Electronics Ch4
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Transcript of Electronics Ch4
NTUEE Electronics – L. H. Lu 4‐1
CHAPTER 4 BIPOLAR JUNCTION TRANSISTORS (BJTs)
Chapter Outline4.1 Device Structure and Physical Operation4.2 Current‐Voltage Characteristics4.3 BJT Circuits at DC4.4 Applying the BJT in Amplifier Design4.5 Small‐Signal Operation and Models4.6 Basic BJT Amplifier Configurations4.7 Biasing in BJT Amplifier Circuits4.8 Discrete‐Circuit BJT Amplifiers
4.1 Device Structure and Physical Operation
Physical structure of bipolar junction transistor (BJT)Both electrons and holes participate in the conduction process for bipolar devicesBJT consists of two pn junctions constructed in a special way and connected in series, back to backThe transistor is a three‐terminal device with emitter, base and collector terminalsFrom the physical structure, BJTs can be divided into two groups: npn and pnp transistors
Modes of operationThe two junctions of BJT can be either forward or reverse‐biasedThe BJT can operate in different modes depending on the junction biasThe BJT operates in active mode for amplifier circuitsSwitching applications utilize both the cutoff and saturation modes
NTUEE Electronics – L. H. Lu 4‐2
Mode EBJ CBJ
Cutoff Reverse Reverse
Active Forward Reverse
Saturation Forward Forward
Operation of the npn transistor in the active mode
Electrons in emitter regions are injected into base due to the forward bias at EBJ
Most of the injected electrons reach the edge of CBJ before being recombined if the base is narrowElectrons at the edge of CBJ will be swept into collector due to the reverse bias at CBJEmitter injection efficiency ( ) = iEn/(iEn+iEp)Base transport factor (T) = iCn/iEnCommon‐base current gain () = iCn/iE = T < 1Terminal currents of BJT in active mode:
iE(emitter current) = iEn(electron injection from E to B) + iEp(hole injection from B to E)iC(collector current) = iCn(electron drift) + iCBO(CBJ reverse saturation current with emitter open) iB(base current) = iB1(hole injection from B to E) + iB2(recombination in base region)
NTUEE Electronics – L. H. Lu 4‐3
Terminal currents:Collector current:Base current: Hole injection into emitter due to forward bias:
Eelectron‐hole recombination in base:
Total base current:
Emitter current:
NTUEE Electronics – L. H. Lu 4‐4
TBETBE VvS
Vv
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Large‐signal model and current gain for BJT in active region
Common‐emitter current gain:
Common‐base current gain:
The structure of actual transistorsIn modern process technologies, the BJT utilizes a vertical structureTypically, is smaller and close to unity while is large
NTUEE Electronics – L. H. Lu 4‐5
)1/()21( 1
2
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B
nB
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C
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DD
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)1/(
Common‐base current gain
iB
iCiE
1
(+1)
1
Common‐emitter current gain
iB
iE
iC(1)
Operation of the npn transistor in the saturation modeSaturation mode: both EBJ and CBJ are forward biasedCarrier injection from both emitter and collector into baseBase minority carrier concentraiton change accordingly leading to reduced slope as vBC increasesCollector current drops from the value in active mode for negative vCBFor a given vBE, iC drops sharply to zero at vCB around 0.5 V and vCE around 0.2 VBJT in saturation: VCEsat = 0.2 VCurrent gain reduces (from to forced):
NTUEE Electronics – L. H. Lu 4‐6
np0
np0exp(vBE/VT) np0exp(vBC/VT)
vBC increases
saturationB
Cforced i
i
Ebers‐Moll modelIn EM model, the EBJ and CBJ are represented by two back to back diodes iDE and iDC
Current transported from one junction to the other is presented by F (forward) and R (reverse)EM model can be used to describe the BJT in any of its possible modes of operationEM model is used for more detailed dc analysisThe diode currents:The terminal currents:
Application of the EM modelThe forward active mode:
The saturation mode:
