Electronic Devices & Systems
Dr Chinmayee Dora
Dept. Electronics & Communication Engineering
Centurion University of Management & Technology
Bhubaneswar
BJT DC Biasing
Biasing
• Biasing: The DC voltages applied to a transistor in order to turn it on so that
it can amplify the AC signal.
Need for DC biasing
If a signal of very small voltage is given to the input of BJT, it cannot be amplified.
Because, for a BJT, to amplify a signal, two conditions have to be met.
• The input voltage should exceed cut-in voltage for the transistor to be ON.
• The BJT should be in the active region, to be operated as an amplifier.
Factors affecting the operating point
The main factor that affect the operating point is the temperature. The operating
point shifts due to change in temperature.
• As temperature increases, the values of ICE, β, VBE gets affected.
• ICBO gets doubled (for every 10o rise)
• VBE decreases by 2.5mv (for every 1o rise)
• So the main problem which affects the operating point is temperature. Hence
operating point should be made independent of the temperature so as to
achieve stability.
Stabilization
• The process of making the operating point independent of temperature changes or variations
in transistor parameters is known as Stabilization.
• Once the stabilization is achieved, the values of IC and VCE become independent of
temperature variations or replacement of transistor. A good biasing circuit helps in the
stabilization of operating point.
Need for Stabilization
Stabilization of the operating point has to be achieved due to the following reasons.
• Temperature dependence of IC
• Individual variations
• Thermal runaway
Operating Point
• The DC input establishes an
operating or quiescent point called
the Q-point.
Three states of Operation
Active or Linear Region Operation
• Base–Emitter junction is forward biased
• Base–Collector junction is reverse biased
Cutoff Region Operation
• Base–Emitter junction is reverse biased
Saturation Region Operation
• Base–Emitter junction is forward biased
• Base–Collector junction is forward biased
DC Biasing Circuits
• Fixed-bias circuit
• Emitter-stabilized bias circuit
• Voltage divider bias circuit
• DC bias with voltage feedback
• Emitter Follower configuration
• Common base configuration
Fixed Bias
Circuit Analysis
The Base Emitter Loop:
• From Kirchhoff’s voltage law:
Solving for base current:
Circuit Analysis
Collector–Emitter Loop
Analyse the circuit and calculate the parameters
Transistor Saturation
• When the transistor is operating in saturation,
current through the transistor is at its maximum
possible value.
•
• Setting VCE= 0V,
• Also, at IC=0mA
Load line Movement
Load line Movement
Emitter Bias Configuration
• The dc bias network beside
contains an emitter resistor to
improve the stability level
over that of the fixed-bias
configuration. The more
stable a configuration, the
less its response will change
due to undesirable changes in
temperature and parameter
variations.
Base Emitter Loop
+VCC - IBRB - VBE - IERE = 0
Replacing IE using IE = (β+ 1)IB
VCC - IBRB - VBE - (β+ 1)IBRE = 0
Grouping terms then provides the following:
-IB(RB + (β+ 1) RE) + VCC - VBE = 0
Multiplying through by (-1), we have
IB(RB + (β+ 1) RE) - VCC + VBE = 0
with IB(RB + (β+ 1) RE) = VCC - VBE
and solving for IB gives
Collector Emitter loop
Analyse the circuit and calculate the
parameters
Effect of β
Load line Analysis
Voltage Divider Bias Configuration
Exact Analysis
Exact Analysis
Example
Approximate Analysis
• We can draw the input stage of a BJT based voltage-
divider network as shown in the figure.
• The resistance Ri may be considered as the resistance
equivalent between base and ground line of the circuit,
and RE as the resistor between emitter and ground.
• From our previous discussions we know that the
resistance reproduced or reflected between
base/emitter of the BJT is expounded by the
equation Ri = (β+ 1)RE.
• If we consider a situation where Ri is considerably
bigger than the resistance R2, will result in IB
relatively smaller than I2 (remember current always
tries to find and move to the direction of minimum
resistance), and thus I2 will turn approximately equal
to I1.
• Considering the approximate value of IB to be
essentially zero in relation to I1 or I2, then I1 = I2, and
R1, and R2 could be regarded as series elements.
Approximate Analysis
• The voltage across R2, which originally would be the base voltage could be evaluated as shown below,
by applying the voltage-divider rule network:
Now since Ri = (β + 1)RE ≅ βRE, the condition that confirms whether the execution of the approximate
method is feasible or not is decided by the equation:
In other words, if β times the value of RE is at least 10 times the value of R2 , the approximate approach
can be applied with a high degree of accuracy.
After VB is evaluated, the VE magnitude could be determined by the equation:
Problem for Practice
COLLECTOR FEEDBACK CONFIGURATION
• This self biasing collector feedback
configuration is another beta dependent
biasing method which requires two resistors
to provide the necessary DC bias for the
transistor.
• The collector to base feedback configuration
ensures that the transistor is always biased in
the active region regardless of the value of
Beta (β).
• The DC base bias voltage is derived from
the collector voltage VC, thus providing good
stability.
Base Emitter Loop
Analysis
• The collector voltage provides the bias for the base-emitter junction. The negative
feedback creates an “offsetting” effect that tends to keep the Q-point stable.
• If IC tries to increase, it drops more voltage across RC, thereby causing VC to decrease.
• When VC decreases, there is a decrease in voltage across RB, which decreases IB.
• The decrease in IB produces less IC which, in turn, drops less voltage across RC and thus
offsets the decrease in VC.
Collector Emitter Loop
Problem
Emitter Follower Configuration
Analysis
