Ch. 21--Semi-Conductors and Radios

293

FIGURE 21.1

lattice of Germanium atoms(dashes signify shared valence electrons)

Ge Ge Ge Ge

Ge Ge Ge Ge

Ge Ge Ge Ge

Chapter 21

SEMI-CONDUCTORS AND RADIOS

A.) Semiconductors--"p-types" and "n-types":

1.) By themselves, Germa-nium (Ge) atoms bond in thesame way all insulators do--cova-lently. Each Germanium atomhas four valence electrons whichare shared with four Germaniumneighbors. See Figure 21.1.

2.) A curious thing hap-pens when Gallium (Ga)--a fivevalence electron atom--is intro-duced into a structure whoseatomic composition is made up ofGermanium.

a.) Each Germaniumatom around the Galliumshares a single electron

FIGURE 21.2a

Germanium doped with Gallium

Ge Ge Ge Ge

Ge Ge Ge

Ge Ge Ge Ge

Ga

extra electron

with its oddball neighbor (theGa), leaving the Gallium withone electron more than wouldhave been the case if aGermanium had occupied thespot (see Figure 21.2a).

Note: Placing impurities intoan otherwise pure insulator is called"doping."

b.) Because Gallium's va-lence shell holds only eight elec-trons, this extra electron doesn'tfit into the valence orbital andhas to occupy an energy orbitalfurther out. In so doing, it is

294

FIGURE 21.2b

n-type semi-conductor in an external electric field

Ge GeGa

extra electron will break loose and migrate to the left in an external electric field

external electric field

FIGURE 21.3a

hole

Ge Ge Ge Ge

Ge Ge

Ge Ge Ge Ge

As

Germanium doped with Arsenic: the hole (the absence of an electron) acts like a positive charge and migrates to the right when subjected to an electric field oriented to the right

Ge

only loosely bound to the positive Gallium nucleus.

c.) Due to this looseattraction, a small ex-ternal electric field isall that is needed tobreak the extra electronaway from its hostatom. Once free, theelectron can migratethrough the structurejust as do electrons inconductors (see Figure21.2b).

d.) Bottom line: Ifwe take an insulator(Germanium for exam-ple) and place the right impurities in it (Gallium in this case), we willend up with an insulator that has conductor-like characteristics--amaterial through which free charge can migrate if an electric field isapplied.

3.) A doped insulator like this is called a semi-conductor. In the caseoutlined above, negative charges (electrons) move through the material. Assuch, the material is called an n-type semi-conductor.

4.) A positive-charge-version ofthe semi-conductor can be generatedusing Arsenic (As) as the impurity.Arsenic has three valence electrons.Introducing it into a structure whoseatomic composition is made up ofGermanium yields the following:

a.) Each Germaniumatom around the Arsenicwants to share a single elec-tron with its oddball neighbor(the As). After doing so,though, the Arsenic still hasone less electron than wouldhave been the case if a Germa-nium had occupied the spot.

Ch. 21--Semi-Conductors and Radios

295

FIGURE 21.3b

electron in E. field migrates into hole on left leaving a hole on the right-- the motion makes it look as though the hole has migrated to the right

GeAs

p-type semi-conductor in an external electric field

Ge

external electric fieldapparent motion of hole

FIGURE 21.4

R load

n-type semi-conductor

p-type semi-conductor

In other words,there is a hole wherean electron should be(see Figure 21.3a).

b.) Placing a smallelectric field acrossthis structure (Figure21.3b) motivates elec-trons from neighbor-ing atoms to migrateup-field into the vacan-cies.

c.) When an elec-tron leaves a Germani-um atom to fill a hole,it leaves a hole of itsown behind. That newvacancy makes the area around that Germanium less negative--hence, MORE POSITIVE--than otherwise would have been (there isone less electron present).

Put another way, holes act like positive charges in the sense thatwhere a hole exists, the area around it is more positive than it wouldotherwise have been.

d.) Bottom line: When an electric field is applied to this kind ofsemi-conductor, holes move through the structure. As such, thecharge carriers are called holes, they are associated with positivecharge, and the material is called a p-type semi-conductor.

