Output Output Stage Stage and Powerand Power...
Transcript of Output Output Stage Stage and Powerand Power...
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Output Output Stage Stage and Powerand PowerAmplifiersAmplifiersAmplifiersAmplifiers
Lecture # 9Lecture # 9
MostMost importantimportant functionfunction ofof thethe outputoutput stagestage ofof anan
What is Expected from Output Stage ?What is Expected from Output Stage ?
pp pp ggamplifieramplifier isis toto provideprovide lowlow outputoutput resistanceresistance sosothatthat itit cancan deliverdeliver thethe outputoutput signalsignal toto thethe loadloadwithoutwithout anyany gaingain lossloss..
OutputOutput isis thethe lastlast stagestage soso itit dealsdeals normallynormally withwithlargelarge signalsignal modelmodel soso thethe smallsmall signalsignal modelsmodels
llll dd li blli bl bb dd ii
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usuallyusually dodo notnot applicableapplicable oror cannotcannot bebe usedused inin aastraightstraight mannermanner..
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GoodnessGoodness ofof thethe outputoutput stagestage designdesign isis measuredmeasured
THDTHD
GoodnessGoodness ofof thethe outputoutput stagestage designdesign isis measuredmeasuredasas ““TotalTotal HarmonicHarmonic DistortionDistortion (THD)(THD)”” whichwhich thethefinalfinal stagestage produceproduce andand itit isis rmsrms valuevalue ofof thetheoutputoutput signalsignal excludingexcluding thethe fundamentalfundamentalcomponentcomponent (expressed(expressed asas aa %%ageage ofof thethe rmsrms ofof thethefundamental)fundamental)..
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ThTh h llh ll hh ii tt d lid li thth i di d
ChallengeChallenge
TheThe challengechallenge herehere isis toto deliverdeliver thethe requiredrequiredpowerpower toto thethe loadload inin anan efficientefficient mannermanner..
WhichWhich essentiallyessentially meaningmeaning thatthat thethe powerpowerdissipationdissipation shouldshould bebe asas lowlow asas possiblepossible (the(thejunctionjunction temperaturetemperature remainsremains underunder controlcontrolwhichwhich cancan resultresult inin damagingdamaging thethe transistor)transistor)
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whichwhich cancan resultresult inin damagingdamaging thethe transistor)transistor)..
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High PowerHigh Power
WhenWhen wewe saysay “High“High Power”Power” itit generallygenerally meansmeans 11 WW..
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ItIt isis basedbased onon thethe collectorcollector currentcurrent waveformwaveform whenwhen aa signalsignalisis appliedapplied atat thethe inputinput..
ClassificationClassification
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CC II ˆat Biased >0at Biased =CI
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ClassificationClassification
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0at Biased =CICI of valuezeronon at Biased
Class A Output StageClass A Output Stage
satCECCo
BEIo
LE
VVv
vvv
iIi
1max
1
1
−=
−=
+=
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Transfer CharacteristicsTransfer Characteristics
Maximum
satCECCo
Lo
QVVv
QIRv
22min
1min
saturationin when
off is when
+−=
−=Minimum
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L
satCECC
satCECCL
satCECCo
RVV
I
VVIR
Q
2
2
22min
+−≥
+−=−
The absolute lowest output voltage can only be achieved provided the bias current I is greater than the magnitude of the corresponding load current.
Exercise 14.1 & 14.2Exercise 14.1 & 14.2
Li =
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11
11
1
ln
E
Te
C
ETBEBE
LE
iVr
iiVvv
iIi
=
⎟⎟⎠
⎞⎜⎜⎝
⎛+=
+=1BEio vvv −=
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Signal Wave FormsSignal Wave Forms
Maximum signal waveform in the class A output stage, when biased at current I (it will swing from +VCC to –VCC , neglecting vCEsat .
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Signal Wave FormsSignal Wave Forms
When Q1 is in saturation the output voltage is approximately, VCC (when no signal is applied).
oCCCE vVv −=1
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Signal Wave FormsSignal Wave Forms
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AsAs amplifieramplifier isis biasedbiased atat II soso thethe currentcurrent inin collectorcollector ofof QQ11 willwill havehave thisthis shapeshape..
