Transistor Biasing and Stability Factor | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

 Page 1


 
Biasing 
We known that transistor can operate in any of three regions of operation namely cutoff, active region 
and saturation. To operate  the transistor in these regions the two junction of a transistor should be 
forward or reversed biased as shown in table 
Region of operation Base Emitter Junction Collector base junction Application 
Cut off Reversed bias Reversed bias As a switch 
Active Forward bias Reversed bias Amplifier 
Saturation Forward bias Forward bias As a switch 
In order to do so, we need to connect external DC power supplies with correct polarities & magnitude. 
This process is called as biasing of transistor. 
Voltage divider bias (VDB) 
The most famous circuit based on the emitter-bias prototype is called voltage divider bias. You can 
recognize it by the voltage divider in the base circuit. 
Accurate VDB Analysis 
The Key idea is for the base current to be much smaller than the current through the voltage divider. 
When the condition is satisfied, the voltage divider holds the base voltage almost constant and equal to 
the unloaded voltage out of the voltage divider. This Produces a solid Q point under all operating 
conditions 
VDB load line & Q point 
The load line is drawn through saturation and cut off. The Q point lies on the load line with the exact 
location determined by the biasing. Large variations in current gain have almost no effect on the Q point 
because this type of bias sets up a constant value of emitter current. 
Two –Supply emitter bias 
This design uses two power supplies: one positive and the other negative . The idea to set up a constant 
value of  „emitter current?.  
 Other types of bias 
This section introduced negative feedback, a phenomenon that exits when an increase in an output 
quantity , produces decreases in an input quantity. It is brillent idea that led to voltage-divider bias. The 
other type of bias cannot use enough –ve feedback, so they fail to attain the performance level to voltage-
divider bias. 
PNP Transistors 
These pnp devices have all current & voltages reversed from their npn counterparts. They may be used 
with negative power supplies; more commonly, they are used with +ve power supplies in an upside-down 
configuration. 
Reverse Feedback ratio 
If some percentage of an amplifier?s output signal is connected to the input, so that the amplifier amplifies 
part of its own output signal, we have what is known as feedback. Feedback comes in two 
varieties: positive (also called regenerative), and negative (also called degenerative). Positive feedback 
reinforces the direction of an amplifier?s output voltage change, while negative feedback does just the 
opposite. 
 
 
Page 2


 
Biasing 
We known that transistor can operate in any of three regions of operation namely cutoff, active region 
and saturation. To operate  the transistor in these regions the two junction of a transistor should be 
forward or reversed biased as shown in table 
Region of operation Base Emitter Junction Collector base junction Application 
Cut off Reversed bias Reversed bias As a switch 
Active Forward bias Reversed bias Amplifier 
Saturation Forward bias Forward bias As a switch 
In order to do so, we need to connect external DC power supplies with correct polarities & magnitude. 
This process is called as biasing of transistor. 
Voltage divider bias (VDB) 
The most famous circuit based on the emitter-bias prototype is called voltage divider bias. You can 
recognize it by the voltage divider in the base circuit. 
Accurate VDB Analysis 
The Key idea is for the base current to be much smaller than the current through the voltage divider. 
When the condition is satisfied, the voltage divider holds the base voltage almost constant and equal to 
the unloaded voltage out of the voltage divider. This Produces a solid Q point under all operating 
conditions 
VDB load line & Q point 
The load line is drawn through saturation and cut off. The Q point lies on the load line with the exact 
location determined by the biasing. Large variations in current gain have almost no effect on the Q point 
because this type of bias sets up a constant value of emitter current. 
Two –Supply emitter bias 
This design uses two power supplies: one positive and the other negative . The idea to set up a constant 
value of  „emitter current?.  
 Other types of bias 
This section introduced negative feedback, a phenomenon that exits when an increase in an output 
quantity , produces decreases in an input quantity. It is brillent idea that led to voltage-divider bias. The 
other type of bias cannot use enough –ve feedback, so they fail to attain the performance level to voltage-
divider bias. 
PNP Transistors 
These pnp devices have all current & voltages reversed from their npn counterparts. They may be used 
with negative power supplies; more commonly, they are used with +ve power supplies in an upside-down 
configuration. 
Reverse Feedback ratio 
If some percentage of an amplifier?s output signal is connected to the input, so that the amplifier amplifies 
part of its own output signal, we have what is known as feedback. Feedback comes in two 
varieties: positive (also called regenerative), and negative (also called degenerative). Positive feedback 
reinforces the direction of an amplifier?s output voltage change, while negative feedback does just the 
opposite. 
 
