2. Outlines
DC load line
Operating region
Selection of operating point
Operating point
Transistor biasing
Stabilization
Stability factor
Types of biasing ckt
1. Fixed Biasing Circuits (Base Resistor Method)
2. Emitter Bias
3. Biasing with collector-feedback resistor
4. Voltage Divider Biasing
Si Vs. Ge
2
3. DC load line
Load line: locus of operating point on o/p
characteristics of transistor
A line on which operating point moves according to
input ac signal
3
4. Q-point: It is point on load line which represents DC value of VCE and IC in
absence of signal. Best position is mid-point on load line, VCE= ½ VCC
4
7. Operating Point or Quiescent Point
For transistor amplifiers the resulting dc current and voltage
establish an operating point on the characteristics that define
the region that will be employed for amplification of the
applied signal.
Since the operating point is a fixed point on the characteristics,
it is also called the quiescent point (abbreviated Q-point).
By definition, quiescent means quiet, still, inactive. Figure
shows a general output device characteristic with four
operating points indicated.
7
9. Contd...
The biasing circuit can be designed to set the device operation
at any of these points or others within the active region.
The maximum ratings are indicated on the characteristics of
Fig. by a horizontal line for the maximum collector current
ICmax and a vertical line at the maximum collector-to-emitter
voltage VCEmax. The maximum power constraint is defined
by the curve PCmax in the same figure.
At the lower end of the scales are the cutoff region, defined by
IB = 0 , and the saturation region, defined by VCE=VCEsat.
9
10. Need Of Operating Point
Temperature causes the device parameters such as the transistor
current gain (ac) and the transistor leakage current (ICEO) to
change.
Higher temperatures result in increased leakage currents in the
device, thereby changing the operating condition set by the
biasing network. The result is that the network design must also
provide a degree of temperature stability so that temperature
changes result in minimum changes in the operating point.
This maintenance of the operating point can be specified by a
stability factor, S.
10
11. Example
1. For CE configuration Vcc = 10 v, Rc = 8k. Draw DC
load line. Find Q-point for zero signal if base
current is 15μA, β=40
Ic = 1.25mA,
Zero signal Ic=β IB = 0.6mA, VCE = 5.2v
2. In transistor ckt Rc= 5k, quiescent current is
1.2mA. Determine Q-point when Vcc=12 v. how will
point change when Rc is changed from 5k to 7.5k
Operating pt 1: (6 v, 1.2mA)
Operating pt 2: (3 v, 1.2mA)
11
12. Transistor biasing
Faithful amplification: “The process of raising
strength of weak signal without change in its general
shape is known as faithful amplification”
Proper zero signal collector current
Min proper base-emitter voltage at
any instant
Min proper collector-emitter voltage
at any instant
Key
point
for
Faithful
Amplification
1
2
3
12
13. BJT to be biased in its linear or active operating
region the following must be true:
1. Proper zero signal collector current:
Zero sig Ic ≥ max Ic due to signal alone
13
15. 2. Proper min VBE:
The base–emitter junction must be forward-biased (p-region voltage
more positive), with a resulting forward-bias voltage of about 0.3v
(Ge) and 0.7 V (Si).
15
16. 3. Proper min VCE:
The base–collector junction must be reverse-biased (n-region more
positive), with the reverse-bias voltage being any value within the
maximum limits of the device.
β falls if VCE is not proper unfaithful amplification
VCE = 0.5 v(Ge) and 1v (Si)
16
17. Example
An NPN – Si transistor has Vcc = 6v, Rc=2.5k
Find (1) max Ic that can be allowed during
application of signal for faithful amplification (2) min
zero sig collector current required
Ic max = 2mA
Ic min 0 signal = 1mA
17
18. Biasing
The proper flow of zero signal collector current and the
maintenance of proper collector-emitter voltage during
the passage of signal is known as Transistor Biasing
The basic purpose of transistor biasing is to keep the
base-emitter junction properly forward biased and
collector-base junction properly reverse biased during the
application of signal
This can be achieved with a bias battery or associating a
circuit with a transistor
The circuit which provides transistor biasing is known as
biasing circuit
Biasing is very essential for the proper operation of
transistor in any circuit.
18
19. Inherent Variations of Transistor Parameters
Parameters such as β, VBE are not the same for
every transistor even of the same type
The major reason for these variations is
manufacturing techniques have not too much
advanced. For instance, it has not been possible to
control the base width and it may vary, although
slightly, from one transistor to the other even of the
same type.
