battery charging circuit in Figure 14-29. Consider VB = 12 V, VBE = 0.7 V, VS = 20 V, f = 60 Hz. The average battery charging current is to be iD = 2 A. Find the required value of R.

Figure 14-29 Circuit for Problem 14-1.

14-2 Consider the circuit shown in Figure 14-30. A DC voltmeter measures 12 V when connected between C and D. What would an AC voltmeter measures between A and B? Assume an ideal diode.


14-3 Consider the DC power supply given in Figure 14-31. Is this a half-wave or full-wave rectifier? Find the DC current in the load.

VB

R iD

vS(t) = vS sin ωt

A

B D

R

C

1:2



14-4 For the transistor circuit shown in Figure 14-32, assume V1 = 10 V, V2 = 9 V, R1 = 4 Ω, and R2 = 3 Ω.


14-5 For the transistor circuit shown in Figure 14-33, determine the operating region of the BJT when the base voltage VBB is short-circuited.

R = 500 Ω

120 V RMS; 60 Hz

IDC

V2

R1 R2

V1 - vD +



14-6 Determine the DC operating point of the BJT in the circuit of Figure 14-34. Figure 14-34 Circuit for Problem 14-6.

VCC 12 V RB

30 kΩ

0.7 V

B

E

RC 2 kΩ

+-

RE 500Ω

C

VCC 15 V RB

30 kΩ

0.7 V

B

E

RC 1 kΩ

+-

C

VBB 10 V


14-7 Consider the transistor amplifier circuit in Figure 14-35. Find the collector-emitter voltage vCE if the transistor is cutoff. What is the collector current iC if vCE = 10 V, and β = 100?


14-8 Consider the small-signal amplifier shown in Figure 14-22. Assume RS = 1 kΩ, R1 = 5.6 kΩ, R2 = 50 kΩ, RC = 2 kΩ, RL = 1 kΩ, VCC = 15 V. Find the open-circuit base bias voltage VBB, the DC output impedance of the bias network RB, the DC base current IB, the collector DC current IC. Does the transistor operate in the active region?

14-9 Consider the FET circuit in Figure 14-26. What value of vGS corresponds to

iD = 10 mA and vDS = 5 V?

14-10 Consider the FET amplifier in Figure 14-28. What is the drain current if the source is grounded?

10 kΩ

0.7 V

B

E

1 kΩ

+-

C

15 V + vCE - +

vin -


MULTIPLE CHOICE QUESTIONS Choose the right answer:

1. A ______ is a device that converts AC signal to DC signal a. transistor b. rectifier c. diode d. antenna

2. A ______ is a semiconductor device that allows the current to flow in one direction only. a. transistor b. rectifier c. diode d. regulator

3. Transistor technology was first developed during a. 1940s b. 1950s c. 1960s d. 1970s

4. To be used as an amplifier, the BJT must be biased into

a. active region b. cutoff region c. saturation region d. none of the above

5. The input base-emitter current-voltage characteristics of a BJT is similar to that of a. a. resistor b. capacitor c. ideal diode d. pn junction diode


6. One of the following is a nonlinear device a. pn junction diode b. resistor c. capacitor d. inductor

7. Coupling capacitors in amplifier circuits

a. blocks DC signal but pass AC signal. b. blocks AC signal but pass DC signal. c. blocks both DC and AC. d. none of the above

8. A small-signal amplifier must have a DC bias circuit for a. placing the transistor in its amplifying region. b. placing the capacitors in their amplifying region. c. placing the resistors in their amplifying region. d. all the above.

9. As a power supply circuit, the full-wave rectifier does better than the half-wave rectifier a. True b. False

11. To be used as an amplifier, the FET must be biased into

a. cutoff region b. saturation region c. triode region d. non of the above

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Basic Electronics - University of Ottawarhabash/BIECh14.pdf · 1 14 Basic Electronics In this chapter, we lead you through a study of the basics of electronics. After completing the