Learn about the different types of Compensation techniques

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