Quick study academic electronics part 1 600dpi

• Electronic System: An arrangement of components passive elements and/or active devices with a specified input signal producing a defined output signal.. Frequency distortion: Due to th

PART ~

ONE

o

~

PART 1 of FUNDAMENTALS OF ELECTRONIC DEVICES AND BASIC ELECTRONIC CIRCUITS

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ELECTRONIC CIRCUITS

An electronic circuit is an information-bearing signal

processing network formed by interconnections of passive

components and/or active devices

- Passive Componentll: Resistors, capacitors and inductors

- Active Devices (or energy source devices) - transistors,

metal-oxide semiconductors, etc

• Electronic System: An arrangement of components

(passive elements and/or active devices) with a specified

input signal producing a defined output signal

Signal Processing: Functionally, electronic circuits and systems

process the input signal Common processing includes:

- Amplification (magnification)

- Integration

- DitTerentiation

- Filtering: Changing the relative magnitude of different

frequency components of the signal

- Rectification: Selection/rejection of a particular part of

the signal on polarity basis

Other Electronic Circuitll are:

- Harmonic oscillators: Produce sinusoidal waveforms of

desired frequency; or, termed as relaxatlonal oscillators,

their other versions can produce nonsinusoidal wave

forms such as square, impulse, triangular, etc

- Digital circuits: Specific circuits which handle pulsed wave

forms; they can perform computational operations such as

addition, subtraction, mUltiplication, etc in binary form

electrical entity (such as voltage or current) derived

from a transducer (e.g voice signal voltage delivered by a

microphone) Signal processing refers to processing the

electrical signal in a predetermined manner so as to enable

the recovery ofthe information contained in it Signal sources

can be represented by (Fig 1):

_Thevenin's equivalent circuit: Fig I

A signal source represented Thevenin C ircuit

current generator with a shunt

resistance Ro

• Electrical signal is characterized

by: amplitude, frequency and

phase parameters The signal is a

time-varying function representing

time It can be periodic (with a

definite period T, so that frequency f= liT); or, it can be

aperiodic A complex waveform consists of several wave

forms of different frequencies A periodic signal with a

complex envelope (of waveform) has a discrete spectrum

of harmonic (sine/cosine) wave forms of magnitudes

as decided by Fourier series expansion An aperiodic

waveform has a continuous spectrum of harmonic

components as per Fourier integral transform

Examples of signal representation

by Fourier series and Fourier Fig 2

transform: A periodic, continuous Fourier Series

non-sinusoidal signal can be (discrete spectrum)

harmonic (sine and/or cosine)

4A ~ sin(nw.t) Iv(w)1 Dif thc IC ·""" 'J'CC od"um

An aperiodic waveform representing an arbitrary time-varying

signal can be depicted by Fourier transform (Fig 3):

• Fourier series and Fourier transform representations of signals enable a description of the spectral components (frequency components) constituting the signal as shown

Fig_ 3 Fourier

Continuous specttum

ofan aperiodic wave

SIGNAL DISTORTION

Electrical signa! processed by a circuit may undergo three types of distor­

tions: amplitude distortion, frequency distortion and phase distortion

Amplitude distortion: Also known as harmonic or nonlinear distortion, this is caused by the nonlinear transfer function charac­

teristics of the components/devices in the circuit (Fig 4) That is,

an input signal e,(t) will be delivered at the output of the circuit as: ee(t) = ate,(t) + aze,Z(t) = a3e,3(t) + _ , where at az, a3, _, etc are the coefficients of the nonlinear transfer function If Cj(t) is a single

frequency signal, the output will contain higher harmonic components due to square, cubic terms, etc As a result, the output signal wave shape (envelope) will be seen distorted (envelope distortion)

e;( t ~ in~~ eo(t) eo(t ~

e;(t) r o~'

t em I""'-' eo(t) = alei(t) + a2e2i(t) +

Frequency distortion: Due to the presence of capacitive (C) and/or inductive (L) elements in the circuit, a complex signal (composed of a spectrum of several frequency components) will face filtering of its components, inasmuch as the reactances offered by C and/or L elements are frequency-dependent As

a result, the transfer function relating the input and the output would vary as a function of frequency

EX: A voltage amplifier which is expected to provide a constant voltage gain (output voltage to input voltage ratio) for any frequency of the input signal may yield a varying gain versus frequency plot as shown (Fig 5) The drooping of A (gain) versus

f (frequency) curve at high (HF) and low (LF) frequencies is, for example, due to low reactance of the shunt capacitance Cp and high reactance of series capacitance Cs respectively

'

