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1 www.ice77.net
Bipolar Junction Transistors The Bipolar Junction Transistor (BJT) is an active nonlinear device that consists of three terminals. These terminals are called collector (C), base (B) and emitter (E). The BJT is a current-controlled device and it behaves like a switch. Its on state depends on a specific amount of voltage necessary to turn the device on: this threshold voltage is called VBE(ON) or VEB(ON) and it’s about 0.3V for germanium transistors or 0.7V for silicon transistors. BJTs come in two types: NPN and PNP. Under normal operating conditions, in NPN transistors the BE junction is forward-biased while the BC junction is reverse-biased. Similarly, in PNP transistors the EB junction is forward-biased while the CB junction is reverse-biased.
V
0
I
V
0
V1
0Vdc
0
Q1
Q2N2222
I1
0Adc
I
NPN BJT (Q2N2222) The transistor is off until VBE reaches about 0.6V. Then the transistor turns on and there are two possibilities: if VCE is less than 0.2V the transistor is in the saturation region whereas if VCE is more than 0.2V the transistor is in the active/linear region. In the active/linear region the collector current stays fairly constant and that is where the transistor typically operates.
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DC sweep (IC/VCE)
The IC/VCE characteristics are plotted above (IC is on the Y axis and VCE is on the X axis). The graph is a DC sweep generated by a primary DC voltage source (0V to 10V) and a secondary DC current source (10µA to 100µA). The sweep shows 10 different collector currents (all in red), each one of them generated by a specific base current (increments of 10µA). At the bottom, a 10µA base current produces a 1.5mA collector current in the active/linear region.
V _ V 1
0 V 1 V 2 V 3 V 4 V 5 V 6 V 7 V 8 V 9 V 1 0 VV ( I 1 : - )
5 5 0 m V
6 0 0 m V
6 5 0 m V
7 0 0 m V
7 5 0 m V
DC sweep (VBE/VCE)
The VBE/VCE characteristics are shown above (VBE is on the Y axis and VCE is on the X axis). The graph is a DC sweep generated by a primary DC voltage source (0V to 10V) and a secondary DC current source (10µA to 100µA). The sweep shows 10 different voltages across the BE junction (all in green), each one of them generated by a specific base current (increments of 10µA). At the bottom, a 10µA base current sets the voltage of the BE junction to 657mV in the active/linear region.
V_V1
0V 1V 2V 3V 4V 5V 6V 7V 8V 9V 10VIC(Q1)
0A
5mA
10mA
15mA
20mA
active/linear
saturation
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BJT characteristics
N
C
Q1
Q2N2222
IeP
IbP
BNP
C
N
E
Ie
Ib
Q2
Q2N2906
Ic
Ic
E
PNP
B
NPN
CURRENT-VOLTAGE CHARACTERISTICS IN THE ACTIVE/LINEAR REGION DC parameters
A
CEnV
V
SC V
VeII T
BE
1
A
ECnV
V
SC V
VeII T
EB
1 BCE III
BE II )1(
1B
C
I
I (50<β<300)
EBC III 1
E
C
I
I (0.980<α<0.997)
S
CTBE I
InVV ln
AC parameters
C
A
C
CEAo I
V
I
VVr
eT
Cm rrV
Ig
mC
T
E
Te gI
V
I
Vr
emC
T
B
T rgI
V
I
Vr 1
PARAMETRIC SWEEP FOR THE COMMON EMITTER CONFIGURATION
I
0
V1
0Vdc
I
0
0
Q1
Q2N2222
I1
0Adc
V_V1
0V 1V 2V 3V 4V 5V 6V 7V 8V 9V 10VIC(Q1)
0A
5mA
10mA
15mA
20mA
active/linear
saturation
VBE=0.7V VEB=0.7V
NPN PNP
VCE(min)=0.2V VBC(max)=0.5V
+VBE-
+VCE-
-VEB+
+VEC-
TRANSCONDUCTANCE
EARLY EFFECT
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Facts about BJTs
1. The BJT was invented in 1947 by Bardeen, Brattain and Shockley.
2. Early BJTs were made of germanium (Ge). Modern BJTs are made of silicon (Si).
3. An NPN transistor has an n-type collector, a p-type base and an n-type emitter.
Electrons are minority carriers in the base and they define the type of current in
the device.
