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ELEC 301
BJT Bias Circuits
1
B
C E^
E B
C
IB
IC IE
IC IE IB npn^
pnp
BJT Bias Configurations
BJT Current Relationships:
(^
)^
C F F E B F E E F F C BF C E F B C F B^
I I I I I I I I I I I I
β^ β
,^
BJT Bias Configuration
Base-Emitter Loop
Collector-Emitter Loop
Fixed Bias npnR^ B^
I^ B^ R^ C
+VCC I^ C + VCE- V^ BE^ +-
R^ B^
I^ B^ R^ C
I^ C
V + γ
βI^ F^
B
(a)^
(b)
BE B CC B^
V R
V
I^
=^
V^
V^
I^ R
CE^
CC^
C^
C
=^
C BE B CC F CC CE^
R
V R
V
V
V^
⎛^ ⎜⎜⎝
β
Fixed Bias w/ Emitter Feedback npnR^ B^
I^ B^ R^ C
I^ C + VCE- +VCC V^ BE^ +-
= -II EE E
R^ E
E EE BE BB CC^
R
I
V
RI
V^
E F F B F
BE CC
C
R
R
V
V
I
⎛^ ⎜⎜⎝
ELEC 301
BJT Bias Circuits
2
BJT Bias Configuration
Base-Emitter Loop
Collector-Emitter Loop
Fixed Bias w/ Collector Feedback npn
R^ B
R^ C I^ B^
I^ C + VCE- +VCC +V - BE^
I^ CC
I^ CC^
=^ I^ C
+^ I
B
(^
BE B F C F F C
BE BB C B C
BE BB C CC CC
V
R
R
I
V
RI
R
I
I
V
RI
R
I
V
⎤^ +⎥ ⎦
⎡^ ⎢ ⎣
⎛^ ⎜⎜⎝
β^ β
B F C
BE CC F F C^
R
R
V
V
I
β
β β
(^
CE CC
F F
CE C B C
CE C CC CC
V
VR RI
I
I
V
R
I
V
⎛^ ⎜⎜⎝
β^ β
C F F C CC CE^
R
I
V
V^
⎛^ ⎜⎜⎝
β^ β
Fixed Bias wi/ Emitter and Collector Feedback npn
R^ B
R^ C I^ B^
I^ C + VCE- +VCC +V - BE^
I^ CC R^ E
I^ EE
I^ CC^
= I^ C
+ I
B
(^
(^
)^
⎡^ ⎢ ⎣
⎛^ ⎜⎜⎝
B F E C F F C BE
E EE BE BB C B C
E EE BE BB C CC CC
R
R
R
I
V
R
I
V
RI
R
I
I
R
I
V
RI
R
I
V
β^ β
B F E C
BE CC
F F C^
R
R
R
V
V
I
(^
)^
CE E C F F C
E EE CE C CC CC
V
R
R
I
R I V R I V
⎛^ ⎜⎜⎝
β^ β
(^
) E
C F F C CC CE^
R
R
I
V
V^
⎛^ ⎜⎜⎝
β^ β
ELEC 301
BJT Bias Circuits
4
BJT Bias Configuration
Base-Emitter Loop
Collector-Emitter Loop
Self Bias Equivalent Circuit
R R B C I^ B
I^ C + VCE- V^ BE^ +- R^ E^
I^ EE +VCC
= -I^ E
R^ B V^ BBR B V^ = _____________ CC
R^ B R^ B^
R^ =B +VCC
V^ BBR^ B V^ = _____________ CCR
B R^ +B^
R^ B R^ B R^ =B
R^ B1 R^ B Circuit at base of the BJT.
Thevenin equivalent of the circuit at the base of the BJ
⎛^ ⎜⎜⎝
E F F B F C BE
E EE BE BB BB
R
R
I
V
R
I
V
RI
V
β^ β
⎛^ ⎜⎜⎝
E F F C C CC
E EE CC CC CE
R
R
I
V
R
I
RI
V
V
β^ β
(^
)(^
) 1
1
1
1
1
2 1
2 2 1 1
2
⎞⎟⎟⎠
⎛^ ⎜⎜⎝
−
⎞⎟⎟⎠ ⎛⎜⎜⎝
=
− + ⎞⎟⎟⎠ ⎛⎜⎜⎝
=
⎞⎟⎟⎠ ⎛^ ⎜⎜⎝
−
=
F B B
B B
E
BE B B
B CC
F F
B F E
BE BB F F
E F F B F
BE BB C
R R
R R
R
V R R
R R V R
V V
R
R
V V I
β
β β
β
β β
β β β
Fixed Bias w/ Collector Feedback pnp
R^ B
R^ C I B
I^ C - VEC+ -V^ CC
I^ CC
C CC BB EB CC^
R
I
RI
V
V^
where
C B CC^
I
I
I^
and
BF C^
I
I
C CC CC EC^
R
I
V
V^
C F F C CC EC^
R
I
V
V^
⎛^ ⎜⎜⎝
β^ β
ELEC 301
BJT Bias Circuits
5
BJT Bias Configuration
Base-Emitter Loop
Collector-Emitter Loop
Self Bias Circuit pnp
Thevenin Equivalent Voltage:
2 1
(^2) B B
B CC BB^
R
R
R
V
V^
B F E
BB EB F F C^
R
R
V
V
I
β
β β
2 1
2 1
B B
B B B^
R
R
R
R
R^
CC EC EC
F F
CC EC EE
CC
RI
V
RI
RI
V
RI
V
⎛^ ⎜⎜⎝
β^ β
C
E C F F CC EC C^
I
RI
V V R
⎞⎟ ⎟⎠ ⎛^ ⎜⎜⎝
− − =
β^ β
1
Bias StabilityA set of three stability factors is used to quantify the variation in the collector current with respect to the reverse saturation current,base-emitter voltage, and
βF^
.^ The stability factors are:
C^ F C F
C BE C BE V C CO C CO I
I
I
S
I V
I V
S
I I
I I
S
β^
=^
,^
.^
The stability of the bias configuration is quantified with respect to the collector current for the following reasons:
-^
the collector current is dependent on the base current and the collector-emitter voltage, and determines the output signal ofa BJT amplifier,
-^
small variations in
β, F^
I^ CO
, and
Δ V
can result in a large change in BE
I^ C^
The total incremental change in I
for small changes inC
I^ CO
,^ V
, and BE
β F^
is,
Δ I^ CT
=^ S
Δ II
+ CO^
SV^
Δ VBE
+^ S
F^
.^
Equation (3.7-7) clearly shows that in order to keep the change in I
small, the magnitude of the stability factors must also be keptC^
R small. B
R C I B
I C -V - + R E CC I
E
R B