Fig ~ ~H,FreqUenCy DistOrtion @

II fMid-i'\ e c' R "",

• Phase distortion: Considering the input and output signals, their relative phase angle is again decided by C (and/or L) elements present in the circuit Hence, their phase difference is frequency­

dependent For a complex input signal (with a spectrum of frequency components), the phase angle (q,) of the transfer function of the circuit when plotted against frequency is typically

as shown (Fig 5) Except over a midrange offrequencies, q, varies

at low and high frequencies due to series and shunt capacitive elements of the circuit respectively (or respectively due to shunt and series inductive elements, if present)

NOISE Noise: An undesired entity introduced into the signal in the circuit, either caused by various circuit elements or electromagnetic interference coupled to the circuit from exterior sources Noise is

a random fluctuation and affects/corrupts the quality of the signal

For preserving the signal characteristics along the circuit, the noise level should be minimized (high signal-to-noise ratio)

1

CIRCUIT DEVICES

• A diode is a two­ Fig 6 terminal, unilateral

- voltage V

ID: Ideal Diode

(such as a semicon­

V : Cut-in voltage

relationship close to being exponential in the forward bias with its anode kept at positive (+) potential relative to its other (cathode) terminal

In the reverse bias (anode being at negative potential with ,

respect to cathode), there is a small reverse current (unlike

in an ideal diode, wherein, the reverse current is equal to 0

zero) Also, in the forward bias, invariably, there is small voltage Vy (known as threshold or cut-in voltage) until ~ which, there is no current conduction in practical diodes

• Basic applications of diodes:

e) detector or demodulator

Ideally, a diode is a short- V>O

as an open circuit when R

R.!=O reverse biased Its state e:

is set by the breakpoint Practical Diode

R,.= Reverse resistance of the diode FB=Forward bias

• Rectifier: A diode can be used to rectiry the alternating ,

current waveform (with bipolarity) to a one-directional 0 waveform A simple half-wave rectifier is illustrated in Fig

8 The current flows through the load resistor RL only during " positive half-cycle as the diode conducts (forward biased) 1/

Hence, voltage (ee) across RL is one-directional or rectified m

Fig 8 Half-wave Rectifier

Z

Fi g 10

v

A diode circuit can be designed to clip-off the voltage above

a certain value That is, the circuit will limit voltage inputs to

a maximum level The clipper circuit and waveform clipping

are as illustrated (Fig 9)

Fig 9 Diode Clipping Circuit

v.(t) h f \ ~ Vc

A diode bilateral limiter is an extension of the clipping

circuit (Fig 10)

Diode Bilateral Limiter

Demodulator or a detector: This circuit is used to recover

an envelope waveform (of low frequency) which modulates

the amplitude of a high frequency waveform as illustrated

(Fig I I) This process is called detection (of a signal

modulated on a high frequency carrier) in radio systems

Fig 11 LF signal Detection Dctected LF signal

envelope cnvelope: eo(t)

e(t) t lll~~.A/~,':ldl: RL

c( t) ' "'-' ' <:>

+ S r

, I

~

RF carrie

I#J #I ~j I [ltl]~ IIIIIli il] ill] [1]1] ~1

INTRINSIC &

Fig 12

( a Intrin sic Type (b) N-Type (e) P-Type

•

.

•( ) • • ( •• ) e • • - · • ( ) • Hol/ e

• •••••- ( • )• • •• - • •• #! ( •• ) •• - • •• ! - · (e ) ! ­ :

• The atoms of semiconducting fourth group elements (Si

and Ge) have four valence electrons which are shared by

neighboring atoms constituting a strong covalent bonding

(Fig 2 a) which limits the current conduction to available

free-electron flow (at a given T, temperature) as facilitated

by the thermal cnergy-induced transfer of electrons from

valence band to conduction band This corresponds to

intrinsic (pure) state of semiconductors (Fig 12 a )

• A semiconductor (such as Si and Ge) can also be "doped"

with a fi tlh or a third group element to control its electrical

conductivity When a fitlh group element (say P, Sb or As,

with 5 valence electrons) is added, the covalent structure is

completed with 4 valence electrons of P, and the available

as an excess free-electron enhancing current conduction

• N-Type or donor impurity: The added fifth group element

in the doped semiconductor is called an N-type extrinsic

semiconductor (N depicting the negative excess charge

carrier introduced)