4. In an NPN transistor, the emitter is heavily doped (n+) while the collector is lightly
doped (n).
5. BJTs are bipolar devices because their operation depends on electrons and
holes rather than either electrons or holes like in MOSFETs.
6. The emitter takes its name from the fact it is the emitter of charge carriers
(electrons for NPN and holes for PNP).
7. NPN transistors are more popular than PNP transistors because electron mobility
is higher than hole mobility in semiconductors. This translates into higher current
and faster operation.
8. BJTs, when compared to MOSFETs, have higher transconductance (gm) for
same output current (iC versus iD).
9. BJTs are preferred over MOSFETs for most analog applications.
10. BJTs and MOSFETs can be combined into BiCMOS to use advantages of both
technologies.
11. Heterojunction Bipolar Transistors (HBTs) are improved BJTs that can handle
high frequencies. They are made of silicon-germanium (Si-Ge) or aluminum
gallium arsenide (AlGaAs). Essentially, they are high-speed BJTs.
12. BJTs are preferred over MOSFETs for very high-frequency applications such as
radio-frequency circuits for wireless systems.
5 www.ice77.net
Common Emitter Configuration
R1
90k
200.5uA
0V
V
11.30V
0
RC
10k
869.7uA
0
0V
0V
CB
10uFRL
20k0A
0
0
20.00V
CE
470uF
CC
10uF
REDC
1k
875.0uA
V
0V
Rs
500A
20.00V
Vs
FREQ = 1kHzVAMPL = 500mVVOFF = 0V
0A
VCC
Q1
Q2N2222
5.359uA869.7uA
-875.0uA
0
1.313V
R2
10k
195.2uA
1.952V
20.00V
REAC
500
875.0uA
0
VCC
VCC20Vdc
1.070mA
VCC
0V
Common Emitter Configuration
The Common Emitter Configuration features a grounded emitter. However, in the above schematic, two emitter resistors and a capacitor are also present. The components are introduced to ensure stability and avoid clipping of the output waveform. The load is connected to the collector.
Time
0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0ms 4.5ms 5.0msV(CC:2) V(Vs:+)
-8.0V
-4.0V
0V
4.0V
8.0V
Time domain sweep at 1kHz
The voltage gain is greater than 1 and the output is 180° out of phase with respect to the input.
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CE
470uF
V
20.00V
20.00V
0
1.313V
RL
20k0A
0
1.952V
R1
90k
200.5uA
Vs500mVac0Vdc
0A
VCC
0V
REAC
500
875.0uA
V
CB
10uF
VCC
0
20.00V
VCC20Vdc
1.070mA
0V
RC
10k
869.7uA
Q1
Q2N2222
5.359uA869.7uA
-875.0uA
R2
10k
195.2uA
0V
0V
CC
10uF
0
VCC
0
Rs
500A
0V
REDC
1k
875.0uA
0
11.30V
Common Emitter Configuration
Frequency
10mHz 1.0Hz 100Hz 10KHz 1.0MHz 100MHz 10GHzV(CC:2) V(Vs:+)
0V
2.0V
4.0V
6.0V
8.0V
AC sweep from 10mHz to 10GHz
The amplifier is linear from about 2.5Hz to 7.5MHz. The voltage gain in the midband is about 12. REAC increases input resistance and lowers the voltage gain but it can also distort the output. CE lowers the low –3dB frequency point.
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Common Collector Configuration
CE
10uF
R1
20k
169.0uA
V
0V
0V
R2
200k
83.10uA
VCC
0V
Vs
FREQ = 1kHzVAMPL = 500mVVOFF = 0V
0A
RL
20k0A
0
Q1
Q2N2222
85.86uA15.82mA
-15.90mA
0
20.00V
Rs
500A
V
CB
10uF
15.90V
VCC
20.00V
0
0
0V
15.90V
0V
RE
1k
15.90mA
0V
16.62V
20.00V
0
VCC
VCC20Vdc
15.99mA
Common Collector Configuration
The Common Collector Configuration features an AC grounded collector. The load is connected to the emitter.
Time
0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0ms 4.5ms 5.0msV(RL:2)
-500mV
0V
500mVV(Vs:+)
-500mV
0V
500mV
SEL>>
Time domain sweep at 1kHz
The voltage gain is close to 1 and the output is in phase with respect to the input.
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20.00V
0V
CE
10uF
R1
20k
169.0uA
V
0V
20.00V
0V
R2
200k
83.10uA
VCC
15.90V
RL
20k0A
15.90V
0
Q1
Q2N2222
85.86uA15.82mA
-15.90mA
0
20.00V
Rs
500A
Vs500mVac0Vdc
0A
V
CB
10uF
VCC
0
0V
0
16.62V
RE
1k
15.90mA 0V
0
0V
VCC
VCC20Vdc
15.99mA
Common Collector Configuration
Frequency
10mHz 1.0Hz 100Hz 10KHz 1.0MHz 100MHz 10GHz 1.0THzV(Vs:+) V(RL:2)
0V
100mV
200mV
300mV
400mV
500mV
AC sweep from 10mHz to 1THz
The amplifier is linear from about 1.4Hz to 587MHz. The voltage gain in the midband is almost 1. This amplifier is often called Emitter Follower.
9 www.ice77.net
Common Base Configuration
V
CE
100uF
V+6Vdc
4.346mA
R1
60k
81.36uA
V+
Rs
50
0A
1.119V
V
V+
0
CB
100uF
V-
V
0
Q1
Q2N2222
25.43uA
4.265mA
-4.290mA
1.736V
0V
RE
1.5k
4.290mA
6.000V
RC
1k
4.265mA
0
V1
FREQ = 1kHzVAMPL = 50mVVOFF = 0V
0A
434.9mV0
0V
V+
0
0V
RL
20k0A
V-
0V
V--6Vdc
4.290mA
R2
20k
55.93uA
0V
-6.000V
CC
10uF
0V
Common Base Configuration
The Common Base Configuration features an AC grounded base. In the above schematic the two resistors at the base are used to bias the transistor. The load is connected to the collector and the voltage gain is the ratio of the voltage at the output over the voltage at the emitter.
Time
0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0ms 4.5ms 5.0msV(CC:2) V(V1:+) V(CE:1)
-1.0V
-0.5V
0V
0.5V
1.0V
Time domain sweep at 1kHz
The voltage gain is greater than 1 and the output is in phase with respect to the input.
10 www.ice77.net
0VR2
20k
55.93uA
0
1.736V
RC
1k
4.265mA
R1
60k
81.36uA -6.000V
0V
0
0
V+
V+
CB
100uF
Q1
Q2N2222
25.43uA
4.265mA
-4.290mA
434.9mV0
V
0V
CE
100uF
6.000V
V+6Vdc
4.346mA
V+
V-
0V
V250mVac0Vdc
0A
V-
V
0V
RE
1.5k
4.290mA
V
Rs
50
0A
V--6Vdc
4.290mA
0V
1.119V
CC
10uF
0
RL
20k0A
Common Base Configuration
Frequency
10mHz 1.0Hz 100Hz 10KHz 1.0MHz 100MHz 10GHzV(RL:2) V(Rs:1) V(CE:1)
0V
0.5V
1.0V
AC sweep from 10mHz to 10GHz
The amplifier is linear from about 28.5Hz to 22.95MHz. The voltage gain in the midband is about 145 (VC/VE). The highest gain attainable by a BJT amplifier is provided by the Common Base Configuration. The highest bandwidth attainable by a BJT amplifier is provided by the Common Collector Configuration.
