bipolar junction transistor basics
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Dr. D G Borse
B
C
E
Dr. D G Borse
The BJT – Bipolar Junction TransistorNote: Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector layer has Moderate doping.
The Two Types of BJT Transistors:
npn pnp
n p nE
B
C p n pE
B
C
Cross Section Cross Section
B
C
E
Schematic Symbol
B
C
E
Schematic Symbol
• Collector doping is usually ~ 109
• Base doping is slightly higher ~ 1010 – 1011
• Emitter doping is much higher ~ 1017
Dr. D G Borse
BJT Relationships - Equations
B
CE
IE IC
IB
-
+
VBE VBC
+
-
+- VCE
B
CE
IE IC
IB-
+
VEB VCB
+
-
+ -VEC
n p n
IE = IB + IC
VCE = -VBC + VBE
p n p
IE = IB + IC
VEC = VEB - VCB
Note: The equations seen above are for the transistor, not the circuit.
Dr. D G Borse
Figure : Current flow (components) for an n-p-n BJT in the active region. NOTE: Most of the current is due to electrons moving from the emitter through base to the collector. Base current consists of holes crossing from the base into the emitter and of holes that recombine with electrons in the base.
- Electrons+ Holes
VBE
VCB
+-
+
-n+
n
p-
IneIpe
-I co
Bulk-recombination Current
Inc
Dr. D G Borse
Physical Structure• Consists of 3 alternating layers of n- and
p-type semiconductor called emitter (E), base (B) and collector (C).
• Majority of current enters collector, crosses base region and exits through emitter. A small current also enters base terminal, crosses base-emitter junction and exits through emitter.
• Carrier transport in the active base region directly beneath the heavily doped (n+) emitter dominates i-v characteristics of BJT.
Dr. D G Borse
- - - - - - - - -
- - - - - - - -
- - - - - - -- - - - - - - - - -- - - -- -
- - - - - - - - -
-
-
- - - - - - + - - + - -
Recombination
- Electrons
+ Holes
+
_
+
_
C
B
E
n
p
n
+
IB
Ic
IE
VBE
VCB
Dr. D G Borse
Figure: An npn transistor with variable biasing sources (common-emitter configuration).
Inc
IneIpe
For CB Transistor IE= Ine+ Ipe
Ic= Inc- Ico
And Ic= - αIE + ICo
CB Current Gain, α ═ (Ic- Ico) . (IE- 0)
For CE Trans., IC = βIb + (1+β) Ico where β ═ α ,
1- α is CE Gain
ICO
Bulk-recombination
current
Dr. D G Borse
Common-Emitter Circuit Diagram
+_VCC
IC
VCE
IB
Collector-Current Curves
VCE
IC
Active Region
IB
Saturation RegionCutoff Region
IB = 0
Region of Operation
Description
Active Small base current controls a large collector current
Saturation VCE(sat) ~ 0.2V, VCE increases with IC
Cutoff Achieved by reducing IB to 0, Ideally, IC will also be equal to 0.
Dr. D G Borse
BJT’s have three regions of operation:1) Active - BJT acts like an amplifier (most common use)2) Saturation - BJT acts like a short circuit3) Cutoff - BJT acts like an open circuit
BJT is used as a switch by switchingbetween these two regions.
rsat
Vo
_ +
C
B
E
Saturat ion Region Model
Vo
_ +
C
B
E
Active Region Model #1
dc IB
IB
Ro
Vo
_ +
C
B
E
Active Region Model #2
dc IB ICEO
RBB
VCE (V)
IC(mA)
IB = 50 A
IB = 0
30
5 10 15 20 0
0
IB = 100 A
IB = 150 A
IB = 200 A
22.5
15
7.5
Saturation Region
Active Region
Cutoff Region
C
E
B
When analyzing a DC BJT circuit, the BJT is replaced by one of the DC circuit models shown below.
DC Models for a BJT:
Dr. D G Borse
DC and DC
= Common-emitter current gain
= Common-base current gain
= IC = IC
IB IE
The relationships between the two parameters are:
= =
+ 1 1 -
Note: and are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.