• Addition of a third group element (such as B, Ga, In)

curtails a part of covalent bonding (Fig 12 c ) due to valency

(or the available valence electrons) being only three The

vacant space or the " hole" created in the bonding structure is

equivalent to a positive change, ready to accept an electron

Filling of a hole, by an electron, generates hole at a different

site Proliferation of the hole represents equivalently a

positive charge carrier movement Hence, a third-group

element doped semiconductor is designated as a P-type

extrinsic material, P denoting the excess positive charge

carrier equivalence of the holes introduced P-type dopants

are known as acceptors

DEFINITIONS

In solid-state materials the distribution of electrons in the outermost orbit in the atoms (termed as valence electrons)

d e cide the property of the material as of being a conductor

an insulator or a semiconductor

Conductors: In conductors (such as Cu or Ag), there exists

a cloud of free-electrons at temperatures above absolute zero formed by weakly bound valence electrons in the outermost orbits of the atoms When subjected to an electric field force (by applying a voltage across the material), these free­

electrons will flow along the field gradient, constituting an electric current With conductors, the valence band and the conduction band overlap as illustrated in (Fig 13)

Insulators: In insulators (dielectrics) such as polyethylene, the valence electrons are tightly bound to the parent nuclei

of the atoms and are hardly available as mobile electrons to constitute a current flow even at room temperatures That is, there is a wide forbidden gap energy prevailing between the

valence and the conduction bands (Fig 13)

Semiconductors: With semiconducting materials (such as

Si and Ge), the forbidden gap energy is small Therefore, some free-electrons are available in the conduction band for current conduction (but not to a large extent as in conductors)

at room temperature

Fig 13

Conductor Insulator SemiConductor

U nfilled Unfilled

band

" ! Gap « kBT

~ i Gap »kBT ! i(Forbidden band)

~ i F ill cd

~ ~!~ ~c e

¢lttl(t ) Efl®'ifl I£> El:J I£>

Parent Nuclei Parent Nuclei Parent Nuclei

PN JUNCTION

A PN junction is constituted by placing together a P-type and

an N-type semi-conductor.This structure represents a simple semiconductor diode When a PN junction is constituted, the majority carriers, namely electrons of the N-region and the holes of the P-region, could combine at the junction_forming

a depletion layer with almost nil free carriers in the vicinity

of the junction The atoms depleted of the electrons and holes remain in this depletion region, as ions (Fig 14) Also, PN junction formation allows the minority carriers (electrons

of P-region and holes of N-region) to migrate across the junction and combine with ions in the respective regions

Fig 14 PN Junctions

(a) Unbiased PN Junction: I,=(d 4-­

(b) Reverse biased PN Junction: I=IS-ID

V

0

+~:n~nd Ch~~s

ee e T ffi$ ' ffi eee ! tfltfltfl

e e e tfl tfl tfl

Depic t o n L !lyc r

10 , - IS

( c ) Forward biased PN Junction

0

V

t ­

1 N~

-:- 10

[ exp(~T)-ll

JUNCTION DIODES

Fig 15 Types of Semiconductor Diodes #

±+s 4-: ~ Schottky-barrier Diode Zener D.ode Lig emitting Diode (LED)

V ar t ctor Diode Li g t~ scns i tive D i o de

Schottky-barrier diodes: Contact of metal with semiconductor may create a junction with properties similar to PN-junction EX: Al or Pt may act as acceptor material when in contact with N-type silicon Merits: No charge storage is involved facilitating tast switching, and very low forward drop = OV cut-in threshold is obtained

Photo diodes: Reverse saturation current depends on generation of hole-electron pairs by the average thermal energy of the crystal This current can be increased further by

light illumination Diodes with the provision of transmitting

light flux to reach the junction are called photo diodes

Varactor or Varicap diodes: The j unction transition capaci­

tance Cj varies with reverse bias voltage VR

C = C I ±-1! V _ 8 _ log - :L !!

where NA & ND : acceptor and donor doping concentra­ tions; n, intrinsic carrier concentration,Voz O_58V at room temperature; m = 1/2 (abrupt junction), 1/3 (graded junction);

Cj = 10pF to IOOpF for VR = 3V to 25V Forward bias is avoided due to high shunt conductance

BIASING A SEMICONDUCTOR DIODE

• No applied bias: This refers to open-circuit condition in which there is a voltage drop across the depletion region called barrier potential, constituted by the depletion region charges The extent of cross diffusion of majority carrier across the depletion region forming the diffusion current

10 is decided by the barrier voltage level Apart from 10,

there is also a thermally generated minority carrier current (Is) Under open circuit, no external current !lows_ since an equilibrium is maintained by ID= Is ~

• Under reverse bias due to VR applied, ~

the minority carrier current Is (which is Fig 6 a )

independent of the barrier voltage) remains * 1

constant But, the diffusion current 10 V1

will be reduced since VD gets increased 0 V

by VD+ V R Hence, the equilibrium current is: Is-In= Is (Reverse saturation current) The reverse voltage VR uncovers more ions in th

depletion region and widens its width and deple on large

charge concentration Hence, the corresponding deple on layer capacitance Cj Uunction cap citance) is inversely

proportional to VR • Cj = K/V R", (n = 1/3 to 4 for different types of junctions fabricated).With large reverse voltage V R,

depletion layer electric field increases_ whose strength cn

rupture the covalent bonding creating electron-hole p irs

This is a regenerative process (Zener effect) indicated by

a large increase in curent at a constant reverse v ltage

VR=Vz «5V) Under this breakd wn, the current is limited only by an external resistor ( Fi g 1 6a )

