This document discusses different compensation techniques used to stabilize the operating point of transistors against temperature variations. It describes diode compensation, thermistor compensation, and sensistor compensation. Diode compensation can compensate for changes in base-emitter voltage or collector current with temperature. It works by using a diode that produces an opposing temperature-dependent voltage to the transistor's base-emitter junction. Thermistor compensation uses a negative temperature coefficient thermistor to reduce the collector current as temperature increases. Sensistor compensation uses a positive temperature coefficient resistor to decrease the base current as temperature rises.

1. COMPENSATION TECHNIQUES
Dr.N.G.Praveena
Associate Professor/ECE
R.M.K. COLLEGE OF ENGINEERING AND
TECHNOLOGY

2. TECHNIQUE
 Collector –Base bias and Voltage Divider bias
use the negative feedback to do the stabilization
action.
 Negative feedback reduces the amplification of the
signal.
 To overcome this Compensation Technique are
used which is used to stabilize the Q-point.
 Bias Compensation Methods are
(i) Diode compensation technique
(ii) Thermistor compensation
(iii) Sensistor compensation

3. Diode Compensation
Technique
 Diode is used as compensation element for
variation in 𝑽𝑩𝑬 𝒐𝒓 𝑰𝑪𝑶.
 For Silicon Diode , the change of 𝑽𝑩𝑬 with
temperature contributes more towards change in 𝑰𝑪
while change in 𝑰𝑪𝑶 is less effective.
 For Germanium Diode, the change of 𝑰𝑪𝑶 with
temperature contributes more towards change in
𝑰𝑪. while change in 𝑽𝑩𝑬 is less effective.

4. Diode Compensation for VBE
VT = VCC
RB =

5.  Diode and transistor used in the circuit is of same
material and type.
 Voltage across the diode will have the same
temperature coefficient as the base-emitter voltage
of a transistor.
 VBE changes by ∂𝑽𝑩𝑬 with change in temperature,
𝑽𝑫 also changes by ∂ 𝑽𝑫 by same amount (∂ 𝑽𝑫 ≈
∂ 𝑽𝑩𝑬 ) so they both tend to cancel each other.

6. Apply KVL to base circuit
VT – IB RB – VBE – IE RE + VD = 0
VT = IB RB + VBE + (IB + IC ) RE - VD
VT = IB ( RB + RE)+ VBE +IC RE - VD ---(1)
The Standard Equation of Ic is
Ic = β IB + (1 + β) Ico
Rearrange the above equation to get IB
IB =
IC − (1 + β)Ico
β
--------(2)
Sub (2) in (1)
VT =
IC − (1 + β)ICO
𝛃
(RB + RE ) + VBE + IC RE – VD
VT =
IC
𝛃
(RB + RE ) −
(1 + β)ICO
𝛃
(RB + RE ) + VBE + IC RE

7. VT =
IC
𝛃
(RB + RE )−
(1 + β)ICO
𝛃
(RB + RE )+ VBE+IC RE
– VD
VT =
IC
𝛃
[RB +(1+β) RE] −
(1 + β)ICO
𝛃
(RB + RE ) + VBE
– VD
VT =
IC[RB+ (1+β)RE] −(1 + β)ICO(RB+RE )+β(VBE –VD)
β
β VT = IC [RB +(1+β) RE] - (1 + β)ICO (RB + RE ) +β(VBE –VD)
IC [RB +(1+β) RE] = (1 + β)ICO (RB + RE ) +β(VT–VBE+VD)
IC =
(1 + β)ICO(RB+RE ) +β (VT–VBE+VD)
[RB+(1+β)RE]

8. Diode compensation for ICO
 In germanium transistors, changes in 𝑰𝑪𝑶 with
temperature are comparatively larger than silicon.
 The diode is kept in reverse biased condition , in
this case the current flow through the diode is only
the leakage current ( 𝑰𝑶).
 If the diode and the transistor are of same type and
material, then 𝑰𝑶 equal to 𝑰𝑪𝑶.

9. Current through resistance R1 can be written as
I =
𝐕𝐂𝐂 −𝐕𝑩𝑬
𝐑𝟏
=
𝑽𝑪𝑪
𝑹𝟏
Apply KCL at node A,
I = IB + Io
IB = I – Io
The standard equation of Ic is
Ic = β IB + (1 + β) Ico
If β >> 1 we get,
Ic = β I – β Io + β Ico
If Io = Ico we get,
IC = β I
A

10. THERMISTOR COMPENSATION

11.  Negative Temperature Coefficient
 As the temperature increases, the resistance
𝑹𝑻 decreases. Now the current fed through 𝑹𝑻
into 𝑹𝑬 increases. So the voltage developed
across 𝑹𝑬 increases.
 This voltage reverse-biases the emitter junction
and subtracts from the forward bias provided by
resistors 𝑹𝟏 and 𝑹𝟐. As a result, the collect
current reduces.
T RT IRT,IRE VRE IC

12. SENSISTOR COMPENSATION

13.  Positive Temperature Coefficient
 As the temperature increases, 𝑹𝑺 increases
which decreases the current flowing through it.
Hence current through 𝑹𝟐 decreases which
reduces the voltage drop across it.
 Voltage drop across 𝑹𝟐 is the voltage between
base and ground. So 𝑽𝑩𝑬 reduces which
decreases 𝑰𝑩.
T RS IRS,IR2 VR2 ,VBE IB