Gibilisco S. Teach Yourself Electricity and Electronics

Подождите немного. Документ загружается.

INTEGRATED CIRCUITS (ICs), ALSO CALLED CHIPS, HAVE STIMULATED AS MUCH EVOLUTION AS ANY

single development in the history of electronic technology. Most ICs look like gray or black plastic

boxes with protruding pins. The schematic symbols are triangles or rectangles with lines emerging

from them, labeled according to their functions.

Advantages of IC Technology

Integrated circuits have advantages over discrete components (individual transistors, diodes, capaci-

tors, and resistors). Here are some of the most important considerations.

Compactness

An obvious asset of IC design is economy of space. They are far more compact than equivalent cir-

cuits made from discrete components. A corollary to this is the fact that more complex circuits can

be built, and kept down to a reasonable size, using ICs as compared with discrete components.

High Speed

The interconnections among the components within an IC are physically tiny, making high switch-

ing speeds possible. Electric currents travel fast, but not instantaneously. The less time charge carri-

ers need to get from component X to component Y, in general, the more computations are possible

within a given span of time, and the less time is needed for complex operations.

Low Power Consumption

Another advantage of ICs is the fact that they use less power than equivalent discrete-component

circuits. This is crucial if batteries are to be used for operation. Because ICs use so little current, they

produce less heat than their discrete-component equivalents. This translates into better efficiency. It

also minimizes the problems that bedevil equipment that gets hot with use, such as frequency drift

and generation of internal noise.

491

CHAPTER

Integrated Circuits

Reliability

Integrated circuits fail less often, per component-hour of use, than systems that use discrete compo-

nents. This is because all the component interconnections are sealed within the IC case, preventing

corrosion or the intrusion of dust. The reduced failure rate translates into less downtime, or time

during which the equipment is out of service for repairs.

Ease of Maintenance

Integrated-circuit technology lowers maintenance costs. Repair procedures are simplified when fail-

ures occur. Many appliances use sockets for ICs, and replacement is simply a matter of finding the

faulty IC, unplugging it, and plugging in a new one. Special desoldering equipment is used with ap-

pliances having ICs soldered directly to the circuit boards.

Modular Construction

Modern IC appliances use modular construction. In this scheme, individual ICs perform defined

functions within a circuit board; the circuit board or card, in turn, fits into a socket and has a spe-

cific purpose. Computers, programmed with customized software, are used by repair technicians to

locate the faulty card in an appliance. The whole card can be pulled and replaced, getting the appli-

ance back to the consumer in the shortest possible time. Then a computer can be used to trou-

bleshoot the faulty card, getting the card ready for use in the next appliance that happens to come

along with a failure in the same card.

Limitations of IC Technology

No technological advancement ever comes without a downside. Integrated circuits have some limi-

tations that must be considered when designing an electronic system.

Inductors Impractical

While some components are easy to fabricate onto chips, other components defy the IC manufac-

turing process. Inductances, except for extremely low values, are an example. Devices using ICs

must generally be designed to work with discrete inductors, external to the ICs themselves. How-

ever, resistance-capacitance (RC) circuits are capable of doing most things that inductance-

capacitance (LC) circuits can do, and these can exist inside an IC.

High Power Impossible

The small size and low current consumption of ICs comes with an inherent limitation: high-power

amplifiers cannot, in general, be fabricated onto chips. High power necessitates a certain amount of

physical mass and volume. High-power devices and systems invariably generate large amounts of

heat, which must be efficiently radiated or conducted away.

Linear ICs

A linear IC is used to process analog signals such as voices, music, and radio transmissions.

The term linear arises from the fact that, in general, the amplification factor is constant as the

input amplitude varies. That is, the output signal strength is a linear function of the input signal

strength (Fig. 28-1).

492 Integrated Circuits

Operational Amplifier

An operational amplifier, or op amp, is a specialized linear IC that consists of several bipolar transis-

tors, resistors, diodes, and capacitors, interconnected so that high gain is possible over a wide range

of frequencies. An op amp might comprise an entire IC. Or, an IC might consist of two or more op

amps. Thus, you’ll sometimes hear of dual op amps or quad op amps. Some ICs have op amps in ad-

dition to other circuits.

An op amp has two inputs, one noninverting and one inverting, and one output. When a signal

goes into the noninverting input, the output is in phase with the input; when a signal goes into the

inverting input, the output is 180° out of phase with the input. An op amp has two power supply

connections, one for the emitters of the transistors (V

) and one for the collectors (V

). The usual

schematic symbol for an op amp is a triangle. The inputs, output, and power-supply connections

are drawn as lines emerging from it (Fig. 28-2).