B.) Diodes and the Production of DC from AC:

1.) If a p-type and an n-type semi-conductor are joined together and placedin an AC circuit as shown in Figure21.4, the following occurs:

a.) When the right terminalof the power supply is positive (seeFigure 21.5a on the next page), theholes in the p-type semi-conductormove to the left and the electronsin the n-type semiconductor move

296

FIGURE 21.5a

VR

direction of electron conduction

direction of hole conduction

Vp.s.

+-

FIGURE 21.5b

direction of electron conduction

direction of hole conduction

+ -

Vp.s.

depletion zone (acts like circuit break)

VR

reversed polarity

nocurrent

to the right. At the junctionbetween the two semi-conductors, the holes andelectrons combine andneutralize one another.

b.) As there is no appre-ciable voltage drop at thejunction, the entire voltagedrop in the circuit occursacross the resistor. As such,the voltage versus timegraph for the power supplyand the voltage versus timegraph for the resistor looksimilar, as shown in Figure21.5a.

The current through aresistor is proportional to the voltage across the resistor. This tells usthere will be a current in the circuit for this part of the power cycle.

c.) During thesecond half of thepower supply'spower cycle, the leftterminal reverses po-larity and becomespositive (see Figure21.5b). Holes in thep-type semi-conduc-tor move to the rightand electrons in then-type semi-conduc-tor move to the left.This separation ofcharge carriers(holes going one way,electrons going theother) creates a de-pletion zone at thejunction between thetwo materials.

d.) In this case, a large voltage drop will exist across the depletionzone with virtually no voltage drop across the resistor. That means nocurrent flows in the circuit (remember, the voltage drop across aresistor is proportional to the current through the resistor).

Ch. 21--Semi-Conductors and Radios

297

FIGURE 21.5c

Vp.s.

VR

a diode makes AC into DC

FIGURE 21.5d

circuit-symbol for a diode

current can pass this way

current blocked in this direction

Note: In simple terms, a depletion zone acts like a break in the circuit.

e.) Putting everything together, when the AC power supplyexhibits one polarity, we see current in the circuit. When the polaritychanges, the current stops.

f.) Bottom line: Diodes allow cur-rent to flow in one and only one direc-tion. That is, they make DC out of AC.The graphs in Figure 21.5c summa-rize this.

Note 1: A reminder: In an AC circuit,charge carriers jiggle back and forth as thealternating voltage that drives themswitches polarity across the power supply.In a DC circuit, on the other hand, the cur-rent flows in one direction only.

The DC generated by a single diode islumpy, but it is still DC--current is still flow-ing in one direction only.

Note 2: The circuit symbol for a diodeis shown in Figure 21.5d. The arrow points in the direction that current (i.e.,positive charge flow) can pass.

Note 3: A single diode in an ACcircuit is sometimes called a half-waverectifier because it keeps only half the ACsignal as it makes DC.

2.) The Diode Bridge: DC generatedby a single diode is inadequate for tworeasons: 1.) current flows only during half ofthe power cycle (consequence--half thepower available to the circuit is lost), and 2.)the DC is lumpy. To correct these problems:

a.) Consider the diode bridge and power supply shown in Figure21.6 on the next page. With the voltage polarity such that the + termi-nal is on the upper side of the power source (this polarity is shown onthe inside of the circuit diagram), the current in the circuit flowsthrough the load resistor from left to right.

298

diode bridge (full-wave rectifier)

+

-

Rload

current flow for inside polarity

+

-

current flow for outside polarity

FIGURE 21.6

FIGURE 21.7

AC made into lumpy DC via a Full Wave Rectifier

Vload

Vp.s.