NTUEE Electronics – L. H. Lu 4‐7
iDE iDC
FiDERiDC
iCiE
iB
CEB iii SSCRSEF III
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11/
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The cutoff mode ICBO (CBJ reverse current with emitter open‐circuited)ICBO = (1RF)ISCBoth EBJ and CBJ are reverse‐biasedIn real case, reverse current depends on vCB ICEO (CBJ reverse current with base open‐circuited)ICEO = ICBO/(1F)F is always smaller than unity such that ICEO > ICBOCBJ current flows from (C to B) so CBJ is reverse‐biasedEBJ current flows from (E to B) so EBJ is slightly forward‐biased
NTUEE Electronics – L. H. Lu 4‐8
+
E C
B
(2) RISC
ISC (1)
iCiE = 0
iB(3) RISC
RFISC (4)
iC = ICBO = (1RF)ISC (5)iB = (R1)ISC+(1F)iDE = 0 iDE = ISC(1R)/(1F) (5)
E C
B
(2) RISC
ISC (1)
iC
iB = 0
iE
+
(3) iDE
FiDE (4)
iC = ISC+FiDE = ISC(1RF)/(1F) ICEO = ICBO/(1F) (6)
The pnp transistorTransistor structure: emitter and collector are p‐type base is n‐type
Operation of pnp is similar to that of npn
Operation of pnp in the active modeCollector current:Base current:Emitter current:
Large‐signal model and current gain for BJT in active region
Exercise 4.1 (Textbook)Exercise 4.3 (Textbook)
NTUEE Electronics – L. H. Lu 4‐9
TEB VvSC eIi /
/CB ii
BCE iii
Common‐base current gain
iB
iCiE
1
(+1)
1
Common‐emitter current gain
iB
iE
iC(1)
4.2 Current‐Voltage Characteristics
Circuit symbols, voltage polarities and current flowTerminal currents are defined in the direction as current flow in active modeNegative values of current or voltage mean in opposite polarity (direction)
Summary of the BJT current‐voltage relationships in the active modeThe terminal currents for a BJT in active mode solely depend on the junction voltage of EBJThe ratios of the terminal currents for a BJT in active mode are constantThe current directions for npn and pnp transistors are opposite
NTUEE Electronics – L. H. Lu 4‐10
TBE VvSC eIi /
TBE VvSCB eIii /
TBE VvSCE eIii /
TEB VvSC eIi /
TEB VvSCB eIii /
TEB VvSCE eIii /
BCE iii
1
1
pnp transistornpn transistor
Current‐voltage characteristics of BJT
The Early effectAs CBJ reverse bias increases, the effective base width Weff reduces due to the increasing depletionFor a constant junction voltage vBE: The slope of nB(x) increases iC increases Charge storage Qn reduces iB decreases Current gain a and b increases
Early voltage (VA) is used for the linear approximation of Early EffectLinear dependence of iC on vCE:
Exhibit finite output resistance:
NTUEE Electronics – L. H. Lu 4‐11
The iC‐vCB characteristics The iC‐vCE characteristics
nB (0)
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Common‐base output characteristics
iC versus vCB plot with various iE as parameter is known as common‐base output characteristicsThe slope indicates that iC depends to a small extent on vCB Early effect iC increases rapidly at high vCB breakdownBCJ is slightly forward‐biased for 0.4V < vCB < 0
No significant change is observed in iCThe BJT still exhibits I‐V characteristics as in the active mode
BCJ turns on strongly and the iC starts to decrease for vBC < 0.4V I‐V characteristics in the saturation mode and vCEsat is considered a constant ( 0.2 V)
Current gain (): large‐signal iC/iE and small‐signal (incremental) iC/iE
NTUEE Electronics – L. H. Lu 4‐12
breakdownEarly effect
Common‐emitter output characteristics (I) iC versus vCE plot with various vBE as parameter Common‐emitter current gain is defined as = iC / iBThe BCJ turns on with a positive vBC at low vCE
BJT operates in saturation modeThe iC curve has a finite slope due to Early effectThe characteristics lines meet at vCE = VA
VA is called the Early Voltage (~ 50 to 100 V)
Common‐emitter output characteristics (II)Plot of iC versus vCE with various iB as parameterBJT in active region acts as a current source with
high (but finite) output resistanceThe cutoff mode in common‐emitter configuration
is defined as iB = 0Current gain: large‐signal bdc iC/iB and bac iC/iB
NTUEE Electronics – L. H. Lu 4‐13
breakdownEarly effect
