Upload
phamminh
View
223
Download
3
Embed Size (px)
Citation preview
1
BJTBJT asas anan AmplifierAmplifier && aa Switch,Switch, LargeLargeSignalSignal Operation,Operation, GraphicalGraphical Analysis,Analysis,BJTBJT atat DCDC BiasingBiasing BJTBJT SmallSmall SignalSignal
1
BJTBJT atat DC,DC, BiasingBiasing BJT,BJT, SmallSmall SignalSignalOperationOperation Model,Model, HybridHybrid PiPi--Model,Model,TT ModelModel..
Lecture # 7Lecture # 7
Direction of Current Flow & Operation for Direction of Current Flow & Operation for Amplifier ApplicationAmplifier Application
Copyright 2004 by Oxford University Press, Inc.2
TBE VvSC
CEBConnBCBC
BEBEonBEonBE
eIi
VvVVVv
VvVVVv
3.0 Typically, 4.0;
7.0 Typically, 5.0;
0
TEB VvSC
ECCBonnCBCB
EBEBonEBonEB
eIi
VvVVVv
VvVVVv
3.0 Typically, 4.0;
7.0 Typically, 5.0;
0
2
Large Signal Equivalent ModelLarge Signal Equivalent Model
TBE VvSB e
Ii
TEB VvSB e
Ii
Copyright 2004 by Oxford University Press, Inc.3
TBE
TBE
VvS
AO
A
CEVvSC
eI
Vr
V
veIi
1
TEB
TEB
VvS
AO
A
ECVvSC
eI
Vr
V
veIi
1
EbersEbers--Moll ModelMoll Model
)1(
)1(
TBC
TBE
Vv
VvSEDE
eIi
eIi
)1(
)1(
TCB
TEB
Vv
VvSEDE
eIi
eIi
Copyright 2004 by Oxford University Press, Inc.4
)1( SCDC eIi )1( SCDC eIi
Area
Area
EBJ
CBJ
I
I
III
R
F
SE
SC
SSCRSEF
3
Operation in Saturation ModeOperation in Saturation Mode
VVvVVVv
VVvVVVv
BCBConnBCBC
BEBEonBEonBE
6.05.0 Typically, 4.0;
8.07.0 Typically, 5.0;
0
VVvVVVv
VVvVVVv
CBCBonnCBCB
EBEBonEBonEB
6.05.0 Typically, 4.0;
8.07.0 Typically, 5.0;
0
Copyright 2004 by Oxford University Press, Inc.5
VVVv CEsatCE 2.01.0 VVVv CEsatCE 2.01.0
Fforced
BforcedCsat II
Beta forced means that the transistor is operating in saturated mode. The ratio of Beta forward and the forced beta is called overdrivefactor, the more transistor goes into saturation the voltage VCE will also be lower.
Transfer Characteristics of CE AmplifierTransfer Characteristics of CE Amplifier
TI
TITBE
VvSCCCCEO
VvS
VvSC
eIRVvv
eIeIi
The resistance RC has two functions,firstly, to establish the desired DC biasvoltage at the collector and secondconverting the current signal into voltage.
CCCCCEO iRVvv
C
CEsatCCCsat
d
R
VVI
Copyright 2004 by Oxford University Press, Inc.6
(a) Basic common-emitter amplifier circuit. (b) Transfer characteristic of the circuit in (a). The amplifier is biased at a point Q, and a small voltage signal vi is superimposed on the dc bias voltage VBE. The resulting output signal vo appears superimposed on the dc collector voltage VCE. The amplitude of vo is larger than that of vi by the voltage gain Av.
T
CECCv
VvI
Ov
V
VVA
dv
dvA
BEI
4
Example 5.2Example 5.2
Copyright 2004 by Oxford University Press, Inc.7
?
