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Attia, John Okyere. “Transistor Circuits.”
Electronics and Circuit Analysis using MATLAB.
Ed. John Okyere Attia
Boca Raton: CRC Press LLC, 1999
© 1999 by CRC PRESS LLC
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View Full Document CHAPTER TWELVE
TRANSISTOR CIRCUITS
In this chapter, MATLAB will be used to solve problems involving metal
oxide semiconductor field effect
and bipolar junction transistors.
The general
topics to be discussed in this chapter are dc model of BJT and MOSFET,
biasing of discrete and integrated circuits,
and frequency response of
amplifiers.
12.1
BIPOLAR JUNCTION TRANSISTORS
Bipolar junction transistor (BJT) consists of two pn junctions connected back
toback.
The operation of the BJT depends on the flow of both majority and
minority carriers.
There are two types of BJT:
npn and pnp transistors.
The
electronic symbols of the two types of transistors are shown in
Figure 12.1
.
B
E
C
I
E
I
C
I
B
B
C
I
E
I
C
I
B
(a)
(b)
Figure 12.1
(a) NPN transistor
(b) PNP Transistor
The dc behavior of the BJT can be described by the EbersMoll Model.
The
equations for the model are
II
V
V
FE
S
BE
T
=
−
exp
1
(12.1)
V
V
RC
S
BC
T
=
−
exp
(12.2)
© 1999 CRC Press LLC
and
II
I
CF
F
R
=−
α
(12.3)
I
EF
R
R
+
(12.4)
and
()
III
BF
FR
R
+−
11
αα
(12.5)
where
I
ES
and
I
CS
are the baseemitter and basecollector saturation
currents, respectively
R
is
large signal reverse current gain of a commonbase
configuration
F
is large signal forward current gain of the commonbase
configuration.
and
V
kT
q
T
=
(
1
2
.
6
)
where
k
is the Boltzmann’s constant (
k
= 1.381
x 10
23
V.C/
o
K ),
T
is the absolute temperature in degrees Kelvin, and
q
is the charge of an electron (q = 1.602
x 10
19
C).
The forward and reverse current gains are related by the expression
RC
S
FE
S
S
I
==
(12.7)
where
I
S
is the BJT transport saturation current.
The parameters
R
and
F
are influenced by impurity concentrations and
junction depths. The saturation current,
I
S
, can be expressed as
© 1999 CRC Press LLC
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View Full DocumentIJ
A
SS
=
(
1
2
.
8
)
where
A
is the area of the emitter and
J
S
is the transport saturation current density, and it can be
further expressed as
J
qD n
Q
S
ni
B
=
2
(
1
2
.
9
)
where
D
n
is the average effective electron diffusion constant
n
i
is the intrinsic carrier concentration in silicon
(
n
i
= 1.45 x
1
0
10
atoms / cm
3
at 300
o
K)
Q
B
is the number of doping atoms in the base per unit area.
The dc equivalent
circuit of the BJT is based upon the EbersMoll model.
The model is shown in
Figure 12.2
. The current sources
α
RR
I
indicate the
interaction between the baseemitter and basecollector junctions due to the
narrow base region.
In the case of a pnp transistor, the directions of the diodes in
Figure 12.2
are
reversed.
In addition, the voltage polarities of Equations (12.1) and (12.2) are
reversed.
The resulting EbersMoll equations for pnp transistors are
II
V
V
EE
S
EB
T
=
−
exp
1
−
−
RC
S
CB
T
I
V
V
exp
1
(12.10)
V
V
CF
E
S
EB
T
=−
−
exp
1
+
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