 
Input & Output impedances 
It  is the input impedance “seen” by the source driving the input of the 
amplifier.  Z
in
 or Input Resistance is an important parameter in the design 
of a transistor amplifier and as such allows amplifiers to be characterized 
according to their effective input and output impedances as well as their 
power and current ratings. 
For details refer to chapter 3 
Bias Stabilization 
The stability of a system is a measure of the sensitivity of a network to variations in its parameter.  ? 
increases with increase in temperature .  Magnitude of V
BE
 decreases about 7.5 mV per degree Celsius 
(°C) increase in temperature. I
CO
(reverse saturation current): doubles in value for every 10°C increase in 
Temperature 
Stability Factors, S(I
CO
), S(V
BE
), and S(ß) 
A stability factor,S, is defined for each of the parameters affecting bias stability as listed below: 
S(I
CO
)   =   ?I
C
 / ?I
CO
 
S(V
BE
) =  ?I
C
 / ?V
BE
 
S(ß) = ?I
C
 / ? ß  
In each case, the delta symbol signifies change in that quantity. 
BJT Transistor modeling 
A model is the combination of circuit elements , properly chosen, the best approximates the actual 
behavior of a semiconductor device under specific operating conditions. 
The ac equivalent of a network is 
1. Setting all dc sources to zero and replacing them by a short- circuit equivalent 
2. Replacing all capacitors by a short-circuit equivalent. 
3. Removing all elements bypassed by the short-circuit equivalents introduced by steps 1 & 2. 
4. Redrawing the network in a more convenient and logical form. 
 
 
 
Transistor Model  
 
The T- Model (Ebers- Moll model)     The  ? Model 
 
 
 
 
 
 
 
 
 
 
 
 
Page 3


 
Biasing 
We known that transistor can operate in any of three regions of operation namely cutoff, active region 
and saturation. To operate  the transistor in these regions the two junction of a transistor should be 
forward or reversed biased as shown in table 
Region of operation Base Emitter Junction Collector base junction Application 
Cut off Reversed bias Reversed bias As a switch 
Active Forward bias Reversed bias Amplifier 
Saturation Forward bias Forward bias As a switch 
In order to do so, we need to connect external DC power supplies with correct polarities & magnitude. 
This process is called as biasing of transistor. 
Voltage divider bias (VDB) 
The most famous circuit based on the emitter-bias prototype is called voltage divider bias. You can 
recognize it by the voltage divider in the base circuit. 
Accurate VDB Analysis 
The Key idea is for the base current to be much smaller than the current through the voltage divider. 
When the condition is satisfied, the voltage divider holds the base voltage almost constant and equal to 
the unloaded voltage out of the voltage divider. This Produces a solid Q point under all operating 
conditions 
VDB load line & Q point 
The load line is drawn through saturation and cut off. The Q point lies on the load line with the exact 
location determined by the biasing. Large variations in current gain have almost no effect on the Q point 
because this type of bias sets up a constant value of emitter current. 
Two –Supply emitter bias 
This design uses two power supplies: one positive and the other negative . The idea to set up a constant 
value of  „emitter current?.  
 Other types of bias 
This section introduced negative feedback, a phenomenon that exits when an increase in an output 
quantity , produces decreases in an input quantity. It is brillent idea that led to voltage-divider bias. The 
other type of bias cannot use enough –ve feedback, so they fail to attain the performance level to voltage-
divider bias. 
PNP Transistors 
These pnp devices have all current & voltages reversed from their npn counterparts. They may be used 
with negative power supplies; more commonly, they are used with +ve power supplies in an upside-down 
configuration. 
Reverse Feedback ratio 
If some percentage of an amplifier?s output signal is connected to the input, so that the amplifier amplifies 
part of its own output signal, we have what is known as feedback. Feedback comes in two 
varieties: positive (also called regenerative), and negative (also called degenerative). Positive feedback 
reinforces the direction of an amplifier?s output voltage change, while negative feedback does just the 
opposite. 
 