Another mechanism of breakdown at VR>Vz is due to

acquired kinetic energy by minority carriers which can break

covalent bonds by collision This ionization pro ess i, call d

avalanche breakdown, which is irreversible Again current can be limited only by a n external resisto

· FORWARD BIASED PN JUNCTION: The forward bias

voltage VF effectively decreases V0, thereby failitating

10> Is· Therefore, at steady state external current lo-Is flows

10 is decided by the extent of thermal energy VT= koT/q (k8: Boltzmann constant, T: temperature and q: electronic

charge) Corresponding to the reverse saturation current Is_

10=lsexp(VFh]VT) TJ is a scale factor such that I< TJ 2

(1 for Ge, 2 for Silo The forward IF ver ses VF charateristic

is therefore: IF=lslexp(VFh]VT)-II _ VT ~ 0 _ 026 V for

silicon at room temperature ( Fi 16 h )

·CUT-IN VOLTAGE: Semi­

conductor diode has a threshold Fi g 1 b

forward bias volta e below which I SB GeSi the current is negligibly small This ~

threshold is called cut-in voltage VF

Typically, at rOom temperature

and =0 r Schottky-barrier diodes Vi a r oom temperature

(F i g i 6 ) ( SB: Schollky- b amer dIOde)

2

depletion layer gives rise to a high transition Uunction)

capacitance Cj In addition, diffusion of large minority

carriers under forward bias injected across the junction causes

a charge storage effect, attributing a ditTusion capacitance

CD Upon switching conditions (ON to OFF), forward-to­

reverse bias changing warrants the decay of injected minority

_ carriers This decay rate is controlled by (CD+ Cj ) Only

lfter a time t, (storage time) in which the excess charge

is removed, diode voltage drops to zero until reverse

saturation is reached at t, The difference (t,-t,) is called

transition time, which limits high speed switching (In OFF

to ON switching, a similar process takes place except that

the time involved is negligible since stored charge is very

small) PN-junction diode switching Fig 17

characteristics are decided by the RC Reverse biased

time constants specified by the bias Diode

• Light-emitting diodes (LED): When RR Cj

injected minority carriers in a

forward-biased PN junction recombine, energy

is released In Si and Ge, it is in the

form of heat But in GaAs, it is of Forward biased

Diode photon energy at red, yellow or green

wavelengths depending on certain

impurities added +B JCD

• Rectifier diodes: These are intended

for ac-to-dc conversion They are ~

power diodes rated on the basis of

power dissipation considerations and reverse breakdown

voltage rating

• Thermal rating: Specified by maximum allowable junction

temperature (typically, IOD°C for Ge and 175°C for Si

devices) Power dissipation capability of diodes can be

increased by using heat sinks

As shown in Fig.18a, the center-tapped transformer provides

two secondary voltages (with respect to the grounded

center-tap) with 1800 phase difference

This facilitates the diodes DI and Dz to conduct alterna­

tively over each half cycle With a capacitor shunting the load

Vd +Ll.V h Ll.V Vs

RL , R - - - ' - - were ,

(Ll.VlRMS f= 2 x trequency applied a.c.; Ripple factor = V:: '

Peak inverse voltage = 2 x peak secondary voltage

Fi g 180 Full-Wave Rectifier Circuits Fig 18b

AC,upply V VS I~ - VD- +VL

(C i! I~ V hOH

o 2:! O Y 50H Z) V[ T- , iL

without c a pacit o r C

Full- wa v e r ec tin er wi

ce nt er-tap e d transl onn c

FULL-WAVE BRIDGE RECTIFIER

Fig 19a

vet)

c

It

B r id g e circu it f ull-w3\ c rec tifi e Output wavefonn

w i th c ap acitor C

As shown in Fig /9a, this does not need a center-tapped

transformer, but requires 4 diodes Depending on the instan­

taneous voltage polarities at the secondary winding ends,

diode pair (Dz, DJ ) or (D\, D4 ) conduct, facilitating a

full-wave recti tIed full-waveform across L, with the current

flow directions as shown

Fi g 1 9c

Bridge Rectifier ­

Peak inverse rating = Peak secondary voltage

Ripple characteristics: Same as those of full-wave rectifier

with center-tapped transformer

HALF-WAVE RECTIFIER

[n Fig 20a, the transformer TR has a primary coil of Np turns and a secondary coil ofNs turns wound on an iron core