11 www.ice77.net
Multistage BJT amplifier I
C4
10u
C2
10u
1.943V
1.039V
C3
10u
C5
10u
R4
10k
1.032mA
V2
20Vdc
3.699mA
0
VCC
R6
1k
1.039mA
R10
250
1.039mA
0R5
250
1.039mA
0
20.00V
V
20.00V
0V
1.039V
R13
10k
194.3uA
0
0V
1.943V
R15
250
1.039mA
R16
1k
1.039mA
R12
90k
200.6uAVCC
R11
1k
1.039mA
0
9.676V
R14
10k
1.032mA
VCC
R3
10k
194.3uA
0
R7
90k
200.6uA
C7
10u
RL
10k
0A
20.00V
0
0
20.00V
0V
1.299V
R1
500A
C1
10u
0V
V
0V
V1
FREQ = 1kHzVAMPL = 1mVVOFF = 0V
0A
R2
90k
200.6uA0
VCC
Q2
Q2N22226.367uA
1.032mA
-1.039mA
Q3
Q2N22226.367uA
1.032mA
-1.039mA
1.299V
R8
10k
194.3uA
Q1
Q2N22226.367uA
1.032mA
-1.039mA
9.676V
VCC
0
1.299V
0
VCC
C6
10u
1.943V
0
1.039V
R9
10k
1.032mA
VCC
0V
0V
Multistage BJT amplifier I
A sequence of Common Emitter amplifiers can be combined to produce higher gain but reduced bandwidth.
Time
0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0ms 4.5ms 5.0msV(RL:2)
-5.0V
0V
5.0VV(V1:+)
-1.0mV
0V
1.0mV
SEL>>
Time domain sweep at 1kHz
The voltage gain is about 4271 and the output is 180° out of phase with respect to the input.
12 www.ice77.net
0
20.00V
1.943V
V
0
VCC
0V
1.039V
1.039V
VCC
R8
10k
194.3uA1.943V
20.00V
C3
10u
0
Q2
Q2N22226.367uA
1.032mA
-1.039mA
VCC
R13
10k
194.3uA
0
0
0
0V
20.00V
R5
250
1.039mA
1.039V0V
0
VCC
VCCV2
20Vdc
3.699mA 9.676V
0
0
R2
90k
200.6uA
R3
10k
194.3uA
R4
10k
1.032mA
R16
1k
1.039mA
9.676V
R9
10k
1.032mA
1.299V
0
1.299V
R6
1k
1.039mA
Q1
Q2N22226.367uA
1.032mA
-1.039mA
R14
10k
1.032mA
C7
10u
V
0V
C4
10u
R10
250
1.039mA
0
C2
10u
VCC
C6
10u
20.00V
1.299V
R15
250
1.039mA
R12
90k
200.6uA
R11
1k
1.039mA
R1
500A
C5
10u
Q3
Q2N22226.367uA
1.032mA
-1.039mA
V11mVac0Vdc
0A
VCC
0V
C1
10u
0V
R7
90k
200.6uA
0
1.943V
RL
10k
0A
0V
Multistage BJT amplifier I
Frequency
100mHz 1.0Hz 10Hz 100Hz 1.0KHz 10KHz 100KHz 1.0MHz 10MHz 100MHz 1.0GHzV(RL:2) V(R1:1)
0V
200mV
400mV
600mV
800mV
AC sweep from 100mHz to 1GHz
The amplifier is linear from 133Hz to 294kHz. The voltage gain in the midband is about 4271. The gain is high so the bandwidth is reduced.
13 www.ice77.net
Multistage BJT amplifier II
0
Q2
Q2N222218.39uA
3.617mA
-3.636mA
V+
10.37V4.310V
R11
30k 503.5uAR7
50k
86.21uA
Rs
500A
V+
V+20Vdc
9.918mA
3.636V
0V
Q1
Q2N2222
17.42uA2.881mA
-2.898mA
C3
10uF
0
C2
10uF
5.598V
3.188V
0
0
V-
20Vdc
2.423mA
0V
0
V+
R1
40k
403.5uA
R3
5k
2.881mA
V+
020.00V
20.00V
R2
10k
386.1uAV1
FREQ = 1kHzVAMPL = 1mVVOFF = 0V
0A
R10
10k489.4uA
0
R4
100
2.898mA
00
4.227V
R9
10k
2.423mA
20.00V
0V
V+ V-
R12
4k
2.409mA
Q3 Q2N2222
14.13uA
2.409mA-2.423mA
V+
4.894V
R6
150k
104.6uA
3.861V
-20.00V
0
0V
C1
10uF
0V
C4 20uF
C5
10uF
C6
10uF
R8
1k
3.636mA
R5
1k
2.898mA0V
V
V+
2.898VV-
V
0
0V
RL
20k
0A
Multistage BJT amplifier II
The three configurations of the BJT can be combined in sequence. In the above schematic the amplifier has three stages: CE, CC and CB (left to right). The combination of these produces high gain and moderate bandwidth.