Dr. D G Borse
Output characteristics: npn BJT (typical)
VCE (V)
IC(mA)
IB = 50 A
IB = 0
30
5 10 15 20 0
0
IB = 100 A
IB = 150 A
IB = 200 A
22.5
15
7.5
Cdc FE
B
I = = h
I
Note: Two key specifications for the BJT are
Bdc and Vo (or assume Vo is about 0.7 V)
Note: The PE review text sometimes uses dc instead of dc.
They are related as follows:
Input characteristics: npn BJT (typical)
VBE (V)
IB(A)
200
0.5 1.0 0
0
VCE = 0
150
100
50
VCE = 0.5 V
VCE > 1 V
The input characteristics look like the characteristics of a forward-biased diode. Note that VBE varies only slightly,
so we often ignore these characteristics and assume:
Common approximation: VBE = Vo = 0.65 to 0.7V
dcdc
dc
= + 1
• Find the approximate values
of bdc and adc from the graph.
dc
dc
- 1
dc
Dr. D G Borse
Figure: Common-emitter characteristics displaying exaggerated secondary effects.
Dr. D G Borse
Figure: Common-emitter characteristics displaying exaggerated secondary effects.
Dr. D G Borse
Various Regions (Modes) of Operation of BJT
• Most important mode of operation
• Central to amplifier operation
• The region where current curves are practically flat
Active:
Saturation: • Barrier potential of the junctions cancel each other out causing a virtual short (behaves as on state Switch)
Cutoff: • Current reduced to zero
• Ideal transistor behaves like an open switch
* Note: There is also a mode of operation called inverse active mode, but it is rarely used.
Dr. D G Borse
BJT Trans-conductance CurveFor Typical NPN Transistor 1
VBE
IC
2 mA
4 mA
6 mA
8 mA
0.7 V
Collector Current:
IC = IES eVBE/VT
Transconductance: (slope of the curve)
gm = IC / VBE
IES = The reverse saturation current of the B-E Junction.
VT = kT/q = 26 mV (@ T=300oK)
= the emission coefficient and is usually ~1
Dr. D G Borse
Three Possible Configurations of BJT
Biasing the transistor refers to applying voltages to the transistor to achieve certain operating conditions.
1. Common-Base Configuration (CB) : input = VEB & IE
output = VCB & IC
2. Common-Emitter Configuration (CE): input = VBE & IB
output= VCE & IC
3. Common-Collector Configuration (CC) :input = VBC & IB
(Also known as Emitter follower) output = VEC & IE
Dr. D G Borse
Common-Base BJT Configuration
Circuit Diagram: NPN Transistor
+ _ + _
IC IE
IB
VCB VBE
EC
B
VCE
VBEVCB
Region of Operation
IC VCE VBE VCBC-B Bias
E-B Bias
Active IB =VBE+VCE ~0.7V 0V Rev. Fwd.
Saturation Max ~0V ~0.7V -0.7V<VCE<0 Fwd. Fwd.
Cutoff ~0 =VBE+VCE 0V 0V Rev.None/Rev.
The Table Below lists assumptions that can be made for the attributes of the common-base BJT circuit in the different regions of operation. Given for a Silicon NPN transistor.
Dr. D G Borse
Common-Base (CB) Characteristics
Although the Common-Base configuration is not the most common configuration, it is often helpful in the understanding
operation of BJT
Vc- Ic (output) Characteristic Curves
Sa
tura
tio
n R
egio
n
IE
IC
VCB
Active Region
Cutoff
IE = 0
0.8V 2V 4V 6V 8V
mA
2
4
6
IE=1mA
IE=2mA
Breakdown Reg.
Dr. D G Borse
Common-Collector BJT Characteristics
Emitter-Current Curves
VCE
IE
Active Region
IB
Saturation Region
Cutoff RegionIB = 0
The Common-Collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at IC as a function of VCE
and IB we are looking
at IE.