Note: By load resistor, we are talking about any device that requireselectrical power. It could be a light bulb, a Skill saw, or a toaster. Do not bethrown by the wordresistor in this context.It is simply a generalterm used to cover allpossibilities.

b.) With thepower supply po-larity such thatthe + terminal ison the lower sideof the power sup-ply (this is shownon the outside ofthe circuit dia-gram), the cur-rent in the circuitagain flowsthrough the loadresistor from leftto right.

c.) Bottom line: Current flows through the diode bridge in thesame direction no matter what.

d.) A graphical representa-tion of both the voltage across thepower supply as a function oftime and the voltage across theload resistor as a function of timeis shown in Figure 21.7. Noticethat the voltage across theresistor is still lumpy-DC, but weare no longer losing half ourpower in the process.

Diode bridges that flip half anAC wave over (versus blottinghalf the wave out altogether) arecalled full wave rectifiers.

3.) To get rid of the lumpiness:

Ch. 21--Semi-Conductors and Radios

299

Rload

diode bridge with lump- eliminatingcapacitor across load

FIGURE 21.8

C

a.) A capacitor must be placed across the load resistor (see Figure21.8). When the power is firstturned on, the current in thecircuit will rise and thecapacitor will charge up. Thevoltage sooner or later reachesits maximum and begins to de-crease. As it does, the chargedcapacitor begins to dischargethrough the load resistor.

b.) With extra currentflowing through the load resis-tor due to the discharging ca-pacitor, the voltage across theload resistor stays higher thanwould normally be expected,dropping only slightly withtime as the charge on the ca-pacitor diminishes.

FIGURE 21.9

voltage across capacitor keeps voltage across load-resistor artificially high producing a DC signal with ripple

Vload

VDC

ripple

c.) After a time the power cycle returns to higher voltages where-upon the voltage across the resistor increases. At that point, the ca-pacitor also begins to re-charge and the cycle begins all over again. Agraph of the superposition ofall the voltages affecting theresistor is shown in Figure21.9.

d.) Note that the net volt-age across the load resistorshown in Figure 21.9 is notcompletely constant. Thisvariation in AC generated DCis called ripple.

A good power supply willhave less than 2% ripple withcheaper power supplieshaving ripple as large as 8%.In both cases, the variation is comparatively little--so small, in fact,that it is not normally evident even when viewed on an oscilloscope.

e.) Summary: A laboratory power supply generates DC by takingthe 110 volt RMS voltage provided by the wall socket, using a trans-former to step the voltage up or down depending upon the need, andpassing the signal through a diode bridge (full wave rectifier) coupledwith a capacitor wired in parallel to the output terminal (i.e., the

300

sending antenna

ground

transformersignal generator

radio sending unit

FIGURE 21.10

terminal into which the load is to be plugged). The output DC voltagewill have some ripple, but it will be relatively lumpless.

C.) Radio Circuits--Sending Stations:

Note: A simplified circuit for a radio sending unit is shown in Figure21.10. A discussion of its workings is presented below:

1.) Charge flowing up and down anantenna at high frequency creates thefollowing in the vicinity of the antenna:

a.) A rapidly varying (i.e., highfrequency) electric field disturbancewhose direction and magnitudechange constantly, and

b.) A rapidly varying (i.e., highfrequency) magnetic fielddisturbance whose direction andmagnitude change constantly.

Note: The rationale for these twoassertions should be obvious. Electric fields are produced whenever freecharge is present. That is the case here as the antenna becomes positively,then negatively, then positively charged. Magnetic fields are produced bycharge in motion--also the case here as charge runs up and down theantenna.

c.) The alternating magnetic and electric fields around theantenna will be perpendicular to and in synch with one another. Thecombination of the two is called an "electromagnetic disturbance."

2.) If the frequency of this electromagnetic disturbance is high enough(over 500,000 hertz), there will not be enough time for the disturbance tocollapse back onto the antenna before the next build-up begins. As a result,the disturbance will be forced to "flip off" the antenna and move outward at aspeed of 3x108 meters per second--the speed of light.

Ch. 21--Semi-Conductors and Radios

301

Note 1: Both light and radio waves are electromagnetic waves. Theonly structural difference between the two are their frequencies. Opticallight has a frequency range between 4.3x1014 hertz and 7.5x1014 hertz whileradio radiation has a frequency range between .5x106 hertz and 1.7x106 hertz.

