# Common Base Configuration

```2. Bipolar Junction Transistor
BJT Transistor Modeling
Amplication in the ac domain
Steady current established by a dc supply
Effect of a control element on the
system
Transistor circuit
ac analysis
Replacing all DC and capacitors by a short-circuit equivalent
small-signal ac analysis
2. Bipolar Junction Transistor
\
The Importance Parameters
Two-port system
Input impedance:
Output impedance:
The output impedance is determined at the output terminals looking back into the system with the
applied signal set to zero.
2. Bipolar Junction Transistor
Voltage Gain
Current Gain
2. Bipolar Junction Transistor
Phase relationship
For the typical transistor amplifier, the input and output signals are either 180&deg; out
of phase or in phase.
THE re TRANSISTOR MODEL
BJT transistor amplifiers are referred to as current-controlled devices.
Common Base Configuration
re model
2. Bipolar Junction Transistor
Typical values of the Zo are in the mega ohm range
Defining Zo.
(Vo and Vi are in phase)
2. Bipolar Junction Transistor
Determining Zi
If the applied signal is set to zero, then Ic is 0 A,resulting
Impact of re on input impedance.
2. Bipolar Junction Transistor
(Vo and Vi are 180Μ out of phase)
Two-port system
The term “term” or h-parameter was chosen because the mixture of variables( V and I)
Hybrid input equivalent circuit
Hybrid output equivalent circuit
2. Bipolar Junction Transistor
Complete hybrid equivalent circuit
Short-circuit input–impedance parameter:
Open-circuit reverse transfer voltage ratio parameter:
Short-circuit forward transfer current ratio parameter:
The common emitter configuration: Ii =Ib, Io=Ic,Vi=Vbe,Vo=Vce
Ie= Ib + Ic
Common-emitter configuration
The common base configuration: Ii=Ie, Io=Ic,Vi=Veb,Vo=Vcb
Ie= Ib + Ic
2. Bipolar Junction Transistor
Common base configuration
Since hr is normally a relatively small quantity and resistance determined by 1/ho is often large
enough, resulting
Hybrid versus re model
Common-emitter Configuration
2. Bipolar Junction Transistor
The current source of the standard hybrid equivalent circuit is pointing down rather than in
the actual direction as shown in the re model
GRAPHICAL DETERMINATION OF THE h-PARAMETERS
2. Bipolar Junction Transistor
2. Bipolar Junction Transistor
2. Bipolar Junction Transistor
Complete hybrid equivalent circuit for a transistor
1.5kW
10 0 Ib
33mA / V
2. Bipolar Junction Transistor
BJT Small-Signal Analysis
re model
