经典指南：电子学艺术第二版

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"The Art of Electronics" 是一本经典的电子学教材，由 Paul Horowitz 和 Winfield Hill Rowland 合著，由剑桥大学出版社出版。这本书在1980年首次发行，并在1989年推出了第二版，后续多次重印，反映了其在电子工程领域的持久影响力。该书的目的是为读者提供深入理解电子原理和技术的基础，从基础概念如电压、电流和电阻，到高级主题如信号处理和电路分析。在第一章"Foundations"中，作者从介绍基本概念开始，阐述了电压（voltage）、电流（current）和电阻（resistance）的重要性。1.01节详细解释了电压和电流的概念，它们是电路中的基本物理量，用来描述电荷的移动和能量的传输。1.02节探讨了电压和电流之间的关系，尤其是在电阻元件中的行为，强调了欧姆定律的应用，即电压等于电流乘以电阻。 1.03讲授了电压分压器（voltagedividers），这是电路设计中常用的工具，用于分配或调整电压。1.04部分讨论了电源，包括电压源和电流源，这对于电路设计时提供稳定电压或恒定电流至关重要。1.05介绍了Thevenin等效电路的概念，这是一种简化电路的方法，通过它，复杂的电路可以被简化为一个单一的电压源和电阻，便于分析和设计。 1.06小信号电阻的概念涉及在交流电路分析中，电阻如何随着频率变化，这对于理解信号在电路中的衰减和增益行为非常重要。接下来的章节转向信号处理，1.07章节讲解了正弦波信号，这是电信号分析的基础，因为它广泛存在于许多电子设备中，如音频和通信系统。《The Art of Electronics》以其详尽的内容和实用的指导，使读者能够掌握电子学的核心原理和实践技巧，无论是初学者还是经验丰富的工程师，都能从中受益匪浅。这本书不仅涵盖了电子学的基础，还深入探讨了信号处理和电路设计的高级技术，是一本不可或缺的电子工程学习资料。

SOME BASIC TRANSISTOR CIRCUITS

2.02

Transistor

switch

that diode is reverse

biased. Just think of

it as

transistor action.

Property

gives the transistor its useful

ness: A small current flowing into the base

controls a much larger current flowing into

the collector.

Warning:

hFE is not a

good

transistor

parameter; for instance, its value can vary

from 50 to 250 for different specimens of

given transistor type. It also depends upon

the collector current, collector

emitter

voltage, and temperature.

circuit that

depends on a particular value for

hFE

a bad circuit.

Note particularly the effect of property

This means you can't go sticking a voltage

across the base

emitter terminals, because

an enormous current will flow if the base

is more positive than the emitter by more

than about 0.6 to 0.8 volt (forward diode

drop). This rule also implies that an op

erating transistor has VB

0.6

volt

(VB

VBE). Again, polarities are

normally given for

npn

transistors; reverse

them for

pnp.

Let us emphasize again that you should

not try to think of the collector current

as diode conduction. It isn't, because the

collector

base diode normally has voltages

applied across it in the reverse direction.

Furthermore, collector current varies very

little with collector voltage (it behaves like

a not

too

great current source), unlike for

ward diode conduction, where the current

rises very rapidly with applied voltage.

SOME BASIC TRANSISTOR CIRCUITS

2.02

Transistor switch

Look at the circuit in Figure

2.3.

This ap

plication, in which a small control current

enables

much larger current to flow in an

other circuit, is called a transistor switch.

From the preceding rules it is easy to un

derstand. When the mechanical switch is

open, there is no base current. So, from

10V

0.1A

mechanical

switch

Figure

2.3.

Transistor switch example.

rule 4, there is no collector current. The

lamp is off.

When the switch is closed, the base

rises to 0.6 volt (base

emitter diode is in

forward conduction). The drop across

the base resistor is 9.4 volts, so the base

current is

9.4mA. Blind application of rule

4 gives

940mA (for a typical beta

of 100). That is wrong. Why? Because

rule 4 holds only if rule 1 is obeyed; at a

collector current of

lOOmA the lamp has

10 volts across it. To get a higher current

you would have to pull the collector below

ground. A transistor can't do this, and

the result is what's called saturation

the

collector goes as close to ground as it can

(typical saturation voltages are about

0.05-

0.2V, see Appendix G) and stays there. In

this case, the lamp goes on, with its rated

10 volts across it.

Overdriving the base (we used

9.4mA

when 1 .OmA would have barely sufficed)

makes the circuit conservative; in this

particular case it is a good idea, since

a lamp draws more current when cold

(the resistance of a lamp when cold is

to 10 times lower than its resistance at

operating current). Also transistor beta

drops at low collector

base voltages, so

some extra base current is necessary to

bring a transistor into full saturation (see

Appendix G). Incidentally, in a real circuit

you would probably put a resistor from

base to ground (perhaps

10k in this case)

to make sure the base is at ground with

the switch open. It wouldn't affect the

TRANSISTORS

Chapter

operation, because it would sink only

0.06mA from the base circuit.

