Electronic Devices
Mid Term
Lecture - 09
Faculty Name: Dr. Md. Rifat Hazari
Email : rifat@aiub.edu
Reference book:
Electronic Devices and Circuit Theory (Chapter-4)
Robert L. Boylestad and L. Nashelsky , (11th Edition)
Faculty of Engineering
American International University-Bangladesh
Objectives
• Be able to determine the dc levels for the variety of important BJT configurations.
• Understand how to measure the important voltage levels of a BJT transistor
configuration and use them to determine whether the network is operating properly.
• Become aware of the saturation and cutoff conditions of a BJT network and the
expected voltage and current levels established by each condition.
• Be able to perform a load-line analysis of the most common BJT configurations.
• Become acquainted with the design process for BJT amplifiers.
• Understand the basic operation of transistor switching networks.
• Begin to understand the troubleshooting process as applied to BJT configurations.
• Develop a sense for the stability factors of a BJT configuration and how they affect its
operation due to changes in specific characteristics and environmental changes.
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American International University-Bangladesh
BIASING
• Applying DC voltages to the transistor to turn it on so that it can amplify AC
signal.
• Once the desired DC current and voltage levels have been defined, a network
must be constructed that will establish the desired operating point.
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INTRODUCTION
• BJT amplifier design requires knowledge of both the DC and AC.
• BJT needs to be operated in active region used as amplifier.
• BJT operated in cut-off and saturation region is used as a switch.
• The following basic current relationships for a transistor are required for
transistor network analysis:
VBE = 0.7 V
IE = (β+1) IB ≅ IC
IC = β IