Thermal stability & bias compensation

Thermal Stability and Bias compensation techniques

1. Thermal Stability and Bias
Compensation
Mrs.V.SrirengaNachiyar

Ramco Institute of Technology

2. Thermal Stability
•The maximum average power P
D(max) which a
transistor can dissipate depends upon the
transistor construction and may lie in the range
from a few milli-watts to 200w.

•For silicon transistor the temperature is in the
range 150 to 225
o
c and for germanium it is
between 60 to 100
o
c

3. Contd…
•The collector base junction temperature may rise
because of the two reasons:
 Due to rise in ambient temperature
 Due to self heating
•The increase in the collector current increases the
power dissipated at the collector junction. This in
turn further increases the temperature of the
junction and hence increase in collector current.
The process is cumulative and it is referred to as
self heating

4. Contd…
•The excess heat produced at the collector base
junction may even burn and destroy the
transistor. This situation is called “Thermal
Runaway”.

5. Thermal resistance
•The steady state temperature rise at the collector
junction is proportional to the power dissipated at the
junction. It is given as,
δT =
θ.P
D =T
j- T
A
Where T
j – junction temperature in
o
C
T
A – Ambient temperature in
o
C
P
D – Power in watts dissipated at the collector
junction
θ – constant of proportionality

6. Contd…
•The θ, which is constant of proportionality is
referred to as thermal resistance.
θ=
??????
??????−????????????
????????????

•The unit of θ is
o
C/W
•The typical value of θ for a various transistor
vary from 0.2
o
C/W for a high power transistor
with an efficient heat sink to 1000
o
C/W for a low
power transistor
•The maximum collector power P
C allowed for
safe operation is specified at 25
o
C

7. The condition for thermal stability
•The thermal runaway may even burn and
destroy the transistor, it is necessary to avoid
thermal runaway.
•The required condition to avoid thermal
runaway is that the rate at which heat is
released at the collector junction must not
exceed the rate at which the heat can be
dissipated.
•It is given by,

9. Contd…
•This condition must be satisfied to prevent
thermal runaway.
•By proper design of biasing circuit, it is
possible to ensure that the transistor cannot
runaway below a specific amount of ambient
temperature.

14. Compensation techniques
Temperature sensitive devices such as diodes,
transistors are used which provide compensating
voltages and currents to maintain the operating
point constant.
1. Diode compensation for instability due to V
BE.
2. Diode compensation for instability due to I
CO.
3. Thermistor compensation.
4. Sensistor compensation.

15. Diode compensation for instability due
to V
BE.
•For germanium transistor, changes in I
CO with
temperature contributes more problem than for
silicon transistor.
•On the other hand, in a silicon transistor, the
changes of V
BE with temperature posses
significantly to the changes in I
C.
•Thus a diode may be used as compensation
element for variation in V
BE or I
CO.

17. Cont…
•In this case, the diode is kept forward biased
by the diode source V
DD & R
d.
•Apply KVL to the base circuit.

18. Diode compensation for instability due
to I
CO
•The diode D and the transistor are of same
type and same material.

•So the reverse saturation current I
CO and diode
will increase with temperature at same rate as
the transistor collector saturation current I
CO

19. Cont…
•The diode is reverse biased by V
BE, W.K.T in
case of germanium transistor V
BE is 0.3V. So
the current through Diode is reverse saturation
current:
• I
B = I-I
O
I
C = β I
B + (1+β)I
CO

20. Thermistor compensation
•Consider self-bias circuit with thermistor R
T as
a compensating element.
•The thermistor has a negative temperature
coefficient and its resistance decreases
exponentially with increasing temperature.
•Slope of the curve = ∂R
T/ ∂T. This is the
temperature coefficient for thermistor and the
slope is negative.

23. Cont…
•With increase in temperature, R
T decreases.
Hence voltage drop across it also decreases.
•The voltage drop is nothing but the voltage at
the base with respect to ground. Hence V
BE
decreases which reduces I
B.

24. Sensistor Compensation.
•This method uses temperature sensitive
resistive element rather than diodes or
transistors. It has a positive temperature
coefficient.
•Its resistance increases exponentially with
increasing temperature .
•Slope of this curve= ∂R
T/ ∂T.
•Slope is positive.

25. Slope

26. Cont…
•Resistor R
1 can be replaced by sensistor
element R
T in self- bias circuit.
•As temperature increases, R
T increases which
decrease the current flowing through it. Hence
current through R
2 decreases which reduces
the voltage drop across it.
•The voltage drop R
2 is the voltage at the base
with respect to ground. Hence V
BE decreases
which reduces I
B.

28. References
1. Donald. A. Neamen, Electronic Circuits
Analysis and Design, 3rd Edition, Mc Graw
Hill Education (India) Private Ltd., 2010.
2. Robert L. Boylestad and Louis Nasheresky,
―Electronic Devices and Circuit Theory, 11
th

Edition, Pearson Education, 2013.
3. A.P.Godse & U.A. Bakshi,”Electronic
Circuits-I”
4. S.Salivahanan & N.Sureshkumar,”Electronic
Circuits-I”