Note 2: We now have a rational explanation for why light, acting like awave (i.e., a disturbance moving through a medium), can move through thenothingness of space (a mediumless region) from the sun to the earth.

According to Faraday's Law, changing magnetic fields induce electricfields. Although we haven't discussed it, it turns out that changing electricfields induce magnetic fields. In other words, the changing magnetic andelectric fields of an electromagnetic wave support one another.

In short, electromagnetic waves are their own medium.

3.) Every radio station is given one frequency (plus or minus a bit ofslop) between 500,000 and 1,700,000 cycles per second to use for its broadcasts(we will talk about how a station puts its programming information on thatone frequency later).

Note: If you examine an old fashioned radio dial (i.e., one that isn'tdigital), you will find that the dial is marked with the units "10 kHz." If youmultiply a station's call numbers by those units, you will get the station'ssending frequency.

Example: In L.A., KFWB's call numbers are "Channel 98." Multiply98 by 10 kHz and you get 98x10x103 Hz, or 980,000 hertz. That is the singlefrequency the FCC (Federal Communications Commission) has given KFWBfor its transmission.

4.) Looking back at Figure 21.10, we see a single frequency oscillator, atransformer for transferring the signal to the antenna, and a groundedantenna (the grounding is needed to allow charge to freely flow onto and off ofthe antenna).

This is an extremely simplified schematic. None of the circuitryneeded to put information onto the station's single-frequency-wave is shown.Nevertheless, it does generally represent the bare-bones circuit of a station'ssetup.

5.) After flip-off, the EM wave moves out from the station over thecountryside.

302

electromagnetic wave with electric and magnetic field components perpendicular to one another

FIGURE 21.11

x

receiving antenna

ground

transformer linking antenna circuit to tuning circuit

radio receiving unit

FIGURE 21.12

transformer linking tuning circuit to speaker circuit

speakersvariable capacitor

diode

tuning circuit

speaker circuitR

L

a.) If sensors arespread out to record theintensity and directionof the electromagneticwave at various placesat a particular instant,the electric and mag-netic field variationswill look like the graphshown in Figure 21.11.

i.) The electricfield component will change both magnitude and direction fromplace to place as will the magnetic field component.

ii.) The electric and magnetic fields will be in phase with oneanother and perpendicular to one another, and the whole messwill be moving away from the radio antenna at the speed of light.

b.) The fun begins when that EM wave comes in contact with theantenna of your radio.

D.) Radio Circuits--the Receiving End:

Note: The circuit for asimple AM radio is shown inFigure 21.12. A discussion of itsworkings is provided below:

1.) As the incoming elec-tromagnetic wave impinges uponthe receiving antenna of your ra-dio, the charge carriers in the an-tenna respond to the varying EMfield by oscillating up and downthe antenna. The oscillation pat-tern of these charge carriers willmatch identically with the oscilla-tion pattern of the charge carriersthat produced the EM wave on thesending antenna in the first place.

Bottom line: The electro-magnetic wave allows us to trans-

Ch. 21--Semi-Conductors and Radios

303

fer charge jiggle from a sending antenna to a receiving antenna.

2.) Receiving antennas are not selective as to the signals they accept.If there are ten radio stations transmitting in a particular area, the chargeson the antenna will respond to the EM waves coming from each of thosestations. The net jiggle on your antenna will be the superposition of all thewaves impinging on the antenna. We need a way to select out one stationonly. To do so, consider the tuning circuit.