The a.c excitation at the primary is coupled to secondary via magnetic coupling mediated by the iron core

Total Base Current ( ) _ _ Ie _1,1-0:

( ) 18= 10,+18 , -Ir.-Ie - a Ie o:-d,-.­

The diode conducts during positive halt~cycle of secondary voltage as decided by the forward diode characteristics

During negative halfcycle, the diode does not conduct

Vs-Vr The load current iL for V,>Vr; otherwise,

Rs+RI)+RL

iL=0) Here, Vr=VD is the forward voltage drop across the diode (~0_7V for Silo Rs is the secondary winding resistance, Rd= Diode forward resistance; RL= Load resistance

·In Fig 20h, the capacitor across RL is charged to V L (peak) during positive halt~cycle and discharges through RL during negative half-cycle with a time constant 't = RLC VL is

a superposition of a d.c voltage "" VL- Ll V and a ripple

2

voltage (approximately of triangular shape) of peak value

Ll V (V, - Vr )xT , 't = RLC where T=t; f = frequency;

"t:

RMS value of ~V = V = ~

• RIPPLE FACTOR

( dV) .!<VRIVPPI'(RMS) =} (VL-O.M~ x 100%

DC

Given a specific ripple factor, Ll.V is calculated at a given

load voltage Hence, C IS chosen such that: C~ ~

·PEAK-INVERSE VOLTAGE: (of a half-wave rectifier):

During negative halt~cycle, the total voltage drop (reverse bias) across the diode = V,(p k)+Vd,'" 2V,(p k) Hence, the diode should be chosen such that its breakdown voltage » 2V,(p k)

• DIODE DISSIPATION RATING: Maximum diode dissipation is decided by (maximum 10ad-currentJ2 x diode forward resistance Diode power rating should be well in excess of this dissipation level

Fig 20 Rectifier Circuits

v(l) ,0 V R Vs

(:(oC.g.I 'UPPIZ0V YVp I i ': - l rR t ; I ' "~' /I ' , t

nov .50 1l ~ ) -.!:

Np Ns 0 Vs

V V -yo· +V\ VD _I

V ( I ) / - I L

:':::':"= ="=- +-' Y Lp 7 '

VOLTAGE REGULATION

A rectitler circuit delivering a load current (Id,) at a d.c voltage Vd, across a load RL can be represented by an equivalent circuit shown (Fig 21):

Voltag

R is the total source resistance (constituted by forward resistance of diodes and the secondary winding resistance of the transforrner).Vde = (VlI - Id' x Rd [f the load resistance changes (i.e as load current demand increases), Vd, drops

Percentage Regulation (PR) = [ Vdc(no-Io.d) - VdC(fun-load)]X I 00 %

Vde(fu lI_lo.d)

A regulated d.c power supply is designed to offer a desired percentage regulation High pertormance should enable drop in

Vd, minimum from no-load to full-load conditions (i.e PR~O)

3

DIODE ENVELOPE DETECTOR

This is used in AM radio circuits to recover the low frequency audio envelope modulated on a high trequency carrier e(t) = V,II + mcosro",tlcosro"t; ro",: Audio modulating signal frequency; CO,: Carrier frequency, co,»ro",; and m: Depth of modulation

LF signal Detected LF signal e(t) envelope envelope: eo(t)

~ RF fille

DIODE VOLTAGE CLAMPS

A voltage clamp shifts the associated d.c level without changing the signal waveform, e.g positive voltage clamp: V,=V,sinrot; Vr: Diode torward voltage drop D.C Clamping

level =Vdamp Note: RC» w & R » ~ an negatIve clamping can be obtained by reversing the polarity of VRand

the polarity of the diode; see Fig 23

' y VR T I Vd ~ _ ' + t

DIODE CLIPPING CIRCUITS

v(l) t_ _ _- -

L

Volt) t

- - - + 1

vd

V(t) t

vd

vd

R V RV

V =-d_= r-=V(Rd~=l

o R+Rd R+Rd

ON state: (~: diode torward resistance) and (V r' : diode projected cut-in voltage Vr )

ZENER REGULATORS

A simple regulated power supply can be constructed with a zener diode connected in shunt with the load as shown (Fig J5):

, I I

· v" + v +t

0 - Diode T ,· · : V

A C ,c",Ii" i - v : 1 1; 1 11 t R 0V

IOp ~ Clt,:ull ' 1' I I i

/ml11

Ze ner R e gulator i /nl<"

If V UR is the unregulated voltage at the outpul of the diode rectitler circuit, the regulated voltage across RL is given by:

RL (R, VI'R +RVJ

VI

RL(R+R,l+ RR, where R is a resistor which can bc designed to achievc a

given % regulation in conjunction with a zener diode of breakdown voltage Vz sustaining a safe current through it by means of Rz limited by Izm to Izml• When RL~;

R VUR -VL rnn ; a d 1= ILmu + 'Zmin ~ IL mia + Izmn

I

P, (power dissipation in the zener dio e) = Vzl + Rzl'

PR (power dissipation in R) = I'R; Vern" = Rzl zmn + Vz

(VL mn -VL m;.) x 100 0 /

VLmln = Rzlzmln + Vz; R /,

VL mu:

VOLTAGE-DIVIDER BIAS

RR

R = R IIR = _1_ 2_

DEFINITIONS

Bipolar junction transistors are constituted by three

semiconductor regions forming two PN junctions The

semiconductor regions are designated as emitter, base

and collector (Fig 26) There are two types of BJTs,

namely NPN and PNP transistors with the symbols as

shown: The junctions are known as: Emitter-base junction

(EBJ) and Collector-base junction (CBJ); see (Fig 26)

Fi g 26 Bipolar Junction Transistors (BJTs)

n p n

In jected C lle c t e d

e l e ct r o n cll,'C tron s

di ffus i on

I n jec t e d R ec ombin e d

ho le s ~ clcx; trun s

I I B0

B inJe cti o n h le ho le

m B !I ole h lJC( t c d Collec l e d

IE I B + I + ,nj e t c 7 d l o Il Rccomb =

e l e ctrons J hol e s

I

n p •

Forward-biased Reverse-biased .c0­

• p •

Forward-biased Forward-biased .m

• p •

-co-BJT CHARACTERISTICS

ANALYTICAL RELATIONS OF BJT CHARACTERISTICS

V BE VCE IE I IBm I •

UNBIASED BJT

When the junctions are constituted, depletion layers are

formed at the PN junctions with depletion layer potentials

across each of them

BIASED BJT

• In the active mode operation, EBJ is forward-biased and

CBJ is reverse-biased These external bi ings enable the

depletion layer potentials at EBJ and CBJ to be decreased

and increased respectively As a result the following

current-flows are realized (for example, in the NPN device):

Forward bias on EBJ allows electron-injection from emitter

into base and hole-injection from base to emitter These

two injections constitute the emitter current (IE)' Emitted

electrons in the base region (where they are minority

carriers) diffuse across the base with some electrons lost

through recombination and appearing as a part of base

current The collected electrons across the collector drift to

the collector tennina\ The electrons under acceleration (due

to their kinetic energy) may break covalent stn,cture to yield

more carriers That is, in the collector, there is multipli­

cation process prevailing Denoting the fraction of electrons

injected from the emitter as a<1 (emitter efficiency), the

fraction of electrons survived in the difTusion across the

bse (after recombination) as the base-transport factor

(b<I) and the multiplied carrier ratio in the collector as

c»1 (collector multiplication factor), the net, transistor

alpha (a=(a.b.c)<I) The emitter efllciency is decided by

the doping levels in the emitter and the base The base

transport factor is dependent on base width Referring to

Fig 26, emitter current IE = Ic + 10, where

Tota base current

18 =(10, +I H,) =

IE -Ic = -;;: c = - - a - ­

Therefore, Ic a • transistor ~ I =.!£

a fraction of Ic Since lu is essentially decided by

n

reverse saturation current across the EBJ

EQUIVALENT CIRCUIT OF A BJT

Equivalent Circuit ofa BJT

13I B = aIE

Is ep

(Eber-Moll 's Model)

VB

j V T

npn pn p npn p p npn pnp

o cOi.,

<>

C o mm on - b se « ('B ) Comm o -e i tt ( CE)Co mmon-~o ll l or (e e)

VOE changes by :::::12mVloC

i:::::)ic, iE , iu ie=aiE

"Df

ie = Ise vT

v ,

io = ; e v.,

Equation Forms

Current-Bias (Fi g 30)

Vcc = R318Q+ V UE + REIEQ;

ICQ ICQ(I+M ICQ

IEQ= ICQ+ 10Q; Ve[Q= (Vee- lcQRcJ

(Vce -VOQ)~

(I + 13) ICQ ] [

VEQ =R[ ~ ; (VB[ = O.7V)

Fig 30 Operating Point (Q-Point) Detennination

, - - - , - -+ Vcc Ic

Fig 32

(a) Without Emitter Bias Resistor

(b) With Emitter Bias Resistor

including

···· ~ ~ : : ~ / ~

- ~ L - -¥~

lui ' 1"0 "'-/

CE ~ ' , ~ /

i 95

LO

o a[ ; " '

Bnta

lET

! !