Time
0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0ms 4.5ms 5.0msV(C6:1)
-4.0V
0V
4.0VV(V1:+)
-1.0mV
0V
1.0mV
SEL>>
Time domain sweep at 1kHz
The voltage gain is about 3262 and the output is 180° out of phase with respect to the input.
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R1
40k
403.5uA
0
3.188V
V+20Vdc
9.918mA
4.894V
3.636V
R3
5k
2.881mA
C4 20uF
0
Q1
Q2N2222
17.42uA2.881mA
-2.898mA
0V
C5
10uF
4.227V
V+
C2
10uF
-20.00V
Q3 Q2N2222
14.13uA
2.409mA-2.423mA
0
20.00V
V+
R5
1k
2.898mA
V+V
0
C3
10uF
R2
10k
386.1uA
V+
R6
150k
104.6uA0
0V
5.598V
RL
20k
0A
4.310V
3.861V
R9
10k
2.423mA
Rs
500A 0V
V+
C6
10uF
C1
10uFR10
10k489.4uA
0
10.37V
V+
R4
100
2.898mA
0V
0
0
0
V11mVac0Vdc
0A
0
R11
30k 503.5uA
R12
4k
2.409mA
V-
20Vdc
2.423mA
V-
0V
0
V0V
R8
1k
3.636mA
20.00V
R7
50k
86.21uA
V-
0V2.898V
20.00V
Q2
Q2N222218.39uA
3.617mA
-3.636mA
V+
Multistage BJT amplifier II
Frequency
10mHz 1.0Hz 100Hz 10KHz 1.0MHz 100MHz 10GHzV(C6:1) V(V1:+)
0V
1.0V
2.0V
3.0V
4.0V
AC sweep from 10mHz to 10GHz
The amplifier is linear from about 288Hz to 1.602MHz. The voltage gain in the midband is about 3262. The gain is high so the bandwidth is reduced.
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Bipolar Junction Transistor AC/SMALL-SIGNAL ANALYSIS - MIDBAND Common emitter configuration
Q1
40239
Vin
-
0
C
+
0
B
E
Vcc
R3
icR2
+
R1
Rc
-
0
Vout
0
C
+
ic=gmVbe
0
Rc
E
R1
rpi
R2
BVout
r0
-
+
Vout
R3
Vin
-
rR 1 CRrR ||02 mm
e ggrR
13
CcoutCCcout RiVRrRriV 00 || BEin VV
CmBE
CBEm
BE
Cc
in
outV Rg
V
RVg
V
Ri
V
VA
16 www.ice77.net
Bipolar Junction Transistor AC/SMALL-SIGNAL ANALYSIS - MIDBAND Common emitter configuration
0
Rs
Rc
Vcc
Vcc
0
RL
0
Vs
0
R1
R2
Vout
B
Vs
0
Rout
0
RL
Vout
Vpi
C
0
RB=R1||R2 rpi
ic=gmVpi
Rs
ro
Vin
0
E
0
Rin
-
00
Rc
+
rRR Bin || coout RrR ||
VVin Lcomout RRrVgV ||||
Lcom
Lcom
in
outv RRrg
V
RRrVg
V
VA ||||
||||
Properties: high voltage gain, high input resistance, reduced bandwidth (Miller effect)
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Common emitter configuration with degeneration
Rc
Vcc
R2
0
Vs
RL
0
R1
Rs
0
REAC
0
Vout
Vcc
0
REDC
Rc
0
E
rpi
+
RoutRin
0
ro
Ve0
C
+
RL-
0
Vin
ic=Bib
Rs B
RB=R1||R2
-0
Vpi
REAC
Vout
Vs