3.) A transformer is used to transfer signals picked up by the antennafrom the antenna circuit to what is generally called the tuner circuit. Thebasis for the tuner circuit is as follows:

a.) From the last chapter, a driven RLC circuit (i.e., an RLCcircuit in which there is some source of power in the circuit) has thepeculiar characteristic of allowing only one frequency to oscillate freelywithin it. If you will remember, that frequency is called the resonancefrequency and is mathematically equal to:

νres = (1/2π) (1/LC)1/2.

b.) Consider for a moment a radio signal that is transmitting atsome frequency other than the natural frequency of the (R)LC combi-nation in the radio's tuner circuit. The signal will be transferred viathe transformer from the antenna circuit to the tuner circuit where itwill die. Why? Because the signal will be out of step with the naturalcharge/discharge pattern of the (R)LC combination in the circuit (ifthis is not clear, go back to the end of the AC Circuits chapter and re-read the Note concerning "what is really happening with νres").

On the other hand, a signal that has exactly the same frequency asthe (R)LC's natural frequency will build freely within the tuner circuit.In other words, the tuner circuit effectively selects one frequency thatwill flourish while damping out all others.

By varying the capacitance of the capacitor in the tuner circuit (thecapacitor symbol in Figure 21.12 is that of a variable capacitor), thenatural frequency of the tuner current changes and different stationscan be "tuned in."

c.) Bottom line: From the many signals impinging on ourantenna, the tuner circuit selects one frequency to pass on to the nextpart of the radio circuit, the speaker circuit.

304

FIGURE 21.13a

low frequency information signal

high frequency carrier signal

4.) To understand the speaker circuit, we need to discuss two separatetopics. The first is, "How is information put on a radio station's single, high-frequency, electromagnetic wave?"

a.) Let's assume we have a radio station that wants to send outmiddle C over the airwaves (middle C is a 256 hertz sine wave). Thestation has a single, high-frequency signal--say 1,000,000 cycles persecond--to work with (Figure21.13a shows both waves).

b.) In AM radios, informa-tion is put on the station's sin-gle, high-frequency signal(called the carrier wave) by atechnique called amplitudemodulation.

The amplitude of the highfrequency carrier wave ismodulated (varied) so that theenvelope (i.e., the outline of thehigh-frequency wave) exactlymatches the waveform to be sent(in this case, a 256 hertz sine

FIGURE 21.13b

the envelope (contour of carrier wave conforms to the 256 hertz information wave)

amplitude-modulated carrier wave

wave). For clarification:

i.) As has already been pointed out, Figure 21.13a shows boththe low frequency information signal (the 256 hertz middle C sinewave) and the station's high-frequency carrier wave;

ii.) Figure 21.13bshows the station'scarrier-wave amplitudemodulated so as toconform to the outlineof the information-signal.

c.) Speech and musichave more complexfrequency wave forms, butthe amplitude modulationapproach still works. Thatis how AM stations putinformation on theircarrier signal. It is also why AM stations are called AM--the lettersstand for Amplitude Modulation.

Ch. 21--Semi-Conductors and Radios

305

FIGURE 21.14

frequency-modulated carrier-wave

information wave

Note: Although we will not be look-ing at the circuitry for such systems, FMstations encode their information bymodulating not the amplitude of theirsending wave but by modulating theirsending frequency over a band-of-fre-quencies registered to them by the FCC(FM frequencies are typically in the 108

hertz range).An example of a frequency-modu-

lated wave and the decoded message isshown in Figure 21.14.

5.) Speakers: As shown in Figure

coil (attached to cardboard cone)

FIGURE 21.15

fixed magnet

NScone (fixed at edge)

rudimentary speaker

21.15, a speaker is comprised of a fixedmagnet and a coil attached to a cardboardcone that is itself attached to a rigid frameat its edges (this attachment is not shown inthe sketch).

a.) At low frequency (i.e., in theaudio range between 20 and 20,000cycles per second), an AC signal pro-duces an alternating current in thecoil. From the chapter on magneticfields, we know that a current flow-ing in a coil produces a magneticfield down its axis which, in turn,makes one side of the coil's face into aNorth Pole and the other side into a South Pole.

b.) Looking at Figure 21.15, assume the current in the coil makesthe cone-side of the coil into a South Pole and the magnet-side of thecoil into a North Pole. At that instant, the magnet's North Pole and thecoil's magnetic field will repulse one another, pushing the coil andcone outward away from the magnet.

c.) The AC polarity changes. The current in the coil flows in theopposite direction. The current in the coil now makes the cone-side ofthe coil into a North Pole and the magnet-side of the coil into a SouthPole. At that instant, the magnet's North Pole and the coil's magneticfield will attract, pulling the coil and cone inward.

