Analog & Digital Electronics
Course No: PH-218
BJT
Lec-7: Biasing schemes and stability of BJT
Course Instructors:
Dr. A. P. VAJPEYI
Department of Physics,
Indian Institute of Technology Guwahati, India
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Biasing of BJT
Biasing refers to the application of D.C. voltages to setup the operating
point in such a way that output signal is undistorted throughout the whole
operation.
Also once selected properly, the Q point should not shift because of
change of IC due to
(i) β variation due to replacement of the transistor of same type
(ii) Temperature variation
Stabilization
The process of making operating point independent of temperature
changes or variation in transistor parameters is known as stabilization.
Stabilization of operating point is necessary due to
Temperature dependence of IC
Individual variations
Thermal runaway
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Stabilization
Temperature dependence of IC & Thermal runaway
I C = β I B + ( β + 1)I CBO
ICBO is strong function of temperature. A rise of 10oC doubles the ICBO and IC
will increase ( β+1) times of ICBO
The flow of IC produce heat within the transistor and raises the transistor
temperature further and therefore, further increase in ICBO
This effect is cumulative and in few seconds, the IC may become large
enough to burn out the transistor.
The self destruction of an unstablized transistor is known as thermal
runaway.
Stability Factor
The rate of change collector current IC with respect to the collector leakage
current ICBO is called stability factor, denoted by S.
S =(
dI C
)
dI CBO
Lower the value of S, better is
the stability of the transistor.
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Stability Factor
The rate of change collector current IC with respect to the collector leakage
current ICBO at constant β and IB is called stability factor, denoted by S.
I C = β I B + ( β + 1)I CBO
(1)
Differentiating equation (1) w.r.t IC
1= β(
dI B
dI
) + ( β + 1) CBO
dI C
dI C
1= β(
dI B ( β + 1)
)+
dI C
S
S =
( β + 1)
dI
1− β ( B )
dIC
Different biasing schemes
(i) Fixed bias (base resistor biasing)
(ii) Collector base bias
(iii) Emitter bias
(iv) Voltage divider bias
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Fixed Bias
This form of biasing is also called base bias. The single power source
is used for both collector and base of transistor, although separate
batteries can also be used.
D.C. Equivalent
Using KVL in the base-emitter loop
VCC – IBRB –VBE = 0 ; IB = (VCC-VBE)/RB
IC = βIB= β(VCC-VBE)/RB
Using KVL in the collector-emitter loop
VCC – ICRC –VCE = 0; VCE = VCC - ICRC
Q(VCE,IC) is set
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Fixed Bias
Advantages:
Operating point can be shifted easily anywhere in the active region by merely
changing the base resistor (RB).
A very small number of components are required.
Disadvantages:
Poor stabilization
High stability factor(S=β+1 because IB is constant so dIB/dIC =0 ), hence
prone to thermal runaway
Usage:
Due to the above inherent drawbacks, fixed bias is rarely used in linear
circuits (i.e., those circuits which use the transistor as a current source).
Instead, it is often used in circuits where transistor is used as a switch.
How the Q point is affected by changes in VBE and ICBO in fixed bias?
IB = (VCC-VBE)/RB
IC = βIB
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Collector base bias
This configuration employs negative feedback to prevent thermal runaway and
stabilize the operating point.
In this form of biasing, the base resistor RB is connected to the collector instead
of connecting it to the DC source Vcc.
So any thermal runaway will induce a voltage drop across the RC resistor that
will throttle the transistor's base current.
Applying KVL
VCC = (IC+IB)RC + VCE
(1)
VCE = IBRB + VBE
(2)
Since, IC = βIB so from equation (1) & (2)
IB =
V CC − V BE
R B + (1 + β ) R C
Q(VCE,IC) is set
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Collector base bias
Advantages:
Better stabilization compared to fixed bias
Disadvantages:
This circuit provides negative feedback which reduces the gain of the amplifier.
Usage:
The feedback also decreases the input impedance of the amplifier as seen from
the base, which can be advantageous. Due to the gain reduction from feedback, this
biasing form is used only when the trade-off for stability is warranted.
How the bias stability is improved in collector base bias?
If IC becomes larger than design value, it causes an increase voltage drop
across RC hence smaller value of VCE which in turn causes IB to be smaller than
its design value. Since IC = βIB thus IC will also tend to be reduced towards its
original value.
For bais Stabilization : RB<< βRC
IC
V CC − V BE
= β[
]
R B + (1 + β ) R C
If RB<< βRC
IC =
V CC − V BE
RC
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Voltage Divider Bias
This is the most widely used method to provide biasing and stabilization to a
transistor.
In this form of biasing, R1 and R2 divide the supply voltage VCC and voltage
across R2 provide fixed bias voltage VB at the transistor base.
Also a resistance RE is included in series with the emitter that provides the
stabilization.
R1
RC
VB = Voltage across R2
(ignoring base current)
R2
VB = VCC
(R1 + R 2 )
R2
RE
Voltage across R2
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Voltage Divider Bias
R2
VB = VTh = VCC
(R1 + R 2 )
R Th
R 1R 2
= R1 R 2 =
(R 1 + R 2 )
Base-Emitter Loop
VTh − I B R Th − VBE − (β + 1)I B R E = 0
VTh − VBE
or, I B =
R Th + (β + 1)R E
Collector- Emitter Loop
I C = βI B =
β(VTh − VBE )
R Th + (β + 1)R E
VCE = VCC − I C R C − I E R E = VCC − I C R C − (I C + I B )R E
For bais Stabilization : RTh<< (β+1)RE
VTh − VBE
IC ≈
RE
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Stability factor for Voltage Divider Bias
S =
( β + 1)
VTh − IBR Th − VBE − IE R E = 0
dI
1− β ( B )
d I C (1) VTh = IBR Th − VBE − (IB + IC )R E
Differentiating equation (1) w.r.t IC
VTh = IBR Th − VBE − (IB + IC )R E
0 = RTh (
dI B
dI
) + (1 + B ) RE
dI C
dI C
dI B
RE
=−
dIC
RTh + RE
R Th
)
RE
S = ( β + 1)
R
( β + 1 ) + Th
RE
(1 +
S = ( β + 1)
( R Th + R E )
R E ( β + 1 ) + R Th
For stability, S should be small which can be achieved by making RTh/RE
small. For very small RTh/RE ; S = 1 (ideal case)
For very small RTh/RE : R2
RTh current drawan by R2 will be large.
RE
Large Vcc required. Hence comproise is made in selection.
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