There are certain cautions to be ob

served when designing transistor switches:

Choose the base resistor conservatively

to get plenty of excess base current, es

pecially when driving lamps, because of

the reduced beta at low

VCE.

This is

also a good idea for high

speed switching,

because of capacitive effects and reduced

beta at very high frequencies (many mega

hertz). A small

speedup

capacitor is of

ten connected across the base resistor to

improve high

speed performance.

the load swings below ground for

some reason

(e.g., it is driven from ac,

or it is inductive), use a diode in series

with the collector (or a diode in the reverse

direction to ground) to prevent

collector-

base conduction on negative swings.

For inductive loads, protect the transis

tor with a diode across the load, as shown

in Figure

2.4.

Without the diode the in

ductor will swing the collector to a large

positive voltage when the switch is opened,

most likely exceeding the collector

emitter

breakdown voltage, as the inductor tries to

maintain its

current from

Vcc

to the

collector (see the discussion of inductors in

Section 1.3

1).

Figure

2.4.

Always use a suppression diode

when switching an inductive load.

Transistor switches enable you to switch

very rapidly, typically in a small fraction of

a microsecond. Also, you can switch many

different circuits with a single control sig

nal. One further advantage is the possibil

ity of remote cold switching, in which only

dc control voltages snake around through

cables to reach front

panel switches, rather

than the electronically inferior approach

of having the signals themselves traveling

through cables and switches (if you run lots

of signals through cables, you're likely to

get capacitive pickup as well as some sig

nal degradation).

Transistor man

Figure

2.5

presents a cartoon that will help

you understand some limits of transistor

Figure

2.5.

Transistor man

observes the base

current, and adjusts the output rheostat in an

attempt to maintain the output current

ILFE

times larger.

behavior. The little man's perpetual task

in life is to try to keep

hFEIB;

however, he is only allowed to turn the

knob on the variable resistor. Thus he

can go from a short circuit (saturation)

to an open circuit (transistor in the

state), or anything in between, but he isn't

allowed to use batteries, current sources,

etc. One warning is in order here: Don't

think that the collector of a transistor

looks like a resistor. It doesn't. Rather,

it looks approximately like a poor

quality

constant

current sink (the value of current

depending on the signal applied to the

base), primarily because of this little man's

efforts.

Another thing to keep in mind is that,

at any given time, a transistor may be (a)

cut off (no collector current), (b) in the

active region (some collector current, and

collector voltage more than a few tenths

of a volt above the emitter), or (c) in

saturation (collector within a few tenths of

a volt of the emitter). See Appendix

transistor saturation for more details.

2.03

Emitter follower

Figure 2.6 shows an example of an

emitter

follower.

It is called that because the out

put terminal is the emitter, which follows

the input (the base), less one diode drop:

0.6 volt

The output is a replica of the input, but 0.6

to 0.7 volt less positive. For this circuit,

V,,

must stay at +0.6 volt or more, or

else the output will sit at ground. By

returning the emitter resistor to a negative

supply voltage, you can permit negative

voltage swings as well. Note that there is

no collector resistor in an emitter follower.

Figure

2.6.

Emitter

follower.

At first glance this circuit may appear

useless, until you realize that the input

impedance

much larger than the out

put impedance, as will be demonstrated

SOME BASIC TRANSISTOR CIRCUITS

2.03

Emitter follower

shortly.

This means that the circuit re

quires less power from the signal source

to drive a given load than would be the

case if the signal source were to drive the

load directly. Or a signal of some inter

nal impedance (in the ThCvenin sense) can

now drive a load of comparable or even

lower impedance without loss of amplitude

(from the usual voltage

divider effect). In

other words, an emitter follower has cur

rent gain, even though it has no voltage

gain. It has power gain. Voltage gain isn't

everything!

Impedances of sources and loads

This last point is very important and is

worth some more discussion before we

calculate in detail the beneficial effects of

emitter followers. In electronic circuits,

you're always hooking the output of some

thing to the input of something else, as

suggested in Figure 2.7. The signal source

might be the output of an amplifier stage

(with Thevenin equivalent series imped

ance ZOut), driving the next stage or per

haps a load (of some input impedance Zin).

In general, the loading effect of the follow

ing stage causes a reduction of signal, as we

discussed earlier in Section 1.05. For this

reason it is usually best to keep

Zo,t

Zin

(a factor of 10 is a comfortable rule of

thumb).

In some situations it is OK to forgo

this general goal of making the source stiff

compared with the load. In particular, if

the load is always connected

(e.g., within

a circuit) and if it presents a known and

constant

Zi,, it is not too serious if it

loads

the source. However, it is always

nicer if signal levels don't change when

a load is connected. Also, if

Zin varies

with signal level, then having a stiff source

(Zout

Zin) assures linearity, where oth-

erwise the level

dependent voltage divider

would cause distortion.