306

FIGURE 21.16

the diode wipes out the bottom- half of the wave (notice thatthe 256 hertz envelope is still intact)

a diode in the speaker-circuitmakes the amplitude-modulated carrier wave into DC

d.) As the current in the coil alternates at the power source fre-quency, the cone vibrates back and forth at that frequency producingcompression and rarefaction regions in the air pressure around thespeaker. Those pressure variations move out into the room.

As these pressure ridges pass your ear, hairs in the ear vibratedue to the pressure changes, electrical impulses are produced, andyour brain senses sound.

6.) The scenario outlined above works fine as long as the frequency ofthe current is low enough for the cone to respond (cardboard cones do fine inthe audio range). Unfortunately, speakers do not respond to million hertzAC signals--a cardboard cone is too massive and, hence, sluggish to vibrateat that rate--so putting a 1,000,000 hertz signal through a speaker will yieldno net motion and, hence, no sound at all (even if it did produce sound, yourears would not be able to pick it up).

7.) The Speaker Circuit: Theproblem outlined in Part 6 above iscircumvented ingeniously with theuse of a diode. The diode in thespeaker circuit (Figure 21.12) turnsthe high frequency AC signal into aDC signal (see Figure 21.16). It islumpy DC, but it is DC nevertheless.As such, the speaker now has achance of dealing with the informa-tion placed on the carrier wave:

a.) Current will flowthrough the speaker's coil inone direction only, which inturn means the fixed magnetside of the coil will alwayshave the same magneticpolarity. Let's assume, for this situation, its polarity is always North.

b.) The coil and cone cannot respond to every change in thevarying current provided by the signal--the frequency is still too highfor that--but it can follow general trends of current flow. That is, as theaverage current becomes larger and larger, the northness of the coil'sface, on average, increases. This creates a larger and larger averagerepulsion between the coil and the fixed magnet and the cone pushesoutward.

c.) As the average current decreases, the northness of the coil'sface lessens. The repulsion between the coil and the fixed magnet

Ch. 21--Semi-Conductors and Radios

307

becomes smaller and smaller, and the cone relaxes, pulling backinward toward its equilibrium position.

In other words, the cone pulls in or pushes out in conformity to thegeneral current trend of the signal. This trend exactly conforms to theenvelope of the carrier wave--to the very waveform we are trying todecode from the carrier signal.

d.) In short, the half-wave-rectified, high-frequency, amplitudemodulated carrier wave generates in the speaker a slowly varying DCsignal that exactly matches the 256 hertz signal we wanted to send inthe first place. Our speaker, coupled with a diode, has acted as adecoder.

E.) Radio Circuit--Summary/Nutshell:

1.) At the station: A single, high-frequency carrier wave is amplitudemodulated so that its envelope exactly matches the signal-to-be-sent (i.e., theinformation wave). A transformer transfers the signal to the sending an-tenna where an amplitude-varied electromagnetic wave is formed and sentout at the speed of light.

2.) The wave impinges on the receiving antenna of your radio. Thatwave, along with signals from every other radio station in the area, is trans-ferred via transformer from the antenna circuit to the tuner circuit.

A signal whose frequency matches the resonance frequency of thevariable (R)LC tuner circuit is the signal that thrives in the tuner circuit. Allothers die. This selected signal is transferred to the speaker circuit via atransformer.

The signal enters the speaker circuit as a high frequency AC volt-age/current source. A diode eliminates half the signal making it into DC.This DC current flows through the coil in the speaker making the speaker'scoil a variable-magnitude, permanent-pole magnet.

As the average current increases and decreases, the variations in thecurrent cause variations in the polarity of the coil. This causes motion in thecoil which, in turn, causes cone vibration. The resulting frequency of cone-vibration is the frequency of the envelope of the signal (i.e., that of theinformation wave). In doing so, we have successfully manipulated theincoming radio wave into audible sound.