 
Input & Output impedances 
It  is the input impedance “seen” by the source driving the input of the 
amplifier.  Z
in
 or Input Resistance is an important parameter in the design 
of a transistor amplifier and as such allows amplifiers to be characterized 
according to their effective input and output impedances as well as their 
power and current ratings. 
For details refer to chapter 3 
Bias Stabilization 
The stability of a system is a measure of the sensitivity of a network to variations in its parameter.  ? 
increases with increase in temperature .  Magnitude of V
BE
 decreases about 7.5 mV per degree Celsius 
(°C) increase in temperature. I
CO
(reverse saturation current): doubles in value for every 10°C increase in 
Temperature 
Stability Factors, S(I
CO
), S(V
BE
), and S(ß) 
A stability factor,S, is defined for each of the parameters affecting bias stability as listed below: 
S(I
CO
)   =   ?I
C
 / ?I
CO
 
S(V
BE
) =  ?I
C
 / ?V
BE
 
S(ß) = ?I
C
 / ? ß  
In each case, the delta symbol signifies change in that quantity. 
BJT Transistor modeling 
A model is the combination of circuit elements , properly chosen, the best approximates the actual 
behavior of a semiconductor device under specific operating conditions. 
The ac equivalent of a network is 
1. Setting all dc sources to zero and replacing them by a short- circuit equivalent 
2. Replacing all capacitors by a short-circuit equivalent. 
3. Removing all elements bypassed by the short-circuit equivalents introduced by steps 1 & 2. 
4. Redrawing the network in a more convenient and logical form. 
 
 
 
Transistor Model  
 
The T- Model (Ebers- Moll model)     The  ? Model 
 
 
 
 
 
 
 
 
 
 
 
 
 
Type Circuit Calculations Characteristics Where used 
Base bias 
 
I
B
= (V
BB
-0.7V)/R
B
 
 
I
C
=ßI
B
 
 
V
CE
= V
CC
 - I
C
R
C 
Few parts; ß 
dependent; 
fixed base 
current 
Switch; digital 
Emitter bias 
 
V
E
=V
BB
 -0.7V 
I
E
= V
E
/ R
E
 
V
C
= V
C
-I
C
R
C
 
V
CE
=V
C
-V
E 
Fixed emitter 
current; ß 
independent 
I
C 
driver ; 
amplifier 
Voltage divider 
bias 
 
V
B
= 
R
2
V
CC
/(R
1
+ R
2
) 
 
V
E
= V
B
-0.7V 
 
I
E
= V
E
/ R
E
 
 
V
C
= V
CC
-I
C
R
C
 
V
CE
= V
C
 - V
E
 
 
Needs more 
resistors; ß 
independent; 
needs only one 
power supply 
Amplifier 
Two – supply 
emitter bias 
 
V
B =  
0V
 
V
E
= V
B
-0.7V 
V
RE
=V
EE
-0.7V 
I
E
=V
RE
/R
E 
 
V
C
= V
CC
- I
C
R
C
 
 
V
CE
= V
C
- V
E 
Needs  positive 
& negative 
power supplies; 
ß independent; 
Amplifier 
 
 
VDB  Derivations  
 
Base voltage       Emitter voltage 
 
   V
BB
= R
2
V
CC
/(R
1
+ R
2
)   V
E
= V
BB
- V
BE 
 
 
Page 4


 
Biasing 
We known that transistor can operate in any of three regions of operation namely cutoff, active region 
and saturation. To operate  the transistor in these regions the two junction of a transistor should be 
forward or reversed biased as shown in table 
Region of operation Base Emitter Junction Collector base junction Application 
Cut off Reversed bias Reversed bias As a switch 
Active Forward bias Reversed bias Amplifier 
Saturation Forward bias Forward bias As a switch 
In order to do so, we need to connect external DC power supplies with correct polarities & magnitude. 
This process is called as biasing of transistor. 
Voltage divider bias (VDB) 
The most famous circuit based on the emitter-bias prototype is called voltage divider bias. You can 
recognize it by the voltage divider in the base circuit. 
Accurate VDB Analysis 
The Key idea is for the base current to be much smaller than the current through the voltage divider. 
When the condition is satisfied, the voltage divider holds the base voltage almost constant and equal to 
the unloaded voltage out of the voltage divider. This Produces a solid Q point under all operating 
conditions 
VDB load line & Q point 
The load line is drawn through saturation and cut off. The Q point lies on the load line with the exact 
location determined by the biasing. Large variations in current gain have almost no effect on the Q point 
because this type of bias sets up a constant value of emitter current. 
Two –Supply emitter bias 
This design uses two power supplies: one positive and the other negative . The idea to set up a constant 
value of  „emitter current?.  
 Other types of bias 
This section introduced negative feedback, a phenomenon that exits when an increase in an output 
quantity , produces decreases in an input quantity. It is brillent idea that led to voltage-divider bias. The 
other type of bias cannot use enough –ve feedback, so they fail to attain the performance level to voltage-
divider bias. 
PNP Transistors 
These pnp devices have all current & voltages reversed from their npn counterparts. They may be used 
with negative power supplies; more commonly, they are used with +ve power supplies in an upside-down 
configuration. 
Reverse Feedback ratio 
If some percentage of an amplifier?s output signal is connected to the input, so that the amplifier amplifies 
part of its own output signal, we have what is known as feedback. Feedback comes in two 
varieties: positive (also called regenerative), and negative (also called degenerative). Positive feedback 
reinforces the direction of an amplifier?s output voltage change, while negative feedback does just the 
opposite. 
 