: !

litying

ease in

voltage

crease

resed

rational

JF ET

quency mmon­

ook for

• or t rans ­

RI

V BE = Open Circuit Voltage = Vec R + R

1 2

V00 = R810Q + VBE+ RI:lEQ VEQ = IEQRE; VOQ = Vo£.+ Vt:Q;

VCQ= Vce- lcQRc

ICQ [Ro+RI : (I+~))

Fig 3 1

(a) Actual Circuit (b) Thcvcnin's

Equivalent Circuit

+Vcc

+ VCEO

4

BIASING FOR Q-POINT STABILITY

~ BJT circuits are sensitive to temperature, power-supply

fluctuations and variations in (l (or ~) from

piece-Zto-piece Such variations cause Q-point instability

Stabilizing methods include current-bias method,

voltage-bias method and voltage-divider method

W

o

Writing Ic=~lo+(1+~)lcoo, the second term is the

~ leakage current component essentially due to minority

carrier contribution, which is sensitive to temperature

Fig 33

~ Biasing for Q-point Stability

+Vcc

(d)

R2

Ic

Ic

+VCE +VCE

+VE

RE

Rl

Stability factor: S = Me

, Llleo

a Single-resistor with current-biasing:

S = (I + ~) ~Very Large (Poor Stability)

bCurrent-bias with emitter resistor:

I+R31

A R)i stabIlity I+p+ R

E

c Voltage-biasing with collector-to-base resistor:

S I+~

I+~

R3+ Re

dVoltage-divider biasing: S = 1 + (RIIIR2)

RE Recommended design values of S:

- Small signal voltage amplifiers: S - 4 - 5

- Large signal power amplifiers: S - 2

EARLY EFFECT

With the reverse bias on CBJ, the depletion layer would

extend into base (when the bias is increased), thereby

reducing the effective base width (W0)' Hence, base­

transport factor will increase Thus, (l increases and Ic also

av

increases with a reduction in output impedance a eo

Ic The change in base width is termed as base-width

modulation or Early effect Further increase in Ic calls

for excessive injection of electrons from the emitter into

base This enhancement of carriers in the base increases

base conductivity and hence, reduces emitter-efficiency

(conductivity modulation) The result is ( l will decrease

(Fig 34)

Fig 34

( l Base-width Conductivily

HPARAMETER & HYBRID

-• Common emitter configuration (Fig 35):

Vbe = hleib + hrevet!' ic = hfeib + hoevce

hie: CE Short-circuit input resistance = ~ h : CE Open-circuit voltage gain = vb~1

~ib=O

h'e: CE Short-circuit forward current gain = t l

i:lu=o

hoe: CE Open-circuit output admittance =

Fig 35 Common Emitter Configuration

eot=====:::tO

o- - -'- - - + - -o e

gm ~ Transfer (mutual) conductance = ale

av I VCE=constant

OE

10 =IOEO [-I+eXP( :~:)J Ic =h,.Io

where 11=lt02' V = koT =( TOK ) volts

' T q 11600 ko: Boltzmann constant q: Electronic charge aIo _ IOEO VOE _ 10

- - - e x p - - = ­ aVOE 11 VT 11 VT VT

IC

h , al = ~

B Vn: = constant

at 27°C

HYBRID - 1t

Fig 36

Cp Input Capacitance

Co: Output Capacitance

C11 : B-C Capacitance

g'mv" = hfeib

CB CE CC(EF)

Ria ~=h h Ib hie hl.+ hr.RL

r

hi +Rs -1->IOkn

Ro h h-I h-I

Ie oe ob h.e h,

Ai =l=-hfb - hr h,

RL hr.RL

hlb hie t

Fig 37

Grounded Emitter Circuit

Equivalent Circuits of Grounded Emitter Circuit

Rt = RBllr.; Gm

-Gain = GmRo= ~m(R.llr.) ~Voltage

i G v

Current gain = .!!.=- !!! ! : G RI=

i, vl/RI m

RB+rx l+rx/Ro

i=i.i=(~)G v v, VI RI.+R, m O L (R IIR )~

overall voltage gain

Fig 38

RB

-o+V"" with

Equivalent Circuits of Common Emitter Circuit

Vb =Vx +V ~+g",Vx

X

Vx +(g", +~)V.RE: Vb = v x (1+';:-}

v (1+ RE)

.1.=(11 + 1 ) ' R =~ r

r - r Ib \ ~

=r.(I+~)~rx(l+g",RE)·

Input resistance = (1 + ~) x Total resistance looking into the emitter circuit + "Resistance reflection rule."

