Chapter-6 DC biasing-1.ppt

Ppt on dcscsv

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 V
CEand I
Cin
absence of signal. Best position is mid-point on load line, V
CE= ½
VCC
4

5. Operating regions
Cut-off region
Saturation region
5

6. Selection of operating point
6
Near cut-off Near saturation
Active region

7. Operating Point or Quiescent Point
Fortransistoramplifierstheresultingdccurrentandvoltage
establishanoperatingpointonthecharacteristicsthatdefine
theregionthatwillbeemployedforamplificationofthe
appliedsignal.
Sincetheoperatingpointisafixedpointonthecharacteristics,
itisalsocalledthequiescentpoint(abbreviatedQ-point).
Bydefinition,quiescentmeansquiet,still,inactive.Figure
showsageneraloutputdevicecharacteristicwithfour
operatingpointsindicated.
7

8. CE Output Characteristics
8

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=βI
B= 0.6mA, VCE = 5.2v
2.In transistor cktRc= 5k, quiescent current is
1.2mA. Determine Q-point when Vcc=12 v. how will
point change when Rcis changed from 5k to 7.5k
Operating pt1: (6 v, 1.2mA)
Operating pt2: (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

14. Faithful amplification
14

15. 2.Proper min V
BE:
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 V
CE:
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.
βfalls if V
CEis not proper unfaithful amplification
V
CE = 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
amplification19

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 I
Cand VC
E) 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 I
C
2.Individual variations
3.Thermal runaway
20

21. 1. Temperature dependence of IC
I
C= β I
B+ I
CEO= β I
B+ (β + 1) I
CBO
Temp ↑
•I
CBO
Influenced
by Temp
I
CBO ↑
•Rise in 10˚
will double
I
CBO
I
C ↑
•Apply
proper Bias
to set zero
sig Ic
Q –point
change
•Make Ic
constant
inspiteof
changes in
Temp
21

22. 2. Individual variations
The value of β and V
BEare not exactly the same
for any two transistors even of the same type
V
BE 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
↑
I
CBO
↑
I
C↑
β I
CBO
I
C= β I
B+ I
CEO= β I
B+ (β + 1) I
CBO
If no
stabilization
applied
•The self-destruction of an
unstabilisedtransistor 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 I
Cw.r.t.
the collector leakage current I
COat constant β
and I
Bis 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

26. General expression of S-factor
26

27. Types of Biasing Circuits
Itisdesirablethattransistorcktshouldhaveasingle
sourceofsupply—theoneintheoutputcircuit(i.e.
V
CC).Thefollowingarecommonlyusedmethodsof
obtainingtransistorbiasingfromonesourceofsupply
(i.e.V
CC):
1.Fixed Biasing Circuits (Base Resistor Method)
2.Emitter Bias
3.Biasing with collector-feedback resistor
4.Voltage Divider Biasing
Inallthemethods,thesamebasicprincipleis
employedi.e.requiredvalueofI
B(andhenceI
C)is
obtainedfromV
CCinthezerosignalconditions.The
valueofRCisselectedsothatproperminV
CE
maintained
27

28. 1. Base resistor method
The required value of zero
signal I
B(and hence I
C= βI
B)
can be made to flow by
selecting the proper value of
base resistor R
B
V
CCisafixedknownquantityandI
Bis
chosenatsomesuitablevalue.Hence,R
B
canalwaysbefounddirectly,andforthis
reason,thismethodiscalledfixed-bias
method
28

29. Stability factor
In fixed-bias method of biasing, I
Bis independent of I
C
so that dI
B/dI
C= 0. Putting the value of dI
B/ dI
C= 0
in the above expression, we have,
Stability factor, S = β + 1
I
Cchanges (β + 1) times as much as any change in
I
CO. For instance, if β = 100, then S = 101 which
means that I
Cincreases 101 times faster than I
CO
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.I
B= 20 μA
I
C= 1mA
V
CE= 7 v
2. I
B= 40 μA
I
C= 2mA
V
CE= 5 v
31

32. Example
Design base resistor bias
circuit for a CE amplifier
such that operating point
is V
CE= 8V and I
C= 2 mA.
You are supplied with a
fixed 15V d.c.supply and a
silicon transistor with β =
100. Take base-emitter
voltage V
BE= 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 cktuses two separate d.c.voltage sources ; one
positive (+ V
CC) and the other negative (–V
EE).
Normally, the two supply voltages will be equal. For
example, if V
CC= + 20V (d.c.), then V
EE= –20V(d.c.)
There is a resistor RE in the emitter circuit
33

34. Circuit Analysis of Emitter Bias
I
C:
–I
BR
B–V
BE–I
ER
E+ V
EE= 0
∴V
EE= I
BR
B+ V
BE+ I
ER
E
34

35. Circuit Analysis of Emitter Bias
V
CE:
Applying KVL to the collector side:
V
CC–I
CR
C–V
CE–I
CR
E+ V
EE= 0
or V
CE= V
CC+ V
EE–I
C(R
C+ R
E)
35

36. Stability of Emitter bias

36

37. Example
For the emitter bias circuit shown in Fig. find I
E,
I
C,V
Cand V
CEfor β = 85 and V
BE= 0.7V.
I
E =I
C= 1.73mA
V
C= 11.9v
V
CE= 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 R
B
2.This circuit provides some stabilisation of the
operating point as discussed below :
V
CE= V
BE+ V
CB
Temp
↑
I
CO
↑
I
C↑
V
CE
↓
V
CB
↓
I
B↓
I
C↓
V
CC= I
CR
C+ V
CE
39
V
CB = I
BR
B

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.
I
B= 0.096 mA
I
C= 9.6 mA
V
CE= 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

44. Stability of Q-point
44
β↑
V
BE↓
TEMP
Temp ↑ V
BE↓ I
B↑
I
C↑ I
CR
C↑ V
C↓
I
BR
B↓ I
B↓ I
C↓
β↑
I
C=βI
B
↑

45. Voltage Divider Bias Method
Most widely used method
Two resistances R1 and
R2 are connected across
the supply voltage V
CC
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

46. Circuit analysis
I
C:
46

47. Circuit analysis
V
CE:
47

48. Stabilization
Temp
↑
I
C↑
I
CR
E↑
V
BE↓
I
B↓
I
C↓
V
2= V
BE+ I
C*R
E
48

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
Atransistorusespotentialdividermethodofbiasing.R1=50kΩ,R2=10kΩ
andRE=1kΩ.IfVCC=12V,find:
(i)thevalueofIC;givenVBE=0.1V
(ii)thevalueofIC;givenVBE=0.3V.Commentontheresult.
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

52. 52

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

54. Bias Compensation techniques
54

55. 1. Diode compensation for variation in V
BE
55

56. Diode compensation for variation in V
BE
56

57. 2. Diode compensation for variation in I
CO
57

58. 3. Thermistor compensation
58
Negative temperature co-efficient

59. 4. Sensistor compensation
59
Positive temperature co-efficient