Also, since ~ 1, and = IC/IE that means IC~IE
Dr. D G Borse
n p n Transistor: Forward Active Mode Currents
Forward Collector current is
Ico is reverse saturation current
1expT
VBE
VcoI
CI
A910A1810 coI
VT = kT/q =25 mV at room temperature
Base current is given by
1expco
TVBE
V
FF
CI
BI I
50020 F
Emitter current is given by
1expT
VBE
V
F
coIB
IC
IE
I
0.11
95.0
F
FF
is forward common-emitter current gain
is forward common- base current gain
In this forward active operation region,
FB
IC
I
FE
IC
I
VBE
IE=
IC=
IB=
Dr. D G Borse
Various Biasing Circuits used for BJT
• Fixed Bias Circuit• Collector to Base Bias Circuit• Potential Divider Bias Circuit
Dr. D G Borse
The Thermal Stability of Operating Point SIco
The Thermal Stability Factor : SIco
SIco = ∂Ic
∂Ico
This equation signifies that Ic Changes SIco times as fast as Ico
Differentiating the equation of Collector Current IC & rearranging the terms we can write
SIco ═ 1+β
1- β (∂Ib/∂IC)
It may be noted that Lower is the value of SIco better is the stability
Vbe, β
Dr. D G Borse
The Fixed Bias Circuit
15 V
C
E
B
15 V
200 k 1 k
The Thermal Stability Factor : SIco
SIco = ∂Ic
∂Ico
General Equation of SIco Comes out to be
SIco ═ 1 + β
1- β (∂Ib/∂IC)
Vbe, β
Applying KVL through Base Circuit we can write, Ib Rb+ Vbe= Vcc
Diff w. r. t. IC, we get (∂Ib / ∂Ic) = 0
SIco= (1+β) is very large
Indicating high un-stability
Ib
Rb
RC
RC
Dr. D G Borse
The Collector to Base Bias Circuit
The General Equation for Thermal Stability Factor,
SIco = ∂Ic
∂Ico
Comes out to be
SIco ═ 1 + β
1- β (∂Ib/∂IC)
Vbe, β
Applying KVL through base circuit
we can write (Ib+ IC) RC + Ib Rb+ Vbe= Vcc
Diff. w. r. t. IC we get
(∂Ib / ∂Ic) = - RC / (Rb + RC)
Therefore, SIco ═ (1+ β)
1+ [βRC/(RC+ Rb)]
Which is less than (1+β), signifying better thermal stability
VCC
RC
C
E
B
RF
Ic
Ib
VBE+
- IE
Dr. D G Borse
The Potential Devider Bias Circuit
VCC
RC
C
E
B
VCC
R1
RE R2
The General Equation for Thermal Stability Factor, SIco ═ 1 + β
1- β (∂Ib/∂IC)
Applying KVL through input base circuit
we can write IbRTh + IE RE+ Vbe= VTh
Therefore, IbRTh + (IC+ Ib) RE+ VBE= VTh
Diff. w. r. t. IC & rearranging we get
(∂Ib / ∂Ic) = - RE / (RTh + RE)Therefore,
This shows that SIco is inversely proportional
to RE and It is less than (1+β), signifying better thermal stability
VCC
RC
C
E
B
RE
RTh
VTh _ +
Thevenin Equivalent Ckt
IC
Ib
IC
Ib
IC
Thevenins Equivalent Voltage
Self-bias ResistorRth = R1*R2 & Vth = Vcc R2
R1+R2 R1+R2
ThRR
R
E
EIcoS
1
1
Dr. D G Borse
A Practical C E Amplifier Circuit
VCC
RC
C
E
B
VCC
R1
RE R2
Rs Ci
RL
Co
CE vi
vo
+
+
vs
+
_ _
_
io
ii
Common Emitter (CE) Amplifier
Input Signal Source
Dr. D G Borse
BJT Amplifier (continued)
An 8 mV peak change in vBE gives a 5 A change in iB and a 0.5 mA change in iC.
The 0.5 mA change in iC gives a 1.65 V change in vCE .
If changes in operating currents and voltages are small enough, then IC and VCE waveforms are undistorted replicas of the input signal.
A small voltage change at the base causes a large voltage change at the collector. The voltage gain is given by:
The minus sign indicates a 1800 phase shift between input and output signals.