 
Input & Output impedances 
It  is the input impedance “seen” by the source driving the input of the 
amplifier.  Z
in
 or Input Resistance is an important parameter in the design 
of a transistor amplifier and as such allows amplifiers to be characterized 
according to their effective input and output impedances as well as their 
power and current ratings. 
For details refer to chapter 3 
Bias Stabilization 
The stability of a system is a measure of the sensitivity of a network to variations in its parameter.  ? 
increases with increase in temperature .  Magnitude of V
BE
 decreases about 7.5 mV per degree Celsius 
(°C) increase in temperature. I
CO
(reverse saturation current): doubles in value for every 10°C increase in 
Temperature 
Stability Factors, S(I
CO
), S(V
BE
), and S(ß) 
A stability factor,S, is defined for each of the parameters affecting bias stability as listed below: 
S(I
CO
)   =   ?I
C
 / ?I
CO
 
S(V
BE
) =  ?I
C
 / ?V
BE
 
S(ß) = ?I
C
 / ? ß  
In each case, the delta symbol signifies change in that quantity. 
BJT Transistor modeling 
A model is the combination of circuit elements , properly chosen, the best approximates the actual 
behavior of a semiconductor device under specific operating conditions. 
The ac equivalent of a network is 
1. Setting all dc sources to zero and replacing them by a short- circuit equivalent 
2. Replacing all capacitors by a short-circuit equivalent. 
3. Removing all elements bypassed by the short-circuit equivalents introduced by steps 1 & 2. 
4. Redrawing the network in a more convenient and logical form. 
 
 
 
Transistor Model  
 
The T- Model (Ebers- Moll model)     The  ? Model 
 
 
 
 
 
 
 
 
 
 
 
 
 
Type Circuit Calculations Characteristics Where used 
Base bias 
 
I
B
= (V
BB
-0.7V)/R
B
 
 
I
C
=ßI
B
 
 
V
CE
= V
CC
 - I
C
R
C 
Few parts; ß 
dependent; 
fixed base 
current 
Switch; digital 
Emitter bias 
 
V
E
=V
BB
 -0.7V 
I
E
= V
E
/ R
E
 
V
C
= V
C
-I
C
R
C
 
V
CE
=V
C
-V
E 
Fixed emitter 
current; ß 
independent 
I
C 
driver ; 
amplifier 
Voltage divider 
bias 
 
V
B
= 
R
2
V
CC
/(R
1
+ R
2
) 
 
V
E
= V
B
-0.7V 
 
I
E
= V
E
/ R
E
 
 
V
C
= V
CC
-I
C
R
C
 
V
CE
= V
C
 - V
E
 
 
Needs more 
resistors; ß 
independent; 
needs only one 
power supply 
Amplifier 
Two – supply 
emitter bias 
 
V
B =  
0V
 
V
E
= V
B
-0.7V 
V
RE
=V
EE
-0.7V 
I
E
=V
RE
/R
E 
 
V
C
= V
CC
- I
C
R
C
 
 
V
CE
= V
C
- V
E 
Needs  positive 
& negative 
power supplies; 
ß independent; 
Amplifier 
 
 
VDB  Derivations  
 
Base voltage       Emitter voltage 
 
   V
BB
= R
2
V
CC
/(R
1
+ R
2
)   V
E
= V
BB
- V
BE 
 
 
 
Emitter current             Collector current 
 
 
I
E 
= V
E
/ R
E   
  I
C
 ˜ I
E
 
 
 
 
 