eEACBEACBin rRRRrRR 1||1||
ccoout RRrR ||* EACmoo Rgrr 1*
EACbEACbbEACebEACcbbein RriRiriRiriRiiriVVV 11
eEACb rRi 1
Lcbout RRiV ||
eEAC
Lc
eEAC
Lc
eEACb
Lcb
in
outv rR
RR
rR
RR
rRi
RRi
V
VA
||
1
||
1
||
Properties: REAC increases the input resistance and lowers the voltage gain
18 www.ice77.net
Common collector/Emitter follower configuration
RL
Vcc
R2
0
00
Vout
R1
0
Vs
Rs
RE
Vcc
-
Vout
0
Rs
Vpi+
0
RB=R1||R2
Rout
0
ic=Bib
Vin E
RLVs
0
B rpi
C
0
Rin
0
ro RE
LEoeBLEoBin RRrrRRRrrRR ||||1||||||1||
1
||||||
SBEoout
RRrRrR
LEobLEobbin RRrriRRririV ||||1||||1
Loutbout RRiV ||1
LEoe
Lout
LEo
Lout
LEob
Loutb
in
outv RRrr
RR
RRrr
RR
RRrri
RRi
V
VA
||||
||
||||1
||1
||||1
||1
Properties: voltage gain close to 1, very high input resistance, very low output resistance
19 www.ice77.net
Common base configuration
Vcc
Vcc
0
R1
RE
0 Vs
Vout
0Rs
VEE
Rc
0
R2
RL
Rc
ic=aie
0 B
Vout
Vpi ie
-
Rs
0
Vs
Rin
RL
E
0
ro
re
Rout
0
Vin
+
C
0
RE
Actual circuit
20 www.ice77.net
0
Rs
0
Rin
VinE
reVpi
RL
0
REVs
0
ic=aie
0
Rout
C
+
B
-
0
Vout
Rcie
Simplified circuit
eEin rRR || CCoout RRrR ||
eein riVV LCeout RRiV ||
LCm
e
LC
ee
LCe
in
outv RRg
r
RR
ri
RRi
V
VA ||
||||
Properties: very high voltage gain, very low input resistance, high bandwidth
21 www.ice77.net
Bipolar Junction Transistor AC/SMALL-SIGNAL ANALYSIS - LOW FREQUENCY The bandwidth of the BJT is limited in the low frequency range by the external capacitances CB, CC and CE. These capacitances set the lower cutoff frequency fL. Common emitter configuration
0
Vcc
0
RL
Cc
Vcc
0
R2
CB
R1
Rs
Vout
Vs
Rc
0
RB=R1||R2
0 00
rpi
Rs C
Vs
E
ic=gmVpi
Vpi
0
Rc
B
RL
0
+
Cc
ro
0
CB
-
0
BBSB CrRR || CLCoC CRRr ||
n
nL
1
2
LLf
22 www.ice77.net
Common emitter configuration with degeneration
R2 REAC
RL
0
0
Cc
Vcc
0
R1
0
Rs
Vcc
Vs
Rc
CB
0 REDCCE
Vout
CE
rpi
- RL
Rs
0
E
ic=gmVpi
Rc
REAC
RB=R1||R2Vs
Vpi
0
+
0
ro
REDC
0
0
0
CB C CcB
BEACBSBLCoEACBSB CRrRRCRRrRrRR ||||||||
CCLCBS
EACoCLC CRRCRRr
RrRR
1
||||||
EBS
EACEDCELCoBS
EACEDCE CRRr
RRCRRrRRr
RR
1
||||||||
1
||||
n
nL
1
2
LLf
23 www.ice77.net
Common collector configuration
Vs
0
CB
Vcc
Vcc
RL
0
0
R2
R1
CE
Rs
RE
0
Vout
0
Vs
rpiRs CE
RE
+
ic=gmVbe
Vbe
0
-
B
0
ro
0
RL
ECB
0
C
0
RB=R1||R2