2061802060008.0
18065.1~
~~
bevcev
vA
Dr. D G Borse
A Practical BJT Amplifier using Coupling and Bypass Capacitors
• AC coupling through capacitors is used to inject an ac input signal and extract the ac output signal without disturbing the DC Q-point
• Capacitors provide negligible impedance at frequencies of interest and provide open circuits at dc.
In a practical amplifier design, C1 and C3 are large coupling capacitors or dc blocking capacitors, their reactance (XC = |ZC| = 1/wC) at signal frequency is negligible. They are effective open circuits for the circuit when DC bias is considered.
C2 is a bypass capacitor. It provides a low impedance path for ac current from emitter to ground. It effectively removes RE (required for good Q-point stability) from the circuit when ac signals are considered.
Dr. D G Borse
D C Equivalent for the BJT Amplifier (Step1)
• All capacitors in the original amplifier circuit are replaced by open circuits, disconnecting vI, RI, and R3 from the circuit and leaving RE intact. The the transistor Q will be replaced by its DC model.
DC Equivalent Circuit
Dr. D G Borse
A C Equivalent for the BJT Amplifier (Step 2)
• Coupling capacitor CC and Emitter bypass capacitor CE are replaced by short circuits.
• DC voltage supply is replaced with short circuits, which in this case is connected to ground.
R1IIR2=RB
Rin
Ro
Dr. D G Borse
A C Equivalent for the BJT Amplifier (continued)
100kΩ4.3kΩ3
R C
RR
30kΩ10kΩ2
R 1
RB
R
• By combining parallel resistors into equivalent RB and R, the equivalent AC circuit above is constructed. Here, the transistor will be replaced by its equivalent small-signal AC model (to be developed).
All externally connected capacitors are assumed as short circuited elements for ac
signal
Dr. D G Borse
A C Analysis of CE Amplifier1) Determine DC operating point and
calculate small signal parameters
2) Draw the AC equivalent circuit of Amp.
• DC Voltage sources are shorted to ground
• DC Current sources are open circuited
• Large capacitors are short circuits
• Large inductors are open circuits
3) Use a Thevenin circuit (sometimes a
Norton) where necessary. Ideally the
base should be a single resistor + a single
source. Do not confuse this with the DC
Thevenin you did in step 1.
4) Replace transistor with small signal model
5) Simplify the circuit as much as necessary.
Steps to Analyze a Transistor Amplifier
6) Calculate the small signal parameters and gain etc.
Step 1
Step 2
Step 3
Step 4
Step 5 π-model
used
Dr. D G Borse
Hybrid-Pi Model for the BJT
• The hybrid-pi small-signal model is the intrinsic low-frequency representation of the BJT.
• The small-signal parameters are controlled by the Q-point and are independent of the geometry of the BJT.
Transconductance:
qKT
TV
CI
mg TV ,
Input resistance: Rin
mgo
CI
TVor
Output resistance:
CI
CEV
AV
or
Where, VA is Early Voltage (VA=100V for npn)
Dr. D G Borse
Hybrid Parameter Model
hi
hrVohohfIiVi
Ii 2
2'
Io
Vo
1
1'
11 12
21 22
i i o i i r o
o i o f i o o
V h I h V h I h V
I h I h V h I h V
Linear Two port DeviceVi
Ii Io
Vo
Dr. D G Borse
11 12
21 22
0 0
0 0
i i
o ii o
o o
o ii o
V Vh h
V II V
I Ih h
V II V
h-Parameters
h11 = hi = Input Resistanceh12 = hr = Reverse Transfer Voltage Ratioh21 = hf = Forward Transfer Current Ratioh22 = ho = Output Admittance
Dr. D G Borse
The Mid-frequency small-signal models
b
e
hoe
hie
hrevce hfeib vbe
ib ic
vce
c
e
+ _
+ +
_ _
h-parameter model
b
e
rd gmv vbe
ib ic
vce
c
e
+ +
_ _
hybrid- model
r v
+
_
b
e
ib vbe
ib ic
vce
c
e
+ +
_ _
re model
re
fe ac o
Alternate names:
h = = =
m C C
o fe doe
ore ie
m
38.92g = I (Note: Uses DC value of I )
nwhere n = 1 (typical, Si BJT)
1 = h r =
h
h = 0 r = h = g
e BB
o fe
o e ie
re
oe doe
26 mVr = (Note: uses DC value of I )
I
= h
r = h
h = 0
1h = 0, or use r =
h
Three Small signal Models of CE Transistor
Dr. D G Borse
BJT Mid-frequency Analysis using the hybrid-p model:
b
e
rd gmv vi
ii io
vo
c
e
+ +
_ _
mid-frequency CE amplifier circuit
r v
+
_
RC RL RTh vs
+
_
is
RS
A common emitter (CE) amplifier VCC
RC
C
E
B
VCC
R1
RE R2
Rs Ci
RL
Co
CE vi
vo
+
+
vs
+
_ _
_
io
ii
The mid-frequency circuit is drawn as follows:
• the coupling capacitors (Ci and Co) and the
bypass capacitor (CE) are short circuits
• short the DC supply voltage (superposition)• replace the BJT with the hybrid-p model
The resulting mid-frequency circuit is shown below.