ˆ , 0
BE
ivT
CECCv
C
CECCC
v
VAVV
VVA
R
VVIIC = 1 mA
VBE = 0.6908 VAv = -272 V/VVo = 1.36 V∆ VBE = 12 mV∆ VBE = -105.5 mV
Exercise 5.19Exercise 5.19
CCCCC
T
CCv
VVS iN ti
RIVV
V
RIA
Copyright 2004 by Oxford University Press, Inc.8
v
CCE
CCE
A
VVAmplitudeInput
VVSwingNegative
5
Graphical AnalysisGraphical Analysis
CCCCCE RiVv
Copyright 2004 by Oxford University Press, Inc.9
. & between iprelationsh lineae a shows this1
CCECECC
CCC ivv
RR
Vi
This linear relationship can be represented by a straight line.
Graphical AnalysisGraphical Analysis
BBBBBE RiVv CCCCCE RiVv
Copyright 2004 by Oxford University Press, Inc.10
BBBEBB RivV CE
CC
CCC
CCCCC
vRR
Vi
1
6
Bias Point LocationBias Point Location
Copyright 2004 by Oxford University Press, Inc.11
Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB (see Fig. above).
Bias Point LocationBias Point Location
Copyright 2004 by Oxford University Press, Inc.12
Effect of bias-point location on allowable signal swing: Load-line A results in bias point QA with a corresponding VCE which is too close to VCC and thus limits the positive swing of vCE. At the other extreme, load-line B results in an operating point too close to the saturation region, thus limiting the negative swing of vCE.
7
Exercise 5.20Exercise 5.20
Copyright 2004 by Oxford University Press, Inc.13
To use it as switch we utilize the cutoff and saturation modes of operations. A simple circuit used to illustrate the different modes of operations of the BJT.
Operation as a SwitchOperation as a Switch
iRVv
vv
iiR
Vvi
CCCCC
BC
BCB
BEiB
holds. thisas long as biased reversed is CBJ so 4.0
,
R
VVI
V
Vv
VRIV
VvI
I
R
VI
iRVv
C
CEsatCCCsat
CE
EOSIi
BEBEOSBEOSI
BEEOSC
EOSB
C
CCEOSC
CCCCC
)(
)()(
)()(
)(
whichdesiredatoperatetoBJTforcecanonesaturationinthatnotedbeshouldIt
as slightly, changeonly but saturation
intodeeper BJT sendich current wh base theincrease beyond Increasing
).saturation of edge toBJT thedrives vi(where
7.0 assume when we
saturation of Edge 3.0
Copyright 2004 by Oxford University Press, Inc.14
factordriveoverI
I
I
I
EOSB
B
B
Csat
)(
forced
forced. called is
which,desiredat operatetoBJTforcecan one saturationin that noted be shouldIt
8
Example 5.3Example 5.3
B
BEBB
EOSB
B
CsatEOSB
C
CEsatCCCsat
CEsatC
I
vVR
factordriveoverI
I
II
R
VVI
VVV
)(
min)(
2.0
Copyright 2004 by Oxford University Press, Inc.15
BI
IC(sat) = 9.8 mAIB(EOS) = 0.196 mAIB = 1.96 mARB = 2.2 K
Exercise 5.21Exercise 5.21
Copyright 2004 by Oxford University Press, Inc.16
9
Example 5.4Example 5.4
0
E
EE
BEBE
R
VI
VVV
1
1
EB
CCCCC
EC
II
RIVV
II
Copyright 2004 by Oxford University Press, Inc.17
Example 5.5Example 5.5
0
E
EE
BEBE
R
VI
VVV
1
1
EB
CCCCC
EC
E
II
RIVV
II
R
Copyright 2004 by Oxford University Press, Inc.18
10
Example 5.6Example 5.6
Copyright 2004 by Oxford University Press, Inc.19
Exercises 5.22, 5.23 & 5.24Exercises 5.22, 5.23 & 5.24
Copyright 2004 by Oxford University Press, Inc.20
11
Example 5.7Example 5.7
0
EC
E
EE
BEBE
II
R
VI
VVV
Copyright 2004 by Oxford University Press, Inc.21
1
1
EB
CCCCC
EC
II
RIVV
Exercises 5.25 & 5.26Exercises 5.25 & 5.26
Copyright 2004 by Oxford University Press, Inc.22
12
Example 5.8Example 5.8
BC
B
BEBB
II
R
VVI
Copyright 2004 by Oxford University Press, Inc.23
CBE
CCCCC
BC
III
RIVV
Exercises 5.27Exercises 5.27
)(30 CCCC
B
CCCC
RiVVofRange
i
VVR
Copyright 2004 by Oxford University Press, Inc.24
0.042 mA)(3.0 CCCC RiVVofRange
13