Finally, there are two situations where

ZOut

Zi, is actually the wrong thing to

TRANSISTORS

Chapter

t~rst

;iriipl~fwr second ampl~f~er

Figure

2.7.

Illustrating circuit

as a voltage divider.

do: In radiofrequency circuits we usually

match

impedances (Z,,t

Zin), for

reasons we'll describe in Chapter

14. A

second exception applies if the signal being

coupled is a

current rather than a voltage.

In that case the situation is reversed, and

one strives to make

Zi,

Zout

(ZOut

oo,

for a current source).

Input and output impedances of emitter

followers

As you have just seen, the emitter

follower is useful for changing impedances

of signals or loads. To put it bluntly, that's

the whole point of an emitter follower.

Let's calculate the input and output

impedances of the emitter follower. In

the preceding circuit we will consider

to be the load (in practice it sometimes is

the load; otherwise the load is in parallel

with

but with

dominating the parallel

resistance anyway). Make a voltage change

AVB at the base; the corresponding change

at the emitter is

AVE

AVB. Then the

change in emitter current is

(using

IB) The input resistance

AVB /AIB. Therefore

The transistor beta

(hfe) is typically

about

100,

so a low

impedance load looks

like a much higher impedance at the base;

it is easier to drive.

In the preceding calculation, as in Chap

ter

we have used lower

case symbols

such as h

to signify small

signal (incre

mental) quantities. Frequently one con

centrates on the changes in voltages

(or currents) in a circuit, rather than the

steady (dc) values of those voltages (or

currents). This is most common when

these

small

signal

variations represent

a possible signal, as in an audio amplifier,

riding on a steady dc

bias

(see Section

2.05). The distinction between dc cur

rent gain (hFE) and small

signal current

gain (h

,) isn't always made clear, and the

term beta is used for both. That's alright,

since

hfe

hFE (except at very high fre

quencies), and you never assume you know

them accurately, anyway.

Although we used resistances in the

preceding derivation, we could generalize

to complex impedances by allowing

AVB,

AIB, etc., to become complex num

bers. We would find that the same

SOME BASIC TRANSISTOR CIRCUITS

2.03

Emitter follower

transformation rule applies for imped

EXERCISE

2.2

ances:

Zi,

(hf,

l)Zl,,d.

Use

follower with base driven from a voltage

We could do a similar calculation to

divider to provide a stiff source of

volts from

find that the output impedance

zOUt

of an

an available regulated

+I5

volt supply. Load

emitter follower (the impedance looking

current (ma'()

25mA. Choose Your resistor

values so that the output voltage doesn't drop

into the emitter) driven from a source of

than

50,0

under

full

load.

internal impedance

ZsOurce

is given by

Zsource

Zout

hfe

Strictly speaking, the output impedance of

the circuit should also include the parallel

resistance of

but in practice

ZOut

(the

impedance looking into the emitter) dom

inates.

EXERCISE

2.1

Show that the preceding relationship is correct.

Hint: Hold the source voltage fixed, and find

the change in output current for a given change

in output voltage. Remember that the source

voltage is connected to the base through

series resistor.

Because of these nice properties, emit

ter followers find application in many

situations,

e.g., making low

impedance sig

nal sources within a circuit (or at out

puts), making stiff voltage references from

higher

impedance references (formed from

voltage dividers, say), and generally isolat

ing signal sources from the loading effects

of subsequent stages.

Figure

2.8.

npn

emitter follower can source

plenty

of current through the transistor, but

can

sink limited current only through its emitter

resistor.

Important points about followers

Notice (Section 2.01, rule

that in

an emitter follower the

npn

transistor can

only

source

current. For instance, in

the loaded circuit shown in Figure 2.8 the

output can swing to within a transistor

saturation voltage drop of

Vcc

(about

+9.9V),

but it cannot go more negative

than

volts. That is because on the

extreme negative swing, the transistor can

do no more than turn off, which it does at

4.4

volts input

(-5V

output). Further

negative swing at the input results in

backbiasing of the base

emitter junction,

but no further change in output. The

output, for a

volt amplitude sine

wave

input, looks as shown in Figure 2.9.

Input

output

Figure

2.9.

Illustrating

the

asymmetrical cur

rent drive capability of the

npn

emitter fol

lower.

Another way to view the problem is

to say that the emitter follower has low

small

signal output impedance. Its large-

signal output impedance is much larger

(as large as

The output impedance

changes over from its small

signal value to

its large

signal value at the point where the

transistor goes out of the active region (in

this case at an output voltage of

5V).

put this point another way, a low value of

small

signal output impedance doesn't

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经典指南：电子学艺术第二版

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THE ART OF ELECTRONICS 电工学英文原版