1. Common Emitter (CE) Fixed-Bias Configuration
Effect of ro
Zo= ro||Rc
Av=−
ππ||ππ
ππ
Io=
1
ππ
1
1
+
ππ ππ
πΌπ
π½πΌπ =
πΌπ πΌπ
ππ
ππ+ππ
ππ.π½
π½πΌπ →
πΌπ
πΌπ
=
πππ½
ππ+ππ
πππ½
Ai= πΌπ = πΌπ πΌπ = (ππ+ππ) (≅ 1) → π΄π = ππ+ππ
2. Bipolar Junction Transistor
CE Voltage-Divider Bias Configuration
R’=R1||R2=
π1.π2
π1+π2
Ib =
1
π½ππ
1
1
+
π½ππ π′
Zi=R’||π½ππ
Io =π½πΌπ →
Zo = Rc
Ai =
Av =
ππ
− ππ
Effect of ro
Zo = ro||Rc
Av =−
ππ||ππ
ππ
πΌπ
ππ. π½
=
πΌπ ππ + ππ
Ai =
πΌπ
πΌπ
=
πΌπ πΌπ
πΌπ πΌπ
Or
ππ
Ai = −π΄π£ ππ
ππ.π½
= ππ+ππ
π′
π ′ +π½ππ
πΌπ
πΌπ
=
πΌπ →
πΌπ
πΌπ
πΌπ
=
π′
π ′ +π½ππ
=π½
πΌπ
πΌπ πΌπ
πΌπ πΌπ
=
π½π′
π ′+π½ππ
2. Bipolar Junction Transistor
Unbypassed
r
Vi = Ibπ½ re +Ie RE = Ibπ½ re + (π½+1)Ib RE = (π½ e +(π½+1) RE)Ib
Zb =
ππ
πΌπ
r
r
= π½ e +( π½+1)RE = π½( e + RE)
(for π½ &gt;&gt;1)
Ib =
1
ππ
1
1
+
ππ ππ΅
πΌπ
Ai =
πΌπ
=
or
Zo = Rc
ππ
ππ
Vo= − πΌπ. ππ = −π½πΌπ ππ
ππ
ππ
ππ
ππ
=−
ππ
ππ+ππΈ
Effect of ro
For typical parameter values the effect of ro can be ignored
Bypassed
If RE is bypassed by an emitter capacitor CE, then
Zb = π½. re
Av =−
ππ
ππ
πΌπ
πΌπ πΌπ
πΌπ πΌπ
=
ππ΅
= ππ΅+ππ
Io = π½πΌπ → πΌπ = π½
Zi = RB || Zb
Vo= −π½( )Rc → π΄π£ =
πΌπ
πΌπ
r
≅ π½RE (for RE &gt;&gt; e)
Ib =
πΌπ →
π½.ππ΅
ππ΅+ππ
2. Bipolar Junction Transistor
Common Collector ( Emitter Follower) Configuration
2. Bipolar Junction Transistor
COMMON-BASE CONFIGURATION
ππ
Vo= Io Rc = ( Ic )Rc =πΌIe.Rc =πΌ ( ) ππ → π΄π£
re
(since πΌ ≅ 1)
Ie ≅ πΌπ ( for RE&gt;&gt;re)
Io = −πΌπ = − πΌIe = − πΌIi→ π΄π = − πΌ ≅ −1
Effect of
ro
For typical parameter values the effect of ro can be ignored
APPROXIMATE HYBRID EQUIVALENT CIRCUIT
2. Bipolar Junction Transistor
common-emitter
hie =π½re
ο·
hie =π½re
common-base
hoe = 1/ro
hfb =πΌ
Common Emitter (CE) Fixed Bias Configuration
Zi =RB
Zo=RC
||hie
ππ
(βππ
) RC
VO = -IoRC = hfe Ib RC = - hfe
Ib ≅ Ii
Io = hfe Ii
Effect of ro
1
Hoe = ππ
( for RB &gt;&gt; hie
hfe
AV = -
β ππ ππ
βππ
hib = re
2. Bipolar Junction Transistor
1
Zo = βππ || Rc
re =
βππ
βππ
Av = -
ο·
πππ(
π
||πΉπ)
πππ
πππ
CE Voltage – Divider – Bias Configuration
ο· CE Emitter –Bias Configuration
Unbypassed
2. Bipolar Junction Transistor
ο·
Common Collector (Emitter Follower) Configuration
ο· Common Base Configuration
2. Bipolar Junction Transistor
Zi = RE||h ib
Zo = Rc
ππ
Ie =
h ib
ππ
Vo =−πΌπ. ππ = −(βππ. πΌπ)ππ = = −βππ (h ) ππ → π΄π£ = −
ib
Ai =
πΌπ
Ii
βππ.ππ
h ib
= hfb
System Aprroach- Effect Of Rs And RL
Fixed Bias
V
CC
RC
RB
RC
Zi = RB|| π½re
Zo = Rc
ππ
ππΏ||ππ
Av = = −
Vi
re
πΌπ
