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A detailed explanation of bjt bias circuits, covering various configurations and their stability factors. It includes diagrams, equations, and explanations for each configuration, making it a valuable resource for students studying electronics. The document also discusses the importance of bias stability and provides practical insights into circuit design.
Typology: Lecture notes
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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 Current Relationships:
C F F E B F E E F F C BF C E F B C F B^
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^
CE^
CC^
C^
C
C BE B CC F CC CE^
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^
E F F B F
BE CC
C
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
B
BE B F C F F C
BE BB C B C
BE BB C CC CC
B F C
BE CC F F C^
β
β β
CE CC
F F
CE C B C
CE C CC CC
β^ β
C F F C CC CE^
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
B
B F E C F F C BE
E EE BE BB C B C
E EE BE BB C CC CC
B F E C
BE CC
F F C^
CE E C F F C
E EE CE C CC CC
C F F C CC CE^
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
E F F C C CC
E EE CC CC CE
(^
)(^
) 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^
where
C B CC^
and
BF C^
C CC CC EC^
C F F C CC EC^
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^
B F E
BB EB F F C^
β
β β
2 1
2 1
B B
B B B^
CC EC EC
F F
CC EC EE
CC
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
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
, and
can result in a large change in BE
The total incremental change in I
for small changes inC
, and BE
is,
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