Such small variations result in large change in
transistor parameters such as β, VBE
The inherent variations of transistor parameters may
change the operating point, resulting in unfaithful
amplification
19
20. Stabilization
The collector current in a transistor changes rapidly when,
1. The temperature changes,
2. The transistor is replaced by another of the same type.
When the temperature changes or the transistor is replaced, the
operating point (i.e. zero signal IC and VCE) also changes
The process of making operating point independent of
temperature changes or variations in transistor parameters
is known as stabilisation
Need for stabilisation:
1. Temperature dependence of IC
2. Individual variations
3. Thermal runaway
20
21. 1. Temperature dependence of IC
IC = β IB + ICEO = β IB + (β + 1) ICBO
Temp ↑
•ICBO
Influenced
by Temp
ICBO ↑
•Rise in 10˚
will double
ICBO
IC ↑
•Apply
proper Bias
to set zero
sig Ic
Q – point
change
•Make Ic
constant
inspite of
changes in
Temp
21
22. 2. Individual variations
The value of β and VBE are not exactly the same
for any two transistors even of the same type
VBE itself decreases when temperature increases
When a transistor is replaced by another of the
same type, these variations change the
operating point
22
23. 3. Thermal runaway
Temp
↑
ICBO
↑
IC↑
β ICBO
IC = β IB + ICEO = β IB + (β + 1) ICBO
If no
stabilization
applied
• The self-destruction of an
unstabilised transistor is
known as Thermal
runaway
• To avoid thermal runaway -
IC is kept constant
• This is done by causing IB
to decrease automatically
with temperature increase
by circuit modification
• Then decrease in β, IB will
compensate for the increase in
(β + 1) ICBO, keeping IC nearly
constant.
cumulative
23
24. Essentials of a Transistor Biasing Circuit
Transistor biasing is required for faithful
amplification.
The biasing network associated with the transistor
should meet the following requirements :
1. It should ensure proper zero signal collector current.
2. It should ensure that VCE does not fall below 0.5 V for
Ge transistors and 1 V for silicon transistors at any
instant
3. It should ensure the stabilisation of operating point
24
25. Stability Factor
The rate of change of collector current IC w.r.t.
the collector leakage current ICO at constant β
and IB is called stability factor i.e.
To achieve greater thermal stability, it is
desirable to have as low stability factor as
possible. The ideal value of S is 1
Experience shows that values of S exceeding 25
result in unsatisfactory performance
25
27. Types of Biasing Circuits
It is desirable that transistor ckt should have a single
source of supply—the one in the output circuit (i.e.
VCC). The following are commonly used methods of
obtaining transistor biasing from one source of supply
(i.e. VCC ) :
1. Fixed Biasing Circuits (Base Resistor Method)
2. Emitter Bias
3. Biasing with collector-feedback resistor
4. Voltage Divider Biasing
In all the methods, the same basic principle is
employed i.e. required value of IB(and hence IC) is
obtained from VCC in the zero signal conditions. The
value of RC is selected so that proper min VCE
maintained
27
28. 1. Base resistor method
The required value of zero
signal IB (and hence IC = βIB)
can be made to flow by
selecting the proper value of
base resistor RB
VCC is a fixed known quantity and IB is
chosen at some suitable value. Hence, RB
can always be found directly, and for this
reason, this method is called fixed-bias
method
28
29. Stability factor
In fixed-bias method of biasing, IB is independent of IC
so that dIB/dIC = 0. Putting the value of dIB / dIC = 0
in the above expression, we have,
Stability factor, S = β + 1
IC changes (β + 1) times as much as any change in
ICO. For instance, if β = 100, then S = 101 which
means that IC increases 101 times faster than ICO
Due to the large value of S in a fixed bias, it has poor
thermal stability
29
30. Advantages :
1. Circuit is very simple as only one resistance RB is required
2. Biasing conditions can easily be set and the calculations are
simple
3. There is no loading of the source by the biasing circuit since
no resistor is employed across base-emitter junction
Disadvantages :
1. This method provides poor stabilisation. It is because there
is no means to stop a self increase in collector current due to
temperature rise and individual variations. For example, if β
increases due to transistor replacement, then IC also
increases by the same factor as IB is constant.
2. The stability factor is very high. Therefore, there are strong
chances of thermal runaway
Due to these disadvantages, this method of biasing is
rarely employed
30
31. Example
Fig shows biasing with base resistor method.