The gain of an op amp is determined by one or more external resistors. Normally, a resistor is

connected between the output and the inverting input. This is called the closed-loop configuration.

The feedback is negative (out of phase), causing the gain of the op amp to be less than it would be

if there were no feedback (the open-loop configuration). Figure 28-3 is a schematic diagram of a non-

inverting closed-loop amplifier.

The reason for providing negative feedback in an op amp circuit is the fact that without it, the

gain may be too great. Excessive amplifier gain can cause problems. Open-loop op amps are prone

to instability, especially at low frequencies. They also generate a lot of internal noise.

When an RC combination is used in the feedback loop of an op amp, the gain depends on the

input-signal frequency. It is possible to get a lowpass response, a highpass response, a resonant peak, or

a resonant notch using an op amp and various RC feedback arrangements. These four types of re-

sponses are shown in Fig. 28-4.

Linear ICs 493

28-1 In a linear IC, the

relative output is a

linear (straight-line)

function of the relative

input. The solid lines

show examples of linear

IC characteristics. The

dashed curves show

functions not char-

acteristic of properly

operating linear ICs.

28-2 Schematic symbol

for an op amp.

Connections are

discussed in the text.

494 Integrated Circuits

28-4 Gain-versus-frequency

response curves.

At A, lowpass; at B,

highpass; at C,

resonant peak; at D,

resonant notch.

28-5 A differentiator circuit

that uses an op amp.

28-3 A closed-loop op amp

circuit with negative

feedback. If the

feedback resistor is

removed, it becomes

an open-loop circuit.

Op Amp Differentiator

A differentiator is an electronic circuit whose instantaneous output amplitude is proportional to the

rate at which the input amplitude changes. The circuit mathematically differentiates the input sig-

nal. Op amps can be used as differentiator circuits. An example is shown in Fig. 28-5.

When the input to a differentiator is a constant dc voltage, the output is zero (no signal). When

the input amplitude is increasing, the output is a positive dc voltage. When the input decreases, the

output is a negative dc voltage. If the input waveform fluctuates periodically (the usual case), the

output varies according to the instantaneous rate of change of the input amplitude. This results in

an output signal with the same frequency as that of the input signal, although the waveform is often

quite different. A pure sine wave input produces a pure sine wave at the output, but the phase is

shifted 90° to the left (that is,

⁄4 cycle earlier in time). Complex input waveforms can produce a

wide variety of outputs from a differentiator.

Op Amp Integrator

An integrator is an electronic circuit whose instantaneous output amplitude is proportional to the

accumulated input signal amplitude as a function of time. The circuit mathematically integrates

the input signal. The function of an integrator is basically the inverse, or opposite, of the func-

tion of a differentiator circuit. Figure 28-6 shows how an op amp can be connected to obtain an

integrator.

If an integrator circuit is supplied with an input signal waveform that fluctuates periodically

(the usual case), the output voltage varies according to the integral, or antiderivative, of the input

voltage. This results in an output signal with the same frequency as that of the input signal, although

the waveform is likely to be different. A pure sine wave input produces a pure sine wave output, but

the phase is shifted 90° to the right (that is,

⁄4 cycle later in time). Complex input waveforms can

produce many types of output waveforms.

An indefinite rise, either negatively or positively, in output voltage cannot occur in a practical

integrator. If the mathematical integral (in pure theory) of an input function is an endlessly increas-

ing output function, the actual output voltage rises to a certain maximum, either positive or nega-

tive, and stays there. This maximum is less than or equal to the power supply or battery voltage.

Voltage Regulator

A voltage regulator IC acts to control the output voltage of a power supply. This is important with

precision electronic equipment. These ICs are available with various voltage and current ratings.

Typical voltage regulator ICs have three terminals, and because of this, they are sometimes mistaken

for power transistors.

Linear ICs 495

28-6 An integrator circuit

that uses an op amp.

Timer

A timer IC is a specialized oscillator that produces a delayed output. The delay is adjustable to suit

the needs of a particular device. The delay is generated by counting the number of oscillator pulses;

the length of the delay can be adjusted by means of external resistors and capacitors. Timers are

commonly used in circuits such as digital frequency counters, where a precise time interval or win-

dow must be provided.

Multiplexer

A multiplexer IC allows several different signals to be combined in a single channel by means of a

process called multiplexing. An analog multiplexer can also be used in reverse; then it works as a de-

multiplexer. Thus, you’ll sometimes hear or read about a multiplexer/demultiplexer IC.