F.) Amplification:

1.) The radio receiver circuit shown in Figure 21.12 uses no batteries.The speaker in such circuits is a tiny, single, ear-plug type. All the powerneeded to drive it is provided by the electromagnetic wave impinging on the

308

FIGURE 21.17

R load

n-type p-type

+-

electron motion electron

motion

hole motion

n-type

depletion zone (acts like break in the circuit)

receiving antenna. If a larger speaker is required, the signal needs to beamplified.

2.) The semi-conductor ar-rangement shown in Figure 21.17 isessentially that of two diodes back-to-back. Called a transistor, thearrangement will not pass currentin either direction.

Why? Given the polarityshown in the sketch, the electrons inthe n-type semi-conductors willmove to the right and the positiveholes in the p-type semi-conductorwill move to the left. That meanselectrons and holes will combineacross the left semi-conductorjunction, but a depletion zone will becreated along the right junction.

FIGURE 21.18a

bigV

base

still no current

R load

collectoremitter

The depletion zone acts like a break in the circuit, stopping current fromflowing altogether. The exact opposite will occur when the power supplypolarity changes--in that case the depletion zone will be along the leftjunction.

3.) As unlikely as it might seem, clever manipulation of this devicewill create a circuit that acts like an amplifier.

4.) Figure 21.18a shows theabove arrangement with threemodifications: a large DC powersupply replaces the AC powersource, a lead has been attached tothe mid-section semi-conductor, andnames have been given to the threetransistor terminals.

a.) What happens whenthe base is made positive?

b.) In Figure 21.18b (nextpage), the base has been madepositive. Holes near and to theright of the base-connection inthe p-type semi-conductor will

Ch. 21--Semi-Conductors and Radios

309

FIGURE 21.18b

collectoremitter

base + some electrons in the right-side n-type semi-conductor will be attracted to the positive base

some holes in the central p-type semi-conductor will be repulsed by the positive base

by biasing the BASE positive, the depletion-zone partially diminishes allowing current to flow--the greater the bias, the greater the current

be repulsed by thepositive base andwill migrate tothe right. Elec-trons near thebase in the right-side n-type semi-conductor will beattracted to thepositive base andwill migratetoward the left.The holes andelectrons willmeet at thejunction and thedepletion zonewill be compro-mised.

c.) If the baseis made just a little positive, the depletion zone will be diminished onlyslightly and the resulting current in the circuit will be small. If thebase is made quite positive, the depletion zone will diminish greatlyand the current in the upper circuit will be large. There are two addi-tional observations to note:

i.) If the positiveness of the base is somehow made to vary, thecurrent in the upper circuit will vary accordingly.

ii.) Devices like this are extremely sensitive to base-voltagevariations. That is, a very small change in the base voltage willmake a very big change in the circuit current (hence, a big voltagevariation across the load resistor). In other words, if we are clever,we should be able to use this device to amplify a small signal into alarge signal.

5.) Having established what happens when the base is made positive,consider the circuit shown in Figure 21.18c on the next page.

a.) In it a very large DC battery Vbig is placed in the upper circuitalong with a load resistor (a speaker).

310 FIGURE 21.18d

Graphical Representation of Amplifiication Circuit

note that the base voltage is always positive

Vbias

R load

biasV

bigV

= V + V signalbiasbase

V

the original signal, but amplified across the load resistor (ie. the speaker)

VRL

signalV

Vsignal

V = V + Vbase signal bias

FIGURE 21.18c

base

transistor-as-amplifier circuit

R load

collector

emitter

ground

bigV

biasV

Vsignal

base voltage equalsV + V

biassignal

b.) A bias battery Vb isconnected to the base ter-minal in series with a small,AC signal Vs (this is the sig-nal to be amplified). Vb ismade greater than Vsmaking the base voltage (thesum of Vb and Vs) alwayspositive (if this is not clear,look at Figure 21.18d--notethat it has been rotated foreasier viewing and that thegraphs ofthe signalvoltage Vs,the biasvoltage Vb,the net basevoltageVbase, aswell as thenet voltageacross theload resis-tor VR, areall shownin thesketch).