Power DissipationPower Dissipation
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ndissipatiopower ousInstantane
CCED ivp =
ClassClass AA hashas maximummaximum instantaneousinstantaneous powerpower dissipationdissipation whenwhen therethere isis nono inputinput signalsignal andand ififthatthat happenshappens forfor longlong periodperiod ofof timetime thethe transistortransistor QQ11 mustmust withstandwithstand aa continuouscontinuous powerpowerdissipationdissipation ofof VVCCCC xx II..
TheThe PPDD inin QQ11 dependdepend uponupon RRLL:: MaximumMaximum (R(RLL =∞)=∞) oror MinimumMinimum (Short(Short CircuitCircuit RRLL ==00))..IfIf RR isis infinityinfinity meaningmeaning collectorcollector currentcurrent inin QQ isis equalequal toto II whichwhich isis aa constantconstant soso powerpower
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IfIf RRLL isis infinity,infinity, meaningmeaning collectorcollector currentcurrent inin QQ11 isis equalequal toto I,I, whichwhich isis aa constantconstant soso powerpowerdissipationdissipation willwill dependdepend onon thethe valuevalue ofof outputoutput vvoo.. MaximumMaximum PPDD willwill occuroccur whenwhen vvoo==--VVCCCC weweknowknow vvCECE11=V=VCCCC -- vvoo andand ifif wewe putput vvoo==--VVCCCC ,, vvCECE11==22VVCCCC soso ppDD11 == 22VVCCCC xx II..
IfIf RRLL == 00,, meaningmeaning shortshort circuit,circuit, theoretically,theoretically, withwith positivepositive voltagevoltage infiniteinfinite currentcurrent willwilloccuroccur soso itit cancan damagedamage thethe transistortransistor..
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Power Conversion EfficiencyPower Conversion Efficiency
PpowerSupplyPpowerLoad
RIVIP
S
L
==
= −−
2
)()(η
( )
LCCo
CCS
L
o
L
oL
IRVV
IVP
RV
RVP
RIVIP
===
=
==
only when obtained becan efficiency Maximum 41
2
212 22
)
))
η
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LoCCo IRVVVSince ≤≤ ˆ and ˆ
4
Exercise 14.3Exercise 14.3
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Exercise 14.4Exercise 14.4
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Class B Output StageClass B Output Stage
PushPush BENIo vvv −=
PullPull BEPIo vvv +=
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It is biased at zero current and any of the above transistors will conducts It is biased at zero current and any of the above transistors will conducts once a signal is present at the input.once a signal is present at the input.
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Transfer Characteristics of Class B Transfer Characteristics of Class B
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Crossover DistortionCrossover Distortion
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Power Conversion Efficiency in Class BPower Conversion Efficiency in Class B
L
oL R
VP2ˆ
21
=
The current drawn from each supply consists of half sine waves of peak amplitude; oV̂The current drawn from each supply consists of half-sine waves of peak amplitude;
Therefore, average current from both supplies =
So the total supply power = 2 V I =
LR
L
o
RV
ˆ
π
L
oCCS R
VVP
ˆ2
π=
ˆ
4ˆ2
ˆ
21 2
CC
o
o
L
o
S
L
VV
VV
RV
PP πη =
⎟⎟⎞
⎜⎜⎛
⎟⎟⎠
⎞⎜⎜⎝
⎛
==
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It comes out to be 78.5 %.
)maximum( is when efficiency maximum so 4 CCo
CCL
VV
VR
πη
π
=
⎟⎟⎠
⎜⎜⎝
Power Dissipation of Class BPower Dissipation of Class B
It is unlike class A where the maximum power dissipation occurs under quiescent conditions, in class B it is zero at quiescent conditions.Also, as the transistors conducts alternatively so each one dissipate half the power.
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LSD PPP −=
L
oL R
VP2ˆ
21
=
L
oCCS R
VVP
ˆ2
π=
12
Crossover Distortion RemovalCrossover Distortion Removal
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Single Supply Circuit for Class BSingle Supply Circuit for Class B
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Exercise 14.5Exercise 14.5
bysuppliedcurrentPeak
IB
L
vI
PPP
η−+
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max
by suppliedcurrent Peak
D
IB
PvI
Class AB output stage. A bias voltage VBB is applied between the bases of QN and QP, giving rise to abias current IQ . Thus, for small vI, both transistors conduct and crossover distortion is almostcompletely eliminated.
