WORLD'S #1 ACADEMIC OUTLINE PART ONE ~ o ~ PART of FUNDAMENTALS OF ELECTRONIC DEVICES AND BASIC ELECTRONIC CIRCUITS CIRCUITS & SYSTEMS: BASIC DEFINITIONS 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 'r Elec;l~a~~!~aIIC~L ;I~n~~~tion-bearing 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 by a voltage generator v,(t) in + series with a source (internal) + Its t resistance Ro vs(t) S v~t) - Norton's equivalent circuit: A + signal source is depicted by a (a) 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 the wave-shape as a function of (b) 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 frequenc ies 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 transform: A periodic, continuous Fourier Series non-sinusoidal signal can be (discrete spectrum) l represented by a superpo- V(tl m r sition of infmite number +A harmonic (sine and/or cosine) t wave forms: e.g Fourier expansion ofasquarewave:(Fig.2) -A 1- 1-i Iv(w)1 Dif·""" thc IC 'J'CC od"um () 4A ~ sin(nw.t) V t =- L pen Ie cD It m=O n n=(2m+l), w=27t1t=21tf= Fundamental angular frequency I , wav," Olo 3010 5010 w m CIRCUIT DEVICES 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 ~ ~~~~~ ~ Z DIODES: IDEAL & PRACTICAL VERSIONS • A diode is a two Fig terminal, unilateral Idea l diode Actual diode device Ideally, it Fig_ Fourier Transfonn conducts electricity vet) Iv(w)1 in one direction and -4 4 does not allow the o V V current to flow in the opposite direction Continuous specttum Compared in Fig of an aperiodic wave are the current I - voltage V charac teristics of a bilateral SIGNAL DISTORTION element (such as a Electrical signa! processed by a circuit may undergo three types ofdistor resistor R) and of an tions: amplitude distortion, frequency distortion and phase distortion ideal diode Amplitude distortion: Also known as harmonic or nonlinear ID: Ideal Diode • A practical diode distortion, this is caused by the nonlinear transfer function charac Rd: Diode forward resistance (such as a semicon teristics of the components/devices in the circuit (Fig 4) That is, V : Cut-in voltage ductor diode) has an input signal e,(t) will be delivered at the output of the circuit R~: Diode reverse resistance a nonlinear V-I as: ee(t) = ate,(t) + aze,Z(t) = a3e,3(t) + _ , where at az, a3, _, etc are relationship close to the coefficients of the nonlinear transfer function If Cj(t) is a single being exponential in the forward bias with its anode kept at frequency signal, the output will contain higher harmonic components positive (+) potential relative to its other (cathode) terminal due to square, cubic terms, etc As a result, the output signal wave In the reverse bias (anode being at negative potential with shape (envelope) will be seen distorted (envelope distortion) respect to cathode), there is a small reverse current (unlike Fig Amplitude D istortion in an ideal diode, wherein, the reverse current is equal to zero) Also, in the forward bias, invariably, there is small e;(t~in~~ eo(t) eo(t~ e;(t) ro~' voltage Vy (known as threshold or cut-in voltage) until -b ~tput which, there is no current conduction in practical diodes t I""'-' , ~~ em 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 C p and high reactance of series capacitance C s respectively Fig ~ ~ ' H,FreqUenCy D istOrtion II fMid-i'\ ,01" f Frequency response ~ • e Re " e c' R "", LF-Equivalent l l~ f Rc~envor: ~ e®~ Phase response HF-Equivalent • 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) ~ ~ ~ ~ m Z DIODES AS CIRCUIT ELEMENTS • Basic applications of diodes: Fig a) Switching element, b) rectifier Ideal Diode Switch , I ,)~wfo= dip"" OFF d) limiter, e) detector or demodulator ·Switching element: Ideally, a diode is a shortcircuit element under forward bias and behaves as an open circuit when reverse biased Its state is set by the breakpoint ~_-_ ,~, ,,' i~' Fo, OFF " @ , + r ~ ~ , V RB ~B V>O V=Q ~ ~I=O R 1>0 R.!