Short Notes: BJT - Biasing & Stabilization | Analog Circuits - Electronics and Communication Engineering (ECE) PDF Download

 Page 1


BJT - Biasing & Stabiliz ation
Biasing in Bip olar Junction T ransistor (BJT) circuits establishes a stable op erating con-
dition (Q-p oin t) for amplification. Stabilization ensures the Q-p oin t remains consisten t
despite v ariations in temp erature, transistor parameters (ß ), or supply v oltage.
1. BJT Op erating Regions
A BJT op erates in three regions:
• A ctiv e: Used for amplification ( I
C
= ßI
B
, V
CE
>V
CE, sat
).
• Saturation: BJT is fully on (V
CE
˜ 0.2 V).
• Cutoff : BJT is off ( I
C
˜ 0 , I
B
˜ 0 ).
2. Biasing T ec hniques
Biasing sets the DC op erating p oin t in the activ e region. Common metho ds include:
1. Fixed Bias:
• A resistor R
B
connects the base to V
CC
, and R
C
connects the collector.
• Base curren t: I
B
=
V
CC
-V
BE
R
B
.
• Collector curren t : I
C
= ßI
B
.
• Collector-emi tter v oltage: V
CE
= V
CC
-I
C
R
C
.
• Dra wbac k: Sensitiv e to ß v ariations.
2. Collector-to-Base Bias:
• A feedbac k resi stor connects the collector to the base.
• Base v oltage: V
B
= V
CC
-I
C
R
C
.
• Impro v es stabi lit y o v er fixed bias but still dep ends on ß .
3. V oltage Divider Bias (Self-Bias):
• Uses resi stors R
1
and R
2
to form a v oltage divider for the base.
• Base v oltage: V
B
= V
CC
R
2
R
1
+R
2
.
• Emitter v oltage: V
E
= V
B
-V
BE
.
• Emitter curren t: I
E
˜ I
C
=
V
E
R
E
.
• Collector-emitte r v oltage: V
CE
= V
CC
-I
C
(R
C
+R
E
) .
• A dv an tage: High stabilit y against ß and temp erature v ariations.
3. Stabilization
Stabilization minimizes Q-p oin t shifts due to temp erature (I
CBO
, V
BE
), ß v ariations, or
supply c hanges. Key tec hniques include:
1
Page 2


BJT - Biasing & Stabiliz ation
Biasing in Bip olar Junction T ransistor (BJT) circuits establishes a stable op erating con-
dition (Q-p oin t) for amplification. Stabilization ensures the Q-p oin t remains consisten t
despite v ariations in temp erature, transistor parameters (ß ), or supply v oltage.
1. BJT Op erating Regions
A BJT op erates in three regions:
• A ctiv e: Used for amplification ( I
C
= ßI
B
, V
CE
>V
CE, sat
).
• Saturation: BJT is fully on (V
CE
˜ 0.2 V).
• Cutoff : BJT is off ( I
C
˜ 0 , I
B
˜ 0 ).
2. Biasing T ec hniques
Biasing sets the DC op erating p oin t in the activ e region. Common metho ds include:
1. Fixed Bias:
• A resistor R
B
connects the base to V
CC
, and R
C
connects the collector.
• Base curren t: I
B
=
V
CC
-V
BE
R
B
.
• Collector curren t : I
C
= ßI
B
.
• Collector-emi tter v oltage: V
CE
= V
CC
-I
C
R
C
.
• Dra wbac k: Sensitiv e to ß v ariations.
2. Collector-to-Base Bias:
• A feedbac k resi stor connects the collector to the base.
• Base v oltage: V
B
= V
CC
-I
C
R
C
.
• Impro v es stabi lit y o v er fixed bias but still dep ends on ß .
3. V oltage Divider Bias (Self-Bias):
• Uses resi stors R
1
and R
2
to form a v oltage divider for the base.
• Base v oltage: V
B
= V
CC
R
2
R
1
+R
2
.
• Emitter v oltage: V
E
= V
B
-V
BE
.
• Emitter curren t: I
E
˜ I
C
=
V
E
R
E
.
• Collector-emitte r v oltage: V
CE
= V
CC
-I
C
(R
C
+R
E
) .
• A dv an tage: High stabilit y against ß and temp erature v ariations.
3. Stabilization
Stabilization minimizes Q-p oin t shifts due to temp erature (I
CBO
, V
BE
), ß v ariations, or
supply c hanges. Key tec hniques include:
1
• Emitter Resistor (R
E
): Negativ e feedbac k stabilizesI
C
. IfI
C
increases,V
E
= I
E
R
E
rises, reducing V
BE
, th us stabilizing I
C
.
• Thermistor Comp ensation: A temp erature-sensitiv e resistor adjusts bias to coun ter
temp era ture effects.
• Dio de Comp ensation: A dio de in the bias circuit comp ensates for V
BE
v ariations
with te mp erature.
4. Stabilit y F actors
Stabilit y is quan tified b y stabilit y factors:
• Collector Curren t Stabilit y F actor (S
I
CO
):
S
I
CO
=
?I
C
?I
CBO
˜
ß +1
1+ß
R
E
R
B
+R
E
Lo w er S
I
CO
indicates b etter stabilit y .
• Base-Emitter V oltage Stabilit y F actor (S
V
BE
):
S
V
BE
=
?I
C
?V
BE
• Curren t Gain Stabilit y F actor (S
ß
):
S
ß
=
?I
C
?ß
V o ltage divider bias with R
E
pro vides the b est stabilit y (S
I
CO
˜ 1 ).
5. Key Considerations
• Cho ose R
E
to balance stabilit y and A C gain.
• Ensure V
CE
˜ 0.5V
CC
for maxim um signal swi ng.
• Use b ypass capacitors to a v oid A C signal loss across R
E
.
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