BSBLEoB CRRrRRr ||||||
ELEoBSE CRRrrRR ||||||
n
nL
1
2
LLf
24 www.ice77.net
Common base configuration
CB
Vout
CE
R2
0
0
0
Vs
Vcc
RE
RL
VEE
Rs
Vcc
0
R1
Rc
Cc
RL
ro
Vs
0
ECE
RE
0
+
-
Rs
Rc
CcC
0
re
0 B0
Vpi
ic=aie
Actual circuit
25 www.ice77.net
0 B
RLre
CE
Rc
0
Vs-
0
Vpi
Rs
+
E
ic=aie
CcC
0 0
RE
Simplified circuit
EeESE CrRR ||
CLCC CRR
n
nL
1
2
LLf
26 www.ice77.net
Bipolar Junction Transistor AC/SMALL-SIGNAL ANALYSIS - HIGH FREQUENCY The bandwidth of the BJT is limited in the high frequency range by the internal capacitances Cπ and Cμ. These capacitances set the higher cutoff frequency fH. Common emitter configuration
Vcc
00
R1VoutCc
Vcc
Rc
CBRs
0
R2Vs
0
RL
00
B
ro
0 0
+
0
RLRB=R1||R2 rpi
0 0
Rc
0
Cpi
E
CCmu
Vpi
ic=gmVpi
-
Vs
Rs
Actual circuit
Cpi CLCM
0
RB=R1||R2 ro
Rs
-
0
B
0
+
0 0 0
E
0
C
RcVs
Vpi rpi
ic=gm*Vpi
0 00
RL
Simplified circuit
MBSi CCrRR |||| )1( KCCM
LLCoo CRRr |||| CK
CCL
11
n
n
H
1
2
HHf
27 www.ice77.net
Common emitter configuration with degeneration
0
R2
0
Rc
0
Rs
Vcc
RL
0
R1
REDC
REAC
0
CB
Vout
CE
Cc
Vs
Vcc
0
RL
0 0
Rs
REAC0 0
CpiRcRB=R1||R2
rpiVs
-
+
Vbe
Cmu CB
E
roic=gmVbe
Actual circuit
0
CLVs
0 00
Vbeic=gm*Vbe
E
Cpi*
0 0
B
CM
00
RB=R1||R2
0
RL
C
0
+
Rs
-
rpi* ro* Rc
Simplified circuit
EACm
mm Rg
gg
1*
28 www.ice77.net
MBSi CCrRR **||||
EACm Rgrr 1*
EACm Rg
CC
1* )1( KCCM
LLCoo CRRr ||||*
EACmoo Rgrr 1* CK
CCL
11
n
n
H
1
2
HHf
29 www.ice77.net
Common collector configuration
Vs
0
CB
Vcc
Vcc
RL
0
0
R2
R1
CE
Rs
RE
0
Vout
RLRE
0
Vbe
0
E
0000 0
+
B
ic=gmVbeCL
Cpi
Cmu
0
Vs
C
-
Rs rpi
RB=R1||R2 ro
Actual circuit
Rs
0 0 0
RLrpi**
0
ro
0
C
00
B
ic=gmVbe
0
REVs
rpi*
E
0
Cmu CLCM
0 0
RB=R1||R2
Simplified circuit
MBSi CCrRR *||||
K
rr
1* )1( KCCM
LLEoo CRRrr |||||| **
K
rr
11
**
CK
CCL
11
n
n
H
1
2
HHf
30 www.ice77.net
Common base configuration
CB
Vout
CE
R2
0
0
0
Vs
Vcc
RE
RL
VEE
Rs
Vcc
0
R1
Rc
Cc
+
Rs C
00
CmuRcVs
00
RL
ro
ic=aie
-
0
reVpi
0 B
RE Cpi
E
0 Actual circuit
31 www.ice77.net
+
0 0
Cpire
0
Rc
B
ic=aie
Vpi
-
RL
E
0 00
VsRE Cmu
C
0
Rs
Simplified circuit
CrRR eESi ||||
CRR LCo ||
n
n
H
1
2
HHf
Note: the common base configuration is not affected by Miller capacitance because the base of the amplifier is grounded.