Saturation of common‐emitter configurationIn saturation region, it behaves as a closed switch with a small resistance RCEsat
The saturation IV curve can be approximated by a straight line intersecting the vCE axis at VCEoff
The saturation voltage VCEsat VCEoff+ICsatRCEsat
VCEsat is normally treated as a constant of 0.2 V for simplicity regardless the value of iCIncremental b in saturation is lower than that in active region: forced ICsat/IB < Overdrive factor /forced
NTUEE Electronics – L. H. Lu 4‐14
Transistor breakdownTransistor breakdown mechanism: Avalanche breakdown: avalanche multiplication mechanism takes place at CBJ or EBJ Base punch‐through effect: the base width reduces to zero at high CBJ reverse bias
In CB configuration, BVCBO is defined at iE = 0The breakdown voltage is smaller than BVCBO for iE > 0In CE configuration, BVCEO is defined at iB =0The breakdown voltage is smaller than BVCEO for iB > 0Typically, BVCEO is about half of BVCBO
Breakdown of the BCJ is not destructive as long as the power dissipation is kept within safe limitsBreakdown of the EBJ is destructive because it will cause permanent degradation of
NTUEE Electronics – L. H. Lu 4‐15
Exercise 4.13 (Textbook)Exercise 4.14 (Textbook)Example 4.3 (Textbook)Exercise 4.21 (Textbook)
NTUEE Electronics – L. H. Lu 4‐16
4.3 BJT Circuits at DC
BJT operation modesThe BJT operation mode depends on the voltages at EBJ and BCJThe I‐V characteristics are strongly nonlinearSimplified models and classifications are needed to speed up the hand‐calculation analysis
Simplified models and classifications for the operation of the npn BJTCut‐off mode: iE = iC = iB = 0 vBE < 0.5 V and vBC < 0.4 V
Active mode: vBE = 0.7 V and iB : iC : iE = 1: : (1+) vCE > 0.3 V
Saturation mode: vBE = 0.7 V and vCE = 0.2 V iC/iB = forced <
NTUEE Electronics – L. H. Lu 4‐17
Mode EBJ CBJ
Active Forward Reverse
Cutoff Reverse Reverse
Saturation Forward Forward
Inverse Reverse Forward
vBE
vBC
Active ModevBE 0, vBC 0
Saturation ModevBE 0, vBC 0
Inverse ModevBE 0, vBC 0
Cutoff ModevBE 0, vBC 0
npn transistor
vEB
vCB
Active ModevEB 0, vCB 0
Saturation ModevEB 0, vCB 0
Inverse ModevEB 0, vCB 0
Cutoff ModevEB 0, vCB 0
pnp transistor
Equivalent circuit models
NTUEE Electronics – L. H. Lu 4‐18
DC analysis of BJT circuitsStep 1: assume the operation modeStep 2: use the conditions or model for circuit analysisStep 3: verify the solutionStep 4: repeat the above steps with another assumption if necessary
Example 4.4
Example 4.5
NTUEE Electronics – L. H. Lu 4‐19
Example 4.9 (Textbook)
Example 4.11 (Textbook)
NTUEE Electronics – L. H. Lu 4‐20
Exercise 4.22 (Textbook)Exercise 4.23 (Textbook)Exercise 4.24 (Textbook)Exercise 4.25 (Textbook)Exercise 4.28 (Textbook)Example 4.12 (Textbook)
NTUEE Electronics – L. H. Lu 4‐21
4.4 Applying the BJT in Amplifier Design
BJT voltage amplifierA BJT circuit with a collector resistor RC can be used as a simple voltage amplifierBase terminal is used the amplifier input and the collector is considered the amplifier outputThe voltage transfer characteristic (VTC) is obtained by solving the circuit from low to high vBE Cutoff mode:0 V vBE < 0.5 V and iC = 0vO = vCE = VCC
Active mode:vBE > 0.5 V and iC = ISexp(vBE/VT)vO = VCC iCRC = VCC RCISexp(vBE/VT) Saturation:vBE further increasesvCE = vCEsat = 0.2 VvO = 0.2 V
NTUEE Electronics – L. H. Lu 4‐22
Biasing the circuit to obtain linear amplificationThe slope in the VTC indicates voltage gainBJT in active mode can be used as voltage amplificationPoint Q is known as bias point or dc operating point
IC = ISexp(VBE/VT)The signal to be amplified is superimposed on VBE
vBE(t) = VBE+vbe(t)The time‐varying part in vCE(t) is the amplified signalThe circuit can be used as a linear amplifier if: A proper bias point is chosen for gain The input signal is small in amplitude
The small‐signal voltage gainThe amplifier gain is the slope at Q:
Voltage gain depends on IC and RC
Maximum voltage gain of the amplifier
NTUEE Electronics – L. H. Lu 4‐23
CT
CVv
BE
CEv R
VI
dvdvA
BEBE
|| maxvT
CC
T
CECCC
T
Cv A
VV
VVVR
VIA