G =.!! = -g,.v

m VI VI =vb 1+~

Ro••

V V v R Voltage gain: -2.= :l.·-2.=~·(_G )(R IIR )

v, v , VI RI.+Rs L

-R IIR

Ifr.(l + g Rd»R., Av = C_ _ L

r.+R[

S

COMMON BASE (CB)

Z ~ ~ - Fig 39 +Yec

W

~

o

~

COMMON COLLECTOR

(CC) AMPLIFIER

Fig 40 ~ o+Ycc

If RL «

r,(l + 13) + R L(1 + 13) = r + Rdl +

VI=v b ~ v.=vo _~ R - R II~"R

v, Ri.+R,' VI -R,+r,' - E 0 L

v, RID +R, R, +r, RL +Ro

io Vo/RL Ria

AI il v,/(R, +Ri) RL

Rout =REIlrollRl' =r, + :513

I CB 1 CE I CC(EF)

-Rln r +~

• 13 0

rb+l3or l3o(RL + r.)

r r + R,+rb

L oo

Ro rc -+oo

• 130

130

UORL _ UORL

RL +r,

r , +~ 13 r + ~

0 I • 130 t

-130» 1; RL« r

TRANSISTOR AS A SWITCH

Switching states (Fig 4f):

Fig 41 Transistor as a Switch

I8(sal)

i,:

-I

- /18(cUI-<>ft)°

r-R<ls l I Iva 'O;f-;Y"C - - -saI) -;:":"CC=- -=-;:":"CE(CU- '-I-<>ft)

Switching states:

(Low voltage & High current):

Is = IS(sa,); Yo = Y CE(sal) == 1 volt (High voltage & Low current):

Is == 0 Y CE(cut-<>ft) == Y cc Transistor capacitances (Fig 42):

-C~: Junction capacitance at CBJ + Due to depletion layer (::::IOpf)

- C.: Diffusion capacitance at EBJ + Due to storage in the base (::::100-200pf)

Fig 42 Transistor Capacitances

ib

Capacitance effect: High frequency gain is reduced

- Parasitic/stray capacitance ~Due to loads and packaging

( '" 13 II£

-~ cut-off frequency T 21trK(C~ +C.) ~

- U cut-off frequency: (fa) = f~ / (l - u)

NOTE TO STUDENT: Due to its condensed format, us this QuickStudy guide as an Electronics guide, not as a replacement for assigned course work

©2001-2008 BarCharts, Inc Boca Raton, FL

6

Iv 1= V Rul3 -V Rul3

out inl hie in2 hie

If RLI = RL2, You, = G(Vlnl - Vin2) ~ Useful in amplifying differential signals from bridge circuits (Fig 44)

• Signal at Vlnl drives the base current at transistor

• This increases propor­

tionately the collector current of transistor I and voltage across Ru increases; or, the voltage output Vo decreases (since Vo+ VRU = Vee

Fi g 44

= constant) Vinl and Vo are phase opposed Suppose ViDI=O

Signal at Vin2 drives a base current at transistor 2 and increases the collector current of 2 The emitter potential V K

is brute-forced at (Vo I - 0.7) volts

Therefore, increase in emitter current of transistor 2 should correspondingly reduce the emitter (and hence, the collector)

current of transistor I so that the potential across RK , VK remains a constant, brute-forced value Hence, a decrease in the collector current in transistor I should reduce the voltage drop across RL2 • Or, the output voltage Vo should increase

That is, the input signal at transistor 2 (Vln2) when increased, will cause the output voltage to increase

Vlnl ~Inverting input signal

Vlnl ~Noninverting input signal

VO~(Vin2 - Vinl ) Basic differential amplifier enables the mathematical difference Fig 45

operation and can be modi fied nl vee

to perform addition, integration, differentiation, etc Hence, it ~ A Vo

is designated as an operational V in2

amplifier (OP-AMP) Operational amplifiers will be covered in the next guide

Part 2 of this two-part electronics series covers Operational Amplifiers, Unipolar Devices such as FET and JFET, MOSFETs, Relevant Equivalent Circuits and Frequency Response of FETs; Common-Gate Amplifiers, Common­

Source Amplifiers and Common-Drain Amplifiers Look for

it at your bookstore

All ri g ts r ese r v ed N o part of this publicati o n may b e r e pr odu ce d or tr a s ­ mitted in an y form , or by a y e s , e l e ct roni c o r m ec hani ca l in c i lld in g

photoc o py, r eco rdi n g, or an y in f o r mation s to ra ge a nd r et ri ev sy ste m with~

o ut written permi ss ion from the publi s her BarCh a rts, Inc 1208

ISBN-13: 978-157222526-8 ISBN-10: 157222526-2

911~~l,lll~~II~1!IJ~llllllllr ll

u.s $5.95

Authors:

Dr P 8 N eelaka nta

C Eng., FellOW lET