Comparator

A comparator IC has two inputs. It compares the voltages at the two inputs, which are called A and

B. If the voltage at input A is significantly higher than the voltage at input B, the output is about +5

V. This is logic 1, or high. If the voltage at input A is lower than or equal to the voltage at input B,

the output voltage is about +2 V. This is designated as logic 0, or low.

Voltage comparators are available for a variety of applications. Some can switch between low

and high states at a rapid rate of speed, while others are slow. Some have low input impedance, and

others have high impedance. Some are intended for AF or low-frequency RF use; others are fabri-

cated for video or high-frequency RF applications. Voltage comparators can be used to actuate, or

trigger, other devices such as relays and electronic switching circuits.

Digital ICs

A digital IC, also sometimes called a digital-logic IC, operates using two discrete states: high (logic

1) and low (logic 0). Digital logic is discussed in Chap. 26. Digital ICs contain massive arrays of

logic gates that perform Boolean operations at high speed.

Transistor-Transistor Logic

In transistor-transistor logic (TTL), arrays of bipolar transistors, some with multiple emitters, oper-

ate on dc pulses. This technology has several variants, some of which date back to around 1970. A

basic TTL gate is illustrated in Fig. 28-7. This gate uses two NPN bipolar transistors, one of which

is a dual-emitter device. The transistors are always either completely cut off, or else completely satu-

rated. Because of this, TTL is relatively immune to external noise.

Emitter-Coupled Logic

Another bipolar-transistor logic form is known as emitter-coupled logic (ECL). In an ECL device, the

transistors are not operated at saturation, as they are with TTL. This increases the speed. But noise

pulses have a greater effect in ECL, because unsaturated transistors are sensitive to, and can actually

amplify, external signals and noise. The schematic of Fig. 28-8 shows a basic ECL gate using four

NPN bipolar transistors.

496 Integrated Circuits

Metal-Oxide-Semiconductor Logic

Digital ICs can also be constructed using metal-oxide-semiconductor (MOS) devices. N-channel

MOS (NMOS) logic, pronounced “EN-moss logic,” offers simplicity of design, along with high op-

erating speed. P-channel MOS logic, pronounced “PEA-moss logic,” is similar to NMOS logic, but

the speed is slower. An NMOS or PMOS digital IC is like a circuit that uses only N-channel MOS-

FETs, or only P-channel MOSFETs, respectively.

Complementary-metal-oxide-semiconductor (CMOS) logic, pronounced “SEA-moss logic,” em-

ploys both N-type and P-type silicon on a single chip. This is analogous to using both N-channel

Digital ICs 497

28-7 A transistor-transistor

logic (TTL) gate. In

this gate, two NPN

bipolar transistors are

used. Note that one

of the transistors has

two emitters.

28-8 An emitter-coupled

logic (ECL) gate using

four NPN bipolar

transistors.

and P-channel MOSFETs in a circuit. The advantages of CMOS technology include extremely low

current drain, high operating speed, and immunity to noise.

All forms of MOS logic ICs require care in handling to prevent destruction by electrostatic dis-

charges. The precautions are the same as those that are required when handling MOSFETs. All tech-

nical personnel who work with the devices should be grounded. This can be achieved by having

technicians wear metal wrist straps connected to a good electrical ground, and by ensuring that the

relative humidity in the lab is not allowed to get too low. When MOS ICs are stored, the pins

should be pushed into special conductive foam that is manufactured for that purpose.

Component Density

The number of elements per chip in an IC is called the component density. There has been a steady

increase in the number of components that can be fabricated on a single chip. There is an absolute

limit on component density, imposed by the atomic structure of the semiconductor material—

literally, the size of the atoms! Technology has begun to approach that barrier.

Small Scale

In small scale integration (SSI), there are fewer than 10 transistors on a chip. These types of ICs can

carry the largest currents, and can be useful in voltage regulation and other moderate-power appli-

cations.

Medium Scale

In medium scale integration (MSI), there are 10 to 100 transistors per chip. This allows for consider-

able miniaturization, but it is not a high level of component density, relatively speaking. An advan-

tage of MSI (in a few applications) is that fairly large currents can be carried by the individual gates.

Both bipolar and MOS technologies can be adapted to MSI.

Large Scale

In large scale integration (LSI), there are 100 to 1000 transistors per semiconductor chip. This is an

order of magnitude (a factor of 10 times) more dense than MSI. Electronic wristwatches, single-chip

calculators, and simple microcomputers are examples of devices using LSI ICs.