Class ABClass AB
2VvSQPN eIIii TBB===
2
LPN
BENBB
Io
Vvv
iii
vVvv
=+
+=
−+=
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2
ln2lnln
QPN
S
QT
S
PT
S
NT
BBBEPBEN
Iii
II
VIiV
IiV
Vvv
=
=+
=+
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Class AB CharacteristicsClass AB Characteristics
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Output Resistance of Class ABOutput Resistance of Class AB
ePeNout rrR = ||
Tout
N
TeN
iiVR
iVr
+=
=
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PN ii +
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A class AB output stage utilizing diodes for biasing. If the junction area of the output devices, QN and QP, is n times that of the biasing devices D1 and D2, and a quiescent current IQ = nIBIAS flows in the output devices.
Biasing Class ABBiasing Class AB
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Class AB operates in the same way as Class B, the power dissipation is almost identical to Class B. The only difference is that under quiescent conditions the Class AB dissipates VCC IQ per transistor.
Exercise 14.7 & 14.8Exercise 14.7 & 14.8
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Biasing Class AB with VBiasing Class AB with VBE BE MultiplierMultiplier
V
⎟⎟⎠
⎞⎜⎜⎝
⎛+=
+=
=
21
21
1
1
1
)(
RRVV
RRIV
RVI
BEBB
RBB
BER
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⎟⎠
⎜⎝ 1R
A discrete-circuit class AB output stage with a potentiometer used in the VBE multiplier. Thepotentiometer is adjusted to yield the desired value of quiescent current in QN and QP.
Biasing Class AB with VBiasing Class AB with VBE BE MultiplierMultiplier
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Exercise 14.9Exercise 14.9
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Power BJTs differ in size, shape due to the ampere range, power dissipation etc.
Large amount of power is dissipated in the collector-base junction in power BJTs.So the junction temperature increases which should not go beyond somemaximum limit TJmax otherwise it will damage the junction permanently (Forsilicon the range of TJmax is 150 – 200 degrees C.
Properties of Power BJTsProperties of Power BJTs
Suppose a transistor is operating in free air with no special cooling mechanism,heat generated will be conducted away from junction to the casing of thetransistor and from there to air and it can be expressed as:
TJ – TA = PD θJA.
TJ is junction temperature, TA is ambience temperature, PD is power dissipationand θJA is the thermal resistance between junction and ambience.
To keep TJmax minimum, θJA should be as small as possible. (In other words wewish to draw large amount of current but would like not to raise the junction
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g jtemperature above TJmax.
We can represent this thermal-conduction process as an electrical equivalentcircuit.
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Maximum allowable power dissipation versus ambient temperature for a BJT operated in free air. This is known as a “power-de-rating” curve.
PPDD VsVs TemperatureTemperature
JA
AJD
D
AJJA
TTP
PTT
θ
θ
−=
−=
maxmax
0
0max
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The popular TO3 package for power transistors. The case is metal with a diameter of about 2.2 cm; theoutside dimension of the “seating plane” is about 4 cm. The seating plane has two holes for screws tobolt it to a heat sink. The collector is electrically connected to the case. Therefore an electricallyinsulating but thermally conducting spacer is used between the transistor case and the “heat sink ”
Heat Sink & Transistor CaseHeat Sink & Transistor Case
insulating but thermally conducting spacer is used between the transistor case and the heat sink.
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CAJCJA θθθ +=
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Electrical analog of the thermal conduction process when a heat sink is utilized.
Heat Sink & Transistor CaseHeat Sink & Transistor Case
)( SACSJCDAJ
SACSCA
PTT θθθ
θθθ
++=−
+=
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The manufacturer also supplies de-rating curve for power dissipation and the case temperature as shown below and the maximum power dissipation is given as:
Heat Sink & Transistor CaseHeat Sink & Transistor Case
CJD
TTP −= max
maxJC
D θmax
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Maximum allowable power dissipation versus transistor-case temperature.
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Safe Operating Area for BJTsSafe Operating Area for BJTs
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Exercise 14.11Exercise 14.11
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Use of Compound DevicesUse of Compound Devices
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Class AB Output Stage with Darlington PairClass AB Output Stage with Darlington Pair
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Short Circuit ProtectionShort Circuit Protection
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Thermal ShutdownThermal Shutdown
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Example LM380 IC Power AmplifierExample LM380 IC Power Amplifier
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Analysis of LM380Analysis of LM380
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