c.)Examiningthosegraphs tellsthe story.The ACsignalvoltage Vsadds to thebias voltageVbias toproduce thebasevoltage. As

Ch. 21--Semi-Conductors and Radios

311

FIGURE 21.19

base

collectoremitter

circuit symbol foran n-p-n transistor

the bias voltage is always larger than the signal voltage, the basevoltage is always positive.

As the signal voltage increases, the base voltage increases. Thisdiminishes the size of the depletion zone allowing a large current toflow through the load resistor.

As the signal voltage decreases, the base voltage decreases. Thisincreases the size of the depletion zone cutting off current in the uppercircuit and, hence, cutting off current through the load resistor.

In all cases, the ups and downs of the signal at the base controlsthe ups and downs of the potentially larger current through the loadresistor. As such, a small signal (Vs) is reproduced and amplifiedacross the load.

6.) This type of transistor is called an n-p-ntransistor--"n-p-n" because its structure looks like ap-type semi-conductor sandwiched between two n-type semi-conductors. Its symbol is shown in Figure21.19.

Note: We could as well have made our transis-tor a p-n-p type (the symbol for a p-n-p transistor isthe same as shown in Figure 21.19 but with the ar-row pointing in the opposite direction). The finalsetup for the amplifier circuit would have been a little

VsignalbigV

FIGURE 21.20a

AM receiving circuit with amplification

different, but the end result would have been the same.

7.) Bottom line: In ourradio circuit, the signal pro-vided by the radio station isushered into the amplifier-inclusive speaker circuit viathe transformer between thetuner circuit and the transis-tor (see Figure 21.20a). Treat-ing that transformer's sec-ondary coil as though it werea signal source Vs, we end upwith a radio receiver circuitwith amplification.

Note 1: If you willremember, a diode in thespeaker circuit does twothings: it exposes the

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VsignalbigV

FIGURE 21.20b

common emitter configuration

emitter

envelope of the carrier wave, and it makes the signal DC. We still need thediode to expose the envelope, but because the diode makes the signal DC, weno longer need the bias voltage source that normally goes with the transistor(the base will always be positive with the diode-generated DC signal). That iswhy the original amplification circuit looks different from the amplificationcircuit shown in the radio system.

Note 2: A circuit like thisis usually drawn with itsground leads connected asshown in Figure 21.20b. Insuch a case, the emitter iscommon to both the speaker andthe amplifier circuit. Whenconnected this way, the transis-tor is said to be in a commonemitter configuration.

Ch. 21--Semi-Conductors and Radios

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QUESTIONS

There are no quantitative problems to be solved in this chapter as thechapter is predominantly a qualitative discussion of the radio. In studyingfor the radio part of the test, you should be able to concisely explain the follow-ing:

a.) What is a semi-conductor and how are semi-conductors used tomake:

i.) Diodes?ii.) Transistors?

b.) How is a diode used in an electrical circuit, why does it act as itdoes, and what is its circuit symbol?

c.) What are electromagnetic waves and how are they produced?d.) What does a simplified radio-station circuit look like (be able to

draw it)?e.) How does a radio station put information onto its single, high-

frequency carrier signal?i.) What is the difference between AM and FM?

ii.) Why are there no radio waves below 500,000 hertz?f.) What does a simple radio receiving circuit without amplifica-

tion look like (be able to draw it)?g.) How does a radio capture radio waves?h.) How does a radio select a particular radio station frequency to

listen to (i.e., how does the tuner circuit work)?i.) How does a speaker work?j.) How does a radio decode a radio station's signal (i.e., how does

the speaker circuit work)?k.) How does a transistor affect amplification and what is its cir-

cuit symbol?l.) What does a radio-receiving circuit with amplification look like

(be able to draw it)?

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