(i) Determine IC and VCE. Neglect small base-
emitter voltage. Given that β = 50
(ii) If RB in this circuit is changed to 50 kΩ, find the
new operating point
1. IB = 20 μA
IC = 1mA
VCE = 7 v
2. IB = 40 μA
IC = 2mA
VCE = 5 v
31
32. Example
Design base resistor bias
circuit for a CE amplifier
such that operating point
is VCE = 8V and IC = 2 mA.
You are supplied with a
fixed 15V d.c. supply and a
silicon transistor with β =
100. Take base-emitter
voltage VBE = 0.6V.
Calculate also the value of
load resistance that would
be employed. Rc = 3.5 k
RB = 720k
32
33. 2. Emitter Bias circuit
This ckt uses two separate d.c. voltage sources ; one
positive (+ VCC) and the other negative (– VEE).
Normally, the two supply voltages will be equal. For
example, if VCC = + 20V (d.c.), then VEE = – 20V(d.c.)
There is a resistor RE in the emitter circuit
33
34. Circuit Analysis of Emitter Bias
IC:
– IB RB – VBE – IE RE + VEE = 0
∴ VEE = IB RB + VBE + IERE
34
35. Circuit Analysis of Emitter Bias
VCE:
Applying KVL to the collector side:
VCC – IC RC – VCE – IC RE + VEE = 0
or VCE = VCC + VEE – IC (RC + RE)
35
37. Example
For the emitter bias circuit shown in Fig. find IE,
IC,VC and VCE for β = 85 and VBE = 0.7V.
IE = IC = 1.73mA
VC = 11.9v
VCE = 14.6v
37
38. 3. Biasing with collector-feedback resistor
Stability factor, S < (β + 1)
38
39. Advantages
1. It is a simple method as it requires only RB
2. This circuit provides some stabilisation of the
operating point as discussed below :
VCE = VBE + VCB
Temp
↑
ICO
↑
IC↑
VCE
↓
VCB
↓
IB↓
IC↓
VCC = IC RC + VCE
39
VCB = IB RB
40. Disadvantages
1. The circuit does not provide good stabilisation
because stability factor is fairly high, though it is
lesser than that of fixed bias. Therefore, the
operating point does change, although to lesser
extent, due to temperature variations and other
effects.
2. This circuit provides a negative feedback which
reduces the gain of the amplifier as explained
hereafter. During the positive half-cycle of the
signal, the collector current increases. The
increased collector current would result in greater
voltage drop across RC. This will reduce the base
current and hence collector current
40
41. Example
Fig. shows a silicon transistor biased by collector
feedback resistor method. Determine the operating
point. Given that β = 100.
IB = 0.096 mA
IC = 9.6 mA
VCE = VCC − IC RC = 10.4 V
∴ Operating point is 10.4 V, 9.6
mA.
41
42. Example
It is desired to set the operating point at 2V, 1mA by
biasing a silicon transistor with collector feedback
resistor RB. If β = 100, find the value of RB..
42
43. Example
Find the Q-point values (IC and VCE) for the
collector feedback bias circuit shown in Fig
43
45. Voltage Divider Bias Method
Most widely used method
Two resistances R1 and
R2 are connected across
the supply voltage VCC
The emitter resistance
RE provides stabilisation
The voltage drop across
R2 forward biases the EB
junction. This causes the
base current and hence
collector current flow in
zero signal conditions
45
49. Stability factor S
S factor is given by,
Proof
If the ratio R0/RE is very small, then R0/RE can be
neglected as compared to 1 and the stability factor
becomes :
49
50. Example
A transistor uses potential divider method of biasing.R1=50kΩ, R2=10 kΩ
and RE = 1kΩ. If VCC = 12 V, find :
(i) the value of IC ; given VBE = 0.1V
(ii) the value of IC ; given VBE = 0.3V. Comment on the result.
50
Comment: From the above example, it is clear that although VBE varies by
300%, the value of IC changes only by nearly 10%. This explains that in this
method, IC is almost independent of transistor parameter variations
51. Example
The circuit shown in Fig. uses silicon transistor having β = 100.
Find the operating point and stability factor.
51
53. Si Vs. Ge
Parameter Si Ge
ICBO 0.01 μA to 1μA 2 to 15 μA
variation of ICBO
with temperature
ICBO doubles with
each 12°C rise
ICBO doubles with
each 8 to 10°C rise
working
temperature
150°C 70°C
PIV rating of diode 1000V 400V
53