 
Collector  voltage       Collector – emitter voltage  
 
 
 
 
V
C 
=  V
CC
  - I
C
R
C
        V
CE
= V
C  - 
V
E
 
              
 
 
 
 
TSEB (Two supply emitter bias)  Derivations 
 
Base voltage        Emitter current 
 
 
 
     
V
B
˜ 0                                                      V
C
= V
CC
- I
C
R
C
 
                 
 
 
 
 
 
Collector Voltage (TSEB)    Collector-emitter Voltage (TSEB)  
 
 
 
V
C
=V
CC
 -I
C
R
C
                                  
                                                    V
CE
= V
C
+ 0.7V 
 
 
 
Page 5


 
Biasing 
We known that transistor can operate in any of three regions of operation namely cutoff, active region 
and saturation. To operate  the transistor in these regions the two junction of a transistor should be 
forward or reversed biased as shown in table 
Region of operation Base Emitter Junction Collector base junction Application 
Cut off Reversed bias Reversed bias As a switch 
Active Forward bias Reversed bias Amplifier 
Saturation Forward bias Forward bias As a switch 
In order to do so, we need to connect external DC power supplies with correct polarities & magnitude. 
This process is called as biasing of transistor. 
Voltage divider bias (VDB) 
The most famous circuit based on the emitter-bias prototype is called voltage divider bias. You can 
recognize it by the voltage divider in the base circuit. 
Accurate VDB Analysis 
The Key idea is for the base current to be much smaller than the current through the voltage divider. 
When the condition is satisfied, the voltage divider holds the base voltage almost constant and equal to 
the unloaded voltage out of the voltage divider. This Produces a solid Q point under all operating 
conditions 
VDB load line & Q point 
The load line is drawn through saturation and cut off. The Q point lies on the load line with the exact 
location determined by the biasing. Large variations in current gain have almost no effect on the Q point 
because this type of bias sets up a constant value of emitter current. 
Two –Supply emitter bias 
This design uses two power supplies: one positive and the other negative . The idea to set up a constant 
value of  „emitter current?.  
 Other types of bias 
This section introduced negative feedback, a phenomenon that exits when an increase in an output 
quantity , produces decreases in an input quantity. It is brillent idea that led to voltage-divider bias. The 
other type of bias cannot use enough –ve feedback, so they fail to attain the performance level to voltage-
divider bias. 
PNP Transistors 
These pnp devices have all current & voltages reversed from their npn counterparts. They may be used 
with negative power supplies; more commonly, they are used with +ve power supplies in an upside-down 
configuration. 
Reverse Feedback ratio 
If some percentage of an amplifier?s output signal is connected to the input, so that the amplifier amplifies 
part of its own output signal, we have what is known as feedback. Feedback comes in two 
varieties: positive (also called regenerative), and negative (also called degenerative). Positive feedback 
reinforces the direction of an amplifier?s output voltage change, while negative feedback does just the 
opposite. 
 
 
Input & Output impedances 
It  is the input impedance “seen” by the source driving the input of the 
amplifier.  Z
in
 or Input Resistance is an important parameter in the design 
of a transistor amplifier and as such allows amplifiers to be characterized 
according to their effective input and output impedances as well as their 
power and current ratings. 
For details refer to chapter 3 
Bias Stabilization 
The stability of a system is a measure of the sensitivity of a network to variations in its parameter.  ? 
increases with increase in temperature .  Magnitude of V
BE
 decreases about 7.5 mV per degree Celsius 
(°C) increase in temperature. I
CO
(reverse saturation current): doubles in value for every 10°C increase in 
Temperature 
Stability Factors, S(I
CO
), S(V
BE
), and S(ß) 
A stability factor,S, is defined for each of the parameters affecting bias stability as listed below: 
S(I
CO
)   =   ?I
C
 / ?I
CO
 
S(V
BE
) =  ?I
C
 / ?V
BE
 
S(ß) = ?I
C
 / ? ß  
In each case, the delta symbol signifies change in that quantity. 
BJT Transistor modeling 
A model is the combination of circuit elements , properly chosen, the best approximates the actual 
behavior of a semiconductor device under specific operating conditions. 
The ac equivalent of a network is 
1. Setting all dc sources to zero and replacing them by a short- circuit equivalent 
2. Replacing all capacitors by a short-circuit equivalent. 
3. Removing all elements bypassed by the short-circuit equivalents introduced by steps 1 & 2. 
4. Redrawing the network in a more convenient and logical form. 
 