si
iv
s
i
i
o
s
o
svCLoLLmi
ov RZ
ZA
v
v
v
v
v
vARRrRRg
v
vA where, , ,''
R where, 21
RRrRI
vZ
ThThi
ii
, Co
ovo
oo
Rri
vZ
i
i
oi i
iA
An a c Equivalent Circuitro
Dr. D G Borse
Details of Small-Signal Analysis for Gain Av (Using Π-model)
33
RCRC
Ro
rL
R R ,
ivbe
v
bev
ov
ivo
v
vA
LbemRvgv
LR
oI
o
Rs
Rs
LR
orR
CR
bev
mg
ov
3
rB
RS
R
rB
R
LR
mg
vA
rB
RS
R
rB
Ri
v
bev
From input circuit
Dr. D G Borse
C-E Amplifier Input Resistance
• The input resistance, the total resistance looking into the amplifier at coupling capacitor C1, represents the total resistance presented to the AC source.
rRRrBRR
rBR
21xixv
in
)(xixv
Dr. D G Borse
C-E Amplifier Output Resistance
• The output resistance is the total equivalent resistance looking into the output of the amplifier at coupling capacitor C3. The input source is set to 0 and a test source is applied at the output.
CRorC
RR
mgorC
R
xixv
out
bevxvxv
xi
But vbe=0.
since ro is usually >> RC.
Dr. D G Borse
High-Frequency Response – BJT Amplifiers
Capacitances that will affect the high-frequency response:• Cbe, Cbc, Cce – internal capacitances
• Cwi, Cwo – wiring capacitances• CS, CC – coupling capacitors• CE – bypass capacitor
Dr. D G Borse
Frequency Response of AmplifiersThe voltage gain of an amplifier is typically flat over the mid-frequency range, but drops drastically for low or high frequencies. A typical frequency response is shown below.
LM(Avi) = 20log(vo/vi) [in dB]
BW
3dB
20log(Avi(mid))
f
fLOW fHIGH
LM Response for a General Amplifier
For a CE BJT: (shown on lower right)• low-frequency drop-off is due to CE, Ci and Co • high-frequency drop-off is due to device capacitances Cp and Cm
(combined to form Ctotal)• Each capacitor forms a break point (simple pole or zero) with a break
frequency of the form f=1/(2pREqC), where REq is the resistance seen by the capacitor
• CE usually yields the highest low-frequency break which establishes fLow.
Dr. D G Borse
Amplifier Power Dissipation
• Static power dissipation in amplifiers is determined from their DC equivalent circuits.
PDV
CEICV
BEIB
Total power dissipated in C-B and E-B junctions is:
where
Total power supplied is:
BIIII
CI
CCV
SP
12 where ,
2
BEVCB
VCE
V
ER
FEQR
BEV
EQV
BI
RRCC
VI
1 and
211
The difference is the power dissipated by the bias resistors.
