This document discusses thermal stability and bias compensation techniques in transistors. It explains that transistors can overheat due to self-heating or increased ambient temperature. This can cause thermal runaway and destroy the transistor. Various compensation techniques are described to maintain the transistor operating point constant despite temperature variations, including diode compensation to stabilize variations in VBE and ICO, thermistor compensation using a temperature-dependent resistor, and sensistor compensation using a resistor with a positive temperature coefficient. These techniques aim to reduce the base current as temperature increases to prevent thermal runaway.

1. Thermal Stability and Bias
Compensation
Mrs.V.SrirengaNachiyar
Ramco Institute of Technology

2. Thermal Stability
• The maximum average power PD(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 225oc and for germanium it is
between 60 to 100oc

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.PD =Tj- TA
Where Tj – junction temperature in oC
TA – Ambient temperature in oC
PD – 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=
𝑇 𝑗−𝑇 𝐴
𝑃 𝐷
• The unit of θ is oC/W
• The typical value of θ for a various transistor
vary from 0.2oC/W for a high power transistor
with an efficient heat sink to 1000oC/W for a low
power transistor
• The maximum collector power PC allowed for
safe operation is specified at 25oC

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 VBE.
2. Diode compensation for instability due to ICO.
3. Thermistor compensation.
4. Sensistor compensation.

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

17. Cont…
• In this case, the diode is kept forward biased
by the diode source VDD & Rd.
• Apply KVL to the base circuit.

18. Diode compensation for instability due
to ICO
• The diode D and the transistor are of same
type and same material.
• So the reverse saturation current ICO and diode
will increase with temperature at same rate as
the transistor collector saturation current ICO

19. Cont…
• The diode is reverse biased by VBE, W.K.T in
case of germanium transistor VBE is 0.3V. So
the current through Diode is reverse saturation
current:
• IB = I-IO
IC = β IB + (1+β)ICO

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

23. Cont…
• With increase in temperature, RT decreases.
Hence voltage drop across it also decreases.
• The voltage drop is nothing but the voltage at
the base with respect to ground. Hence VBE
decreases which reduces IB.

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= ∂RT/ ∂T.
• Slope is positive.

25. Slope

26. Cont…
• Resistor R1 can be replaced by sensistor
element RT in self- bias circuit.
• As temperature increases, RT increases which
decrease the current flowing through it. Hence
current through R2 decreases which reduces
the voltage drop across it.
• The voltage drop R2 is the voltage at the base
with respect to ground. Hence VBE decreases
which reduces IB.

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, 11th
Edition, Pearson Education, 2013.
3. A.P.Godse & U.A. Bakshi,”Electronic
Circuits-I”
4. S.Salivahanan & N.Sureshkumar,”Electronic
Circuits-I”