=O eq:'''= Practical Diode Switch t O~ v=O, i>O correspond to ' ' on-state For vo RB FB V~Sm.l1 R.q=EqUlvalent represen- ~_-_ ~ tation of the switch -?1~Small -?I>O Rd=Forward resistance - Larg II of the diode ,,~ e ''d~ rna oS R,.= Reverse resistance of the diode FB=Forward bias RB=Reverse bias • Rectifier: A diode can be used to rectiry the alternating , current waveform (with bipolarity) to a one-directional waveform A simple half-wave rectifier is illustrated in Fig 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 Half-wave Rectifier ){t~~rr ~o Z e(,)h VQ ~t ~ e.(1)6 : ~ Diod es a s Circuit Elements continued 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 Diode Clipping Circuit ""~ " + "¢ H I Vc v (t) h ~ f\ ~ Vc t A diode bilateral limiter is an extension of the clipping circuit (Fig 10) Fig 10 Diode Bilateral Limiter v 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 envelope e(t) tlll~~.A/~,':ldl: , I + ~ Sr DEFINITIONS 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 Dctected LF signal cnvelope: eo(t) c( t) ' "'-' ' INTRINSIC & EXTRINSIC SEMICONDUCTORS Fig 12 ( -( ) )- (b) N-Type (e) P-Type ( ) · ! e ! -· • (••) e- •• ( ) ( ) • Hole / : • 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 12a) 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 12a) • 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 valence electrons) is added, the covalent structure is completed with 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 12c) 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 l ".! Gap « kBT ~ band ~!~~ce ~! band) ~i Fillcd b:rcc I£> El:J I£> Efl®'ifl 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 VR + ~:n~nd Ch~~s ee eTffi$ 'ffi eee !tfltfltfl e e e tfl tfl tfl Depiction L!lycr 10 , - IS (c) Forward biased PN Junction VF lP t O-Is + -:- 10 [ N~ exp(~T)-ll # ~ Light-emitting Diode (LED) Types of Semiconductor Diodes ±+s 4-:-Schottky-barrier Diode Zener D.ode 4- ± ~( t Rectifier Diode 111' Li ght~scnsi tive Diode Var.tctor Diode 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 C j varies with reverse bias voltage VR V C = C I ±-1! J U Vu - m V u k T _8_ q N N log - :L !! • n,2 where NA & ND : acceptor and donor doping concentra tions; n, intrinsic carrier concentration,V oz 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 Unfilled conduction ! i (Forbidden ¢lttl (t ) Parent Nuclei •• ••• •• •• .••••••• •• -.• #! -• .••• • • • •• • band Fig 15 band ~ i Gap »kBT w; I#J #I ~j I[ltl]~ IIIIIli il] ill] [1]1] ~1 (a ) Intrinsic Type SemiConductor Unfilled conduction I RL Modulated RF carrier JUNCTION DIODES In solid-state materials the distribution of electrons in the outermost orbit in the atoms (termed as valence electrons) decide the property of the material as of being a conductor an insulator or a semiconductor 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 V R applied, ~ the minority carrier current Is (which is Fig 16a ) *1 independent of the barrier voltage) remains constant But, the diffusion current 10 V1 will be reduced since V D gets increased V by VD+ V R Hence, the equilibrium current is: Is - In= Is (Reverse saturation current) The reverse voltage VR uncovers more ions in the depletion region and widens its width and depletion large charge concentration Hence, the corresponding depletion layer capacitance C j Uunction capacitance) is inversely proportional to V R • Cj = K/V R", (n = 1/3 to for di fferent types of junctions fabricated).With large reverse voltage V R, depletion layer electric field increases_ whose strength can rupture the covalent bonding creating electron-hole pairs This is a regenerative process (Zener effect ) indicated by a large increase in current at a constant rev erse voltage VR=Vz «5V) Under this breakdown, the current is limited only by an external resistor (Fig 16a ) Another mechanism of breakdown at VR>V z is due to acqu ired kinetic energy by minority carriers wh ich can break covalent bonds by collision This ionization process i, called avalanche breakdown, which is irrevers ible Again_current can be limited only by an external resistor · FORWARD BIASED PN JUNCTION : The forward bias voltage VF effectiv ely decreases V0, thereby facilitating 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 < (1 for Ge, for Silo The forward IF verses V F characteristics is therefore: IF = lslexp(VFh]VT)-II _ VT ~ 0_026V for silicon at room temperature (Fig 16h) ·CUT-IN VOLTAGE: Se mi conductor diode has a threshold Fig 16b forward bias voltage below which I SB GeS i the current is negligibly small This threshold is called cut-in voltage VF Typically, at rOom temperature, V i ·,\' \' r O.6\' Vcu._;" = Vy~ 0.2 to r Ge, =0_6 for Si and =0 for Schottky-barrier di odes Vi al room temperature ~ o (Fig i 6b) (SB: Schollky- bamer dIOde) High speed switching diodes: Under forward bias, narrow depletion layer gives rise to a high transition Uunction) capacitance C j 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 (C D+ C j ) 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 3J conditions • Light-emitting diodes (LED): When RR Cj injected minority carriers in a forwardbiased PN junction recombine, energy is released In Si and Ge, it is in the Forward biased form of heat But in GaAs, it is of Diode photon energy at red, yellow or green wavelengths depending on certain + B J CD impurities added • 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 DIODE ENVELOPE DETECTOR HALF-WAVE RECTIFIER [n Fig 20a, the transformer TR has a primary coil of N p 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 ,+1 , ) -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,>V r ; 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 HALF-WAVE RECTIFIER WITH A CAPACITOR FILTER ·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 voltage (approximately of triangular shape) of peak value Ll V (V, - Vr )xT , 't = RLC where T=t; f = frequency; "t: This is used in AM radio circuits to frequency audio envelope modulated on carrier e(t) = V,II + mcosro",tlcosro"t; ro",: signal frequency; CO,: Carrier frequency, Depth of modulation Fig 22 LF signal envelope e(t) Vd-+' Ll.V h Ll.V RL , R - - were L- Vs , f x RL x C f= x trequency applied a.c.; Ripple factor = (Ll.Vl RMS V:: ' Peak inverse voltage = x peak secondary voltage Fig 180 AC,upply (C.i! I ~O V Full-Wave Rectifier Circuits 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 • 21t & R» ~ and ' level =Vdamp Note: RC» w negatIve clamping can be obtained by reversing the polarity of VRand the polarity of the diode; see Fig 23 Fig 23 Diode Voltage Clamps ",,~C~') 'y V+VSI~ -VD- + VL =} (VL-O.M~ x -V load voltage Hence, C IS chosen such that: 100% , , RL Output wavcfonn without capacitor C Full-wave rectiner with center-taped translonncr FULL-WAVE BRIDGE RECTIFIER Ii -': rt ; Rectifier Circuits (Vd~J Fig 24 Diode Clipping Circuits C~ ~ v(l) R nov 501l~) Np Ns V V - yo · p S R "~' I' Volt) vd / +V\ C Bridge circuit full-w3\ c rectifier 21t Output wave fonn with capacitor C As shown in Fig /9a, this does not need a center-tapped transformer, but requires diodes Depending on the instan taneous voltage polarities at the secondary winding ends, diode pairs (Dz, DJ ) or (D\, D4 ) conduct, fac ilitating a fullwave recti tIed waveform across R L, with the current flow directions as shown Fig 19c Full-Wave Bridge Rectifie r Peak inverse rating = Peak secondary voltage Ripple ch aracteristics: Same as those of full-wave rectifier with center-tapped transformer -+ vd R V RV V = -d_ = r- = V(Rd ~= l o R+Rd R+Rd _I ON state: (~: diode torward res istance) and (V r' : diode projected cut-in vol tage Vr ) Vs VD VL VLpexP RC V(I)/ IL t~ Y Lp Np Ns 1t1 ' 21t tiV ZENER REGULATORS t A simple regulated power supply can be constructed with a zener diode connected in shunt with the load as shown (Fig J5): ' A rectitler circuit delivering a load current (I d,) at a d.c voltage Vd, across a load RL can be represented by an equivalent circuit shown (Fig 21): Fig 21 + Voltage Regulation +t Voltage Regulation - Diode A.C IOp~ ·v" ,c",Ii" I T ,· · +v: , I i- v :11; 111 tR ' 1' Clt,:ull I I VI 0V i Zener Regulator /ml11 i/nl