Determining the VTC by graphical analysisProvides more insight into the circuit operationLoad line: the straight line represents in effect the load
iC = (VCCVCE)/RC
The operating point is the intersection point
Locating the bias point QThe bias point (intersection) is determined by properly choosing the load lineThe output voltage is bounded by VCC (upper bound) and VCEsat (lower bound)The load line determines the voltage gainThe bias point determines the headroom or maximum upper/lower voltage swing of the amplifier
NTUEE Electronics – L. H. Lu 4‐24
4.5 Small‐Signal Operation and Models
The collector current and the transconductanceThe total quantities (ac + dc) of the collector current:
Small‐signal approximation: vbe << VT
The transconductance indicates the incremental change of iC versus change of vBEThe transconductance gm is determined by its dc collector current ICGeneral, BJTs have relatively high transconductance compared with FETs at the same current level
The base current and the input resistance at the baseThe total quantities (ac + dc) of the base current:
Small‐signal approximation:
Resistance r is the small‐signal input resistance between base and emitter (looking into the base)
NTUEE Electronics – L. H. Lu 4‐25
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The emitter current and the input resistance at the emitterThe total quantities (ac + dc) of the emitter current:
Small‐signal approximation:
Relation between r and re:
Output resistance accounting for Early effectUse the collector current equation with linear vCE dependence:
The output resistance ro is included to represent Early Effect of the BJTThe resulting ro is typically a large resistance and can be neglected to simplify the analysis
NTUEE Electronics – L. H. Lu 4‐26
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BJT small‐signal modelsTwo models are exchangeable and does not affect the analysis resultThe hybrid‐model Typically used as the emitter is grounded
The T model Typically used as the emitter is not grounded
NTUEE Electronics – L. H. Lu 4‐27
Neglect ro
Neglect ro
4.6 Basic BJT Amplifier Configuration
Three basic configurations
Characterizing amplifiersThe BJT circuits can be characterized by a voltage amplifier model (unilateral model)The electrical properties of the amplifier is represented by Rin, Ro and Avo
The analysis is based on the small‐signal or linear equivalent circuit (dc components not included)
Voltage gain:
Overall voltage gain:
NTUEE Electronics – L. H. Lu 4‐28
Common‐Emitter (CE) Common‐Base (CB) Common‐Collector (CC)
vooL
L
i
ov A
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The common‐emitter (CE) amplifierCharacteristic parameters of the CE amplifier Input resistance: Output resistance: Open‐circuit voltage gain: Voltage gain:
Overall voltage gain:
CE amplifier can provide high voltage gain
Input and output are out of phase due to negative gainLower IC increases Rin at the cost of voltage gainOutput resistance is moderate to highSmall RC reduces Ro at the cost of voltage gain
NTUEE Electronics – L. H. Lu 4‐29
rRin
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)||()||||( LCmoLCmv RRgrRRgA
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The common‐emitter (CE) with an emitter resistanceCharacteristic parameters (by neglecting ro) Input resistance:
Output resistance:
Open‐circuit voltage gain:
Voltage gain:
Overall voltage gain:
Emitter degeneration resistance Re is adoptedInput resistance is increased by adding (1+)Re
Gain is reduced by the factor (1+gmRe)The overall gain is less dependent on It is considered a negative feedback of the amplifier
NTUEE Electronics – L. H. Lu 4‐30
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ee
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The common‐base (CB) amplifierCharacteristic parameters of the CE amplifier (by neglecting ro) Input resistance: Output resistance: Open‐circuit voltage gain: Voltage gain:
Overall voltage gain:
CE amplifier can provide high voltage gain