Dr. D G Borse
Dr. D G Borse
Figure An Emitter follower.
Dr. D G Borse
Figure Emitter follower.
Very high input Resistance
Very low out put Resistance
Unity Voltage gain with no phase shift
High current gain
Can be used for impedance matching or a circuit for providing electrical isolation
An Emitter Follower (CC) Amplifier
Dr. D G Borse
Figure An Emitter follower.
Dr. D G Borse
Figure: An Emitter follower.
Dr. D G Borse
Capacitor Selection for the CE Amplifier
Zc1
jwC Capacitive Reactance XcZc
1wC
where w2f
Xc1R
Br Make X
c10.01 R
Br
for < 1% gain error.
Xc2 0 Make X
c21 for <1% gain error.
Xc3R
3 Make X
c30.01 R
3
for <1% gain error.
The key objective in design is to make the capacitive reactance much smaller at the operating frequency f than the associated resistance that must be coupled or bypassed.
Dr. D G Borse
Summary of Two-Port Parameters forCE/CS, CB/CG, CC/CD
Dr. D G Borse
A Small Signal h-parameter Model of C E - Transistor
= h11
Vce*h12
Dr. D G Borse
A Simple MOSFET Amplifier
The MOSFET is biased in the saturation region by dc voltage sources VGS and VDS = 10 V. The DC Q-point is set at (VDS, IDS) = (4.8 V, 1.56 mA) with VGS = 3.5 V.
Total gate-source voltage is: gsvGS
VGS
v
A 1 V p-p change in vGS gives a 1.25 mA p-p change in iDS and a 4 V p-p changein vDS. Notice the characteristic non-linear I/O relationship compared to the BJT.
Dr. D G Borse
Eber-Moll BJT Model
The Eber-Moll Model for BJTs is fairly complex, but it is valid in all regions of BJT operation. The circuit diagram below shows all the components of the Eber-Moll Model:
E C
B
IRIF
IE IC
IB
RIERIC
Dr. D G Borse
Eber-Moll BJT Model
R = Common-base current gain (in forward active mode)
F = Common-base current gain (in inverse active mode)
IES = Reverse-Saturation Current of B-E Junction
ICS = Reverse-Saturation Current of B-C Junction
IC = FIF – IR IB = IE - IC
IE = IF - RIR
IF = IES [exp(qVBE/kT) – 1] IR = IC [exp (qVBC/kT) – 1]
If IES & ICS are not given, they can be determined using various
BJT parameters.
Dr. D G Borse
Small Signal BJT Equivalent CircuitThe small-signal model can be used when the BJT is in the active region.
The small-signal active-region model for a CB circuit is shown below:
iBr
iE
iCiB
B C
E
r = ( + 1) * VT
IE
@ = 1 and T = 25C
r = ( + 1) * 0.026
IE
Recall:
= IC / IB
Dr. D G Borse
The Early Effect (Early Voltage)
VCE
ICNote: Common-Emitter Configuration
-VA
IB
Green = Ideal IC
Orange = Actual IC (IC’)
IC’ = IC VCE + 1
VA
Dr. D G Borse
Early Effect Example
Given: The common-emitter circuit below with IB = 25A, VCC = 15V, = 100 and VA = 80.
Find: a) The ideal collector current
b) The actual collector current
Circuit Diagram
+_VCC
IC
VCE
IB
b = 100 = IC/IB
a)
IC = 100 * IB = 100 * (25x10-6 A)
IC = 2.5 mA
b) IC’ = IC VCE + 1 = 2.5x10-3 15 + 1 = 2.96 mA
VA 80
IC’ = 2.96 mA
Dr. D G Borse
Breakdown VoltageThe maximum voltage that the BJT can withstand.
BVCEO =The breakdown voltage for a common-emitter biased circuit. This breakdown voltage usually ranges from ~20-1000 Volts.
BVCBO = The breakdown voltage for a common-base biased circuit. This breakdown voltage is usually much higher than BVCEO and has a minimum value of ~60 Volts.