Example 5.9Example 5.9
Copyright 2004 by Oxford University Press, Inc.25
Example 5.10Example 5.10
BCC
BB RRR
VV
2
EEBEB
BBE
BEBBE
EB
EEBEBBBBB
BBBB
BB
RIVV
RR
VVI
II
RIVRIV
RRR
RR
)]1/([
1
)||( 21
21
Copyright 2004 by Oxford University Press, Inc.26
14
Exercise 5.28Exercise 5.28
10028.1
28.1%
. of in terms write
0
C
BE
EEBEBBB
iChange
ii
RiVRiV
1.28 mA
Copyright 2004 by Oxford University Press, Inc.27
Example 5.11Example 5.11
Copyright 2004 by Oxford University Press, Inc.28
15
Exercises 5.29 & 5.30Exercises 5.29 & 5.30
Copyright 2004 by Oxford University Press, Inc.29
Example 5.12Example 5.12
Copyright 2004 by Oxford University Press, Inc.30
16
Exercise 5.31Exercise 5.31
Copyright 2004 by Oxford University Press, Inc.31
Biasing BJT Amplifier CircuitsBiasing BJT Amplifier Circuits
Copyright 2004 by Oxford University Press, Inc.32
Two obvious schemes for biasing the BJT: (a) by fixing VBE; (b) by fixing IB. Both result in wide variations in IC and hence in VCE and therefore are considered to be “bad.” Neither scheme is recommended.
17
Biasing BJT Amplifier CircuitsBiasing BJT Amplifier Circuits
)]1/([
1
BEBBE
EB
RR
VVI
II
Copyright 2004 by Oxford University Press, Inc.33
Classical biasing for BJTs using a single power supply: (a) circuit; (b) circuit with the voltage dividersupplying the base replaced with its Thévenin equivalent. The small changes in the VBE are swept awayby large value of VBB. Also
1
)]1/([
BE
BE
RR
RR
Example 5.13Example 5.13
Copyright 2004 by Oxford University Press, Inc.34
18
Exercise 5.32Exercise 5.32
Copyright 2004 by Oxford University Press, Inc.35
Biasing BJT Amplifier Circuit with 2 PSBiasing BJT Amplifier Circuit with 2 PS
Copyright 2004 by Oxford University Press, Inc.36
Biasing the BJT using two power supplies. Resistor RB is needed only if the signal is to be capacitively coupled to the base. Otherwise, the base can be connected directly to ground, or to a grounded signal source, resulting in almost total -independence of the bias current.
19
Exercise 5.33Exercise 5.33
Copyright 2004 by Oxford University Press, Inc.37
Biasing BJT Amplifier Circuit with Feedback Biasing BJT Amplifier Circuit with Feedback ResistorResistor
Copyright 2004 by Oxford University Press, Inc.38
(a) A common-emitter transistor amplifier biased by a feedback resistor RB. (b) Analysis of the circuit in (a).
20
Exercise 5.34Exercise 5.34
Copyright 2004 by Oxford University Press, Inc.39
Biasing using Constant Current SourceBiasing using Constant Current SourceThis has advantage that emitter current is independent of beta and base resistance, so base resistance can be made large to havehigh input resistance with out affecting the bias stability. It is more simplified design.The circuit in (b) utilizes a matched transistors Q1 and Q2 (Q1 is connected as a diode), both transistors have high beta values, ifwe neglect both base current the current through Q1 is:
As both transistors have same VBE so their collector current will be equal; I = IREF.
By keeping V greater than ( V + V ) we can guarantee that Q2 remains in active region and the current remains constant
R
VVVI BEEECC
REF
)(
By keeping V greater than (-VEE + VBE), we can guarantee that Q2 remains in active region and the current remains constant.
Copyright 2004 by Oxford University Press, Inc.40
(a) A BJT biased using a constant-current source I. (b) Circuit for implementing the current source I.