ππ
Ii = Is→ π΄π = Ii = −π΄π£ ππΏ
Voltage Divider Bias
Vi =
ππ
ππ+ππ
Avs =
ππ
Vs
=
ππ  →
ππ ππ
Vs Vi
ππ
ππ
=
=
ππ
ππ+ππ
ππ
Zi+Rs
Av
RL
2. Bipolar Junction Transistor
CE Unbypass Emitter Bias
Emitter Follower Network
R’E = RE||RL
Vs –πΌπ.Rs – πΌππ½ππ − (π½+1)Ib R’E = 0 → πΌπ =
Ie
π·π°π =
ππ
Rs
+ππ+π ′ πΈ)
β
ππ
Rs+π½ππ+(π½+1)π′πΈ
≅
ππ
Rs+π½(ππ+π ′ πΈ)
2. Bipolar Junction Transistor
Vo = Rs
β
π′ πΈ
+ππ+π ′ πΈ
Zo = RE ||
(
Rs
β
vs → Avs = Rs
β
π′ πΈ
Zb = π½ππ + (π½ + 1)π′ πΈ ≅ π½(ππ + π′ πΈ)
+ππ+π ′ πΈ
+ ππ )
Zi = RB ||Zb
Common Base Network
Av ≅
Philips Semiconductors
NPN general purpose transistors
ππΏ||ππ
re
Product specification
BC 107;BC 108;BC;109
2. Bipolar Junction Transistor
PINNING
FEATURES
PIN
DESCRIPTION
ο·
ο·
1
emitter
2
base
3
Collector,connected the case
Low current (max. 100mA)
Low voltage ( max. 45 V)
APPLICATIONS
ο·
General purpose switching and
amplification
DESCRIPTIONS
NPN transistor in a TO-18;SOT 18 metal package
PNP complement : BC177
Characteristic
T = 25 C, unless other wise specified
symbol
parameters
IcBc
Collector cut-off current
IeBc
hFE
hFE
VCEsat
VBEsat
VBE
Emitter cut-off current
DC current gain
BC 107A; BC108A
BC107B; BC108B; BC109B
BC109C; BC109C
DC current gain
BC 107A; BC108A
BC107B; BC108B;BC109B
BC109C; BC109C
conditions
IE = 0; VCB = 20 V
IE = 0; VCB = 20 V; T = 125 C
Ic = 0; VEB = 5 V
Ic = 10μA;VcE = 5 V
Ic = 2 mA;VcE = 5 V
Collector emitter saturation voltage Ic = 10 mA; IB = 0,5 mA
Ic = 100 mA; IB = 5 mA
Base emitter saturation voltage
Ic = 10 mA; IB = 0,5 mA;note 1
Ic = 100 mA; IB = 5 mA;note 1
Base emitter voltage
Ic = 2 mA; VcE = 5 V; note 2
Ic = 10 mA; VcE = 5 V; note 2
min
-
type
-
max
15
15
unit
nA
-
-
50
nA
40
100
90
150
270
-
110
200
420
180
290
520
220
450
800
550
-
90
200
700
900
620
-
250
600
700
770
μA
mV
mV
mV
mV
mV
mV
2. Bipolar Junction Transistor
Cc
Ce
fT
F
F
Collector capacitance
Emitter capacitance
transition frecuency
Noise figure
BC109B;BC109C
Noise figure
BC 107A; BC108A
BC107B; BC108B; BC108C
BC109B; BC109C
NOTES
IE = ie = 0; VCB =10 V; f = 1 MHZ
IC= ic = 0; VEB =0,5 V; f = 1 MHZ
IC= 10mA; VCB =5 V; f = 100 MHZ
Ic = 200μA;VcE = 5 V;Rs = 2 kΩ
f = 30 Hz to 15.7 kHz
Ic = 200μA;VcE = 5 V;Rs = 2 kΩ;
F = 1kHz;B = 200Hz
1. VBEsat decrease by about 1.7 mV/K with increasing temperature
2. VBE decrease by about 2 mV/K with increasing temperature
100
2.5
9
-
6
-
pF
pF
MHz
-
-
4
dB
-
-
10
4
dB
dB
2. Bipolar Junction Transistor
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