Input and output are in‐phase due to positive gainInput resistance is very lowA single CB stage is not suitable for voltage amplificationOutput resistance is moderate to highSmall RC reduces Ro at the cost of voltage gainThe amplifier is no longer unilateral if ro is included
NTUEE Electronics – L. H. Lu 4‐31
ein rR
Co RR
Cmvo RgA
)||( LCmv RRgA
)||( LCmsige
ev RRg
RrrG
The common‐collector (CC) amplifierCharacteristic parameters of the CC amplifier (by neglecting ro) Input resistance: Output resistance: Open‐circuit voltage gain:
Overall voltage gain:
CC amplifier is also called emitter follower. Input resistance is very highOutput resistance is very lowThe voltage gain is less than but can be close to 1CC amplifier can be used as voltage bufferIt is noted that, in the analysis, the amplifier is not unilateral
NTUEE Electronics – L. H. Lu 4‐32
))(1( Lein RrR
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1))(1(
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4.7 Biasing in BJT Amplifier Circuits
DC bias for BJT amplifierThe amplifiers are operating at a proper dc bias pointLinear signal amplification is provided based on small‐signal circuit operationThe DC bias circuit is to ensure the BJT in active mode with a proper collector current IC
The classical discrete‐circuit bias arrangementA single power supply and resistors are neededThevenin equivalent circuit: VBB = VCCR2/(R1+R2) RB = R1||R2
BJT operating point:
RC is chosen to ensure the BJT in active (VCE > VCEsat)
A two‐power‐supply version of the classical bias arrangementTwo power supplies are neededSimilar dc analysisBJT operating point:
NTUEE Electronics – L. H. Lu 4‐33
)/11(/
EB
BEBBC RR
VVI
)/11(/
EB
BEEEC RR
VVI
Biasing using a collector‐to‐base feedback resistorA single power supply is neededRB ensures the BJT in active (VCE > VBE 0.7 V)
BJT operating point:
The value of the feedback resistor RB affects the small‐signal gain
Biasing using a constant‐current sourceThe BJT can be biased with a constant current source IThe resistor RC is chosen to operate the BJT in active modeThe current source is typically implemented by a BJT current mirrorCurrent mirror circuit: Both BJT transistors Q1 and Q2 are in active mode Assume current gain is very high:
When applying to the amplifier circuit, the voltageV has to be high enough to ensure Q2 in active
Exercise 4.49 (Textbook)Exercise 4.50 (Textbook)
NTUEE Electronics – L. H. Lu 4‐34
)/11(/
CB
BECCC RR
VVI
RVVVII BEEECC
REF
4.8 Discrete‐Circuit BJT Amplifiers
Circuit analysis:DC analysis: Remove all ac sources (short for voltage source and open for current source) All capacitors are considered open‐circuit DC analysis of BJT circuits for all nodal voltages and branch currents Find the dc current IC and make sure the BJT is in active mode
AC analysis: Remove all dc sources (short for voltage source and open for current source) All large capacitors are considered short‐circuit Replace the BJT with its small‐signal model for ac analysis The circuit parameters in the small‐signal model are obtained based on the value of IC
NTUEE Electronics – L. H. Lu 4‐35
Complete amplifier circuit DC equivalent circuit AC equivalent circuit
The common‐emitter (CE) amplifier
The common‐emitter amplifier with an emitter resistance
NTUEE Electronics – L. H. Lu 4‐36
The common‐base (CB) amplifier
The common‐collector (CC) amplifier
NTUEE Electronics – L. H. Lu 4‐37
The amplifier frequency responseThe gain falls off at low frequency band due to the effects of the coupling and by‐pass capacitorsThe gain falls off at high frequency band due to the internal capacitive effects in the BJTsMidband: All coupling and by‐pass capacitors (large capacitance) are considered short‐circuit All internal capacitive effects (small capacitance) are considered open‐circuit Midband gain is nearly constant and is evaluated by small‐signal analysis The bandwidth is defined as BW = fH – fL A figure‐of‐merit for the amplifier is its gain‐bandwidth product defined as GB = |AM|BW
NTUEE Electronics – L. H. Lu 4‐38
Exercise 4.52 (Textbook)Exercise 4.53 (Textbook)Exercise 4.54 (Textbook)Exercise 4.55 (Textbook)
NTUEE Electronics – L. H. Lu 4‐39