 
 
Transistor Model  
 
The T- Model (Ebers- Moll model)     The  ? Model 
 
 
 
 
 
 
 
 
 
 
 
 
 
Type Circuit Calculations Characteristics Where used 
Base bias 
 
I
B
= (V
BB
-0.7V)/R
B
 
 
I
C
=ßI
B
 
 
V
CE
= V
CC
 - I
C
R
C 
Few parts; ß 
dependent; 
fixed base 
current 
Switch; digital 
Emitter bias 
 
V
E
=V
BB
 -0.7V 
I
E
= V
E
/ R
E
 
V
C
= V
C
-I
C
R
C
 
V
CE
=V
C
-V
E 
Fixed emitter 
current; ß 
independent 
I
C 
driver ; 
amplifier 
Voltage divider 
bias 
 
V
B
= 
R
2
V
CC
/(R
1
+ R
2
) 
 
V
E
= V
B
-0.7V 
 
I
E
= V
E
/ R
E
 
 
V
C
= V
CC
-I
C
R
C
 
V
CE
= V
C
 - V
E
 
 
Needs more 
resistors; ß 
independent; 
needs only one 
power supply 
Amplifier 
Two – supply 
emitter bias 
 
V
B =  
0V
 
V
E
= V
B
-0.7V 
V
RE
=V
EE
-0.7V 
I
E
=V
RE
/R
E 
 
V
C
= V
CC
- I
C
R
C
 
 
V
CE
= V
C
- V
E 
Needs  positive 
& negative 
power supplies; 
ß independent; 
Amplifier 
 
 
VDB  Derivations  
 
Base voltage       Emitter voltage 
 
   V
BB
= R
2
V
CC
/(R
1
+ R
2
)   V
E
= V
BB
- V
BE 
 
 
 
Emitter current             Collector current 
 
 
I
E 
= V
E
/ R
E   
  I
C
 ˜ I
E
 
 
 
 
 
 
Collector  voltage       Collector – emitter voltage  
 
 
 
 
V
C 
=  V
CC
  - I
C
R
C
        V
CE
= V
C  - 
V
E
 
              
 
 
 
 
TSEB (Two supply emitter bias)  Derivations 
 
Base voltage        Emitter current 
 
 
 
     
V
B
˜ 0                                                      V
C
= V
CC
- I
C
R
C
 
                 
 
 
 
 
 
Collector Voltage (TSEB)    Collector-emitter Voltage (TSEB)  
 
 
 
V
C
=V
CC
 -I
C
R
C
                                  
                                                    V
CE
= V
C
+ 0.7V 
 
 
 
 
Long & Short Questions 
 
 
Q.1. What is meant by transistor –biasing ? Define Stability factor.   
Or 
 What do you understand by transistor by transistor biasing ? Why it  is necessary ?  
Transistor Biasing is the process of setting a transistors DC operating voltage or current 
conditions to the correct level so that any AC input signal can be amplified correctly by the 
transistor. 
 Necessary of transistor biasing 
? To active an transistor, biasing is essential. For proper working it is essential to 
apply to apply voltages of correct polarity across its two junctions. 
?  If it is not biased correctly it would work inefficiently and produce distortion in 
the output signal  
? Q-point is not middle Output signal is distorted  & the signal is clipped 
? Further  for various applications , BJT is biased as shown in table 
 
? I
n
 
o
In order to have these applications , we need to connect external DC power 
supplies with correct polarities & magnitude. This process is called as biasing of 
transistor. 
Stability Factor 
The stability of Q point of transistor amplifier depends on the following three parameters : 
1. Leakage current I
CO
 2. ß
dc
 3. Base to emitter voltage 
The effect of these parameters can be expressed mathematically by defining the stability factors 
1. Stability factor  S =
??? ?? ??? ????  
   Constant V
BE
 & ß
dc
 
 
This represents the change in collector current due to change in reverse saturation 
current I
CO
 .
The other two parameters that means V
BE
 & ß
dc
 are assumed to be 
constant. 
2. Stability factor S? = 
??? ?? ??? ????  
      Constant I
CO
 & ß
dc
 
   
Region of operation Base Emitter Junction Collector base junction Application 
Cut off Reversed bias Reversed bias As a switch 
Active Forward bias Reversed bias Amplifier 
Saturation Forward bias Forward bias As a switch