Very-Large-Scale Devices

Very-large-scale integration (VLSI) devices have from 1000 to 1,000,000 transistors per chip. This

can be up to three orders of magnitude more dense than LSI. Microcomputers and memory ICs are

made using VLSI.

Ultra-Large-Scale Devices

You might sometimes hear of ultra-large-scale integration (ULSI). Devices of this kind have more

than 1,000,000 transistors per chip. The principal uses for this technology are in the fields of high-

level computing, supercomputing, robotics, and artificial intelligence (AI).

498 Integrated Circuits

IC Memory

Binary digital data, in the form of high and low levels (logic ones and zeros), can be stored in mem-

ory ICs. These devices can take various physical forms.

Random Access

A random access memory (RAM) chip stores binary data in arrays. The data can be addressed (selected)

from anywhere in the matrix. Data is easily changed and stored back in RAM, in whole or in part.

A RAM chip is sometimes called a read/write memory.

An example of the use of RAM is a word-processing computer file that you are actively work-

ing on. This paragraph, this chapter, and in fact the whole text of this book was written in semicon-

ductor RAM in small sections before being incrementally stored on the computer hard drive, and

ultimately on external media.

There are two major categories of RAM: dynamic RAM (DRAM) and static RAM (SRAM). A

DRAM chip contains transistors and capacitors, and data is stored as charges on the capacitors. The

charge must be replenished frequently, or it will be lost through discharge. Replenishing is done au-

tomatically several hundred times per second. An SRAM chip uses a flip-flop to store the data. This

gets rid of the need for constant replenishing of charge, but the tradeoff is that SRAM ICs require

more elements than DRAM chips to store a given amount of data.

With any RAM chip, the data in it will vanish when power is removed, unless some provision is

made for memory backup. The most common means of memory backup is the use of a small cell or bat-

tery with a long shelf life. Modern IC memories need so little current to store their data that a backup

battery lasts as long in the circuit as it would on the shelf. Memory that disappears when power is re-

moved is called volatile memory. If memory is retained when power is removed, it is nonvolatile.

Read Only

By contrast to RAM chips, the data in a read-only memory (ROM) chip can be easily accessed, in

whole or in part, but not easily written over. A standard ROM chip is programmed at the factory.

This permanent programming is known as firmware. But there are also ROM chips that you can

program and reprogram yourself. The data contents of ROM chips are generally nonvolatile. That

means a power failure is no cause for concern.

An erasable programmable ROM (EPROM) chip is an IC whose memory is of the read-only

type, but that can be reprogrammed by a certain procedure. It is more difficult to rewrite data in an

EPROM than in a RAM; the usual process for erasure involves exposure to ultraviolet (UV). An

EPROM IC can be recognized by the presence of a transparent window with a removable cover,

through which the UV is focused to erase the data. The IC must be taken from the circuit in which

it is used, exposed to UV for several minutes, and then reprogrammed.

The data in some EPROM chips can be erased by electrical means. Such an IC is called an elec-

trically erasable programmable read-only memory (EEPROM) chip. These do not have to be removed

from the circuit for reprogramming.

Quiz

Refer to the text in this chapter if necessary. A good score is at least 18 correct. Answers are in the

back of the book.

Quiz 499

1. Because of the small size of ICs compared with equivalent circuits made from discrete

components,

(a) more heat is generated.

(b) higher power output is possible.

(d) fewer calculations need to be done per unit time.

2. Which of the following is not an advantage of ICs over discrete components?

(a) Higher component density

(b) Ease of maintenance

(d) Lower current consumption

3. In which of the following devices would you never find an IC as the main component?

(a) The final amplifier in a large radio broadcast transmitter

(b) The microprocessor in a personal computer

(d) A low-power audio amplifier

4. Which, if any, of the following component types is not practical for direct fabrication on a

chip?

(a) Resistors

(b) Capacitors

(d) All three of the above component types are practical for direct fabrication on a chip.

5. An op amp circuit usually employs negative feedback to

(a) maximize the gain.

(b) control the gain.

(d) Forget it! Op amp circuits never have negative feedback.

6. Suppose a communications channel carries several signals at once. Which type of IC might be

used to single one of the signals out for reception?

(a) An op amp

(b) A timer

(d) A multiplexer/demultiplexer

7. Which type of IC is commonly used to determine whether two dc voltages are the same or

different?

(a) An op amp

(b) A timer

500 Integrated Circuits