Breakdown Voltage is Determined By:
• The Base Width
• Material Being Used
• Doping Levels
• Biasing Voltage
Dr. D G Borse
Potential-Divider Bias Circuit with Emitter FeedbackMost popular biasing circuit.Problem: bdc can vary over a wide range for BJT’s (even with the same part number)
Solution: Adding the feedback resistor RE. How large should RE be? Let’s see.
Substituting the active region model into the circuit to the left and analyzing the circuit yields the following well known equation:
VCC
RC
C
E
B
VCC
R1
RE R2
VCC
RC
C
E
B
RE
RTh
VTh _ +
2Th CC Th 1 2
1 2
RV = V and R = R R
R + R
dc Th o CEO Th EC
Th dc E
CEO dc CBO
V - V + I R + R I =
R + + 1 R
where I = + 1 I
ICEO has little effect and is often
neglected yielding the simpler relationship:
dc Th oC
Th dc E
V - V I =
R + + 1 R
Test for stability: For a stable Q-point w.r.t. variations in bdc choose:
Th dc ER << + 1 R Why? Because then
dc Th o dc Th o Th oC dc
Th dc E dc E E
V - V V - V V - V I = (independent of )
R + + 1 R + 1 R R
Voltage divider biasing circuit with emitter feedback
Replacing the input circuit by a Thevenin equivalent circuit yields:
Dr. D G Borse
PE-Electrical Review Course - Class 4 (Transistors)
Example : Find the Q-point for the biasing circuit shown below.The BJT has the following specifications:
bdc = 100, rsat = 100 W (Vo not specified, so assume Vo = 0.7 V)15 V
C
E
B
15 V
200 k 1 k
Example : Repeat Example 3 if RC is changed from 1k to 2.2k.
Dr. D G Borse
PE-Electrical Review Course - Class 4 (Transistors)
Example Determine the Q-point for the biasing circuit shown.The BJT has the following specifications:
bdc varies from 50 to 400, Vo = 0.7 V, ICBO = 10 nA
Solution:
Case 1: bdc = 50 C
E
B
18 V
30 k
15 k
10 k
8 k
18 V
Case 2: bdc = 400 Similar to Case 1 above. Results are: IC = 0.659 mA, VCE =
6.14 V Summary:
dc IC VCE
50400
Dr. D G Borse
BJT Amplifier Configurations
and Relationships:
Using the hybrid-p model.
VCC
RC
C
E
B
VCC
R1
RE R2
Rs Ci
RL
Co
CE vi
vo
+
+
vs
+
_ _
_
io
ii
Common Emitter (CE) Amplifier
'o L' '
vi m L m L 'o L
'L d C L d C L E L
'i Th E Th o L
m
Th S o d C d C E
o
i i ivs vi vi vi
s i s i s i
CE CB CC
1 + RA -g R g R
r + 1 + R
R r R R r R R R R
1Z R r R r R r + 1 + R
g
r + R RZ r R r R R
1 +
Z Z ZA A A A
R + Z R + Z R + Z
i i iI vi vi vi
L L L
P vi I vi I vi I
Th 1 2
Z Z ZA A A A
R R R
A A A A A A A
where R = R R
VCC
RC
E
R2
RE
Rs Ci
RL
Co
C2
vi vo
+
+
vs
+
_
_ _
io ii
Common Base (CB) Amplifier
R1
C
B
VCC
C
E
B
VCC
R1
RE R2
Rs C i
vi
+
vs
+
_
_
RL
Co
vo
+
_
io
ii
Common Collector (CC) Amplifier (also called “emitter-follower”)
Note: The biasing circuit is the same for each amplifier.
Dr. D G Borse
Figure 4.16 The pnp BJT.
Dr. D G Borse
Figure : Common-emitter characteristics for a pnp BJT.
Dr. D G Borse
Figure 4.18 Common-emitter amplifier for Exercise 4.8.
Dr. D G Borse
Figure : BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices ata temperature of 300K.
Dr. D G Borse
Figure 4.19b BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices ata temperature of 300K.
Dr. D G Borse
Figure: BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices ata temperature of 300K.
Dr. D G Borse
Figure : Bias circuit Examples
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