21
Exercise 5.35Exercise 5.35
Copyright 2004 by Oxford University Press, Inc.41
Small Signal OperationSmall Signal Operation
CB
CE
VvSC
RIVVV
II
II
eII TBE
So we first set vbe to zero, and following values of current and voltage are determined for DC.
beBEBE vVv
CCCCCEC RIVVV
beT
beCC
Tbe
VvCC
VvVVSC
VvVS
VvSC
mVvV
vIi
Vv
eIi
eeIi
eIeIi
Tbe
TbeTBE
TbeBETBE
10 if only valid is this1
suppose weif
)(
Collector Current & Transconductance
Copyright 2004 by Oxford University Press, Inc.42
(a) Conceptual circuit to illustrate the operation of the transistor as an amplifier. (b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis.
bemT
beCc
cCT
beCCC
T
vgV
vIi
iIV
vIIi
V
22
TransconductanceTransconductance
Copyright 2004 by Oxford University Press, Inc.43
Linear operation of the transistor under the small-signal condition: A small signal vbe with a triangular waveform is superimposed on the dc voltage VBE. It gives rise to a collector signal current ic, also of triangular waveform, superimposed on the dc current IC. Here, ic = gmvbe, where gm is the slope of the iC–vBE curve at the bias point Q.
Base, Emitter current and GainBase, Emitter current and Gain
bBB
beT
CCcCCB
iIi
vV
IIiIii
is;emitter and base into looking resistanceinput signal Small
1
CcCCcCCCCCcCCCCCCCC
eebee
E
Te
e
bee
eEcCcCC
E
B
T
b
be
RiVRiRIVRiIVRiVv
rrriirrr
I
Vr
i
vr
iIiIiIi
i
I
Vr
i
vr
)()(
1 & as & between iprelationsh findcan We
iprelationsh ealternativanother
is;emitter into lookingemitter & basebetween resistance signal Small
iprelationsh ealternativanother
Copyright 2004 by Oxford University Press, Inc.44
Cmbe
Cbem
be
cv
CbemCcc
Rgv
Rvg
v
vA
RvgRiv
voltagesignal small The
23
Exercise 5.37 & 5.38Exercise 5.37 & 5.38
Copyright 2004 by Oxford University Press, Inc.45
HybridHybrid--ππ ModelModel
The most widely used model.
Copyright 2004 by Oxford University Press, Inc.46
Two slightly different versions of the simplified hybrid- model for the small-signal operation of the BJT. The equivalent circuit in (a) represents the BJT as a voltage-controlled current source (a transconductance amplifier), and that in (b) represents the BJT as a current-controlled current source (a current amplifier).
24
The T ModelThe T Model
Copyright 2004 by Oxford University Press, Inc.47
Two slightly different versions of what is known as the T model of the BJT. The circuit in (a) is a voltage-controlled current source representation and that in (b) is a current-controlled current source representation. These models explicitly show the emitter resistance re rather than the base resistance rfeatured in the hybrid- model.
Example 5.14Example 5.14
Copyright 2004 by Oxford University Press, Inc.48
(a) circuit; (b) dc analysis; (c) small-signal model.
25
Example 5.15Example 5.15
Copyright 2004 by Oxford University Press, Inc.49
Example 5.16Example 5.16
Copyright 2004 by Oxford University Press, Inc.50
(a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit.
26
Example 5.16Example 5.16
Distortion in output signal due to transistor cutoff. Note that it is assumed that no distortion due to the transistor nonlinear characteristics is occurring.
Copyright 2004 by Oxford University Press, Inc.51
Input and output waveforms for the circuit of Fig. 5.55. Observe that this amplifier is non-inverting, a property of the common-base configuration.
Small Signal Model to Account for Early EffectSmall Signal Model to Account for Early Effect
We have seen that the collector current not only depend on vBE but also on vCE, so the dependence on vCE can be modelled byassigning a finite output resistance to the controlled current source in the hybrid pi model.
)(
C
A
C
CEAo I
V
I
VVr
Copyright 2004 by Oxford University Press, Inc.52
The hybrid- small-signal model, in its two versions, with the resistance ro included.
)||( oCbemo rRvgv
27
Exercise 5.40Exercise 5.40
Copyright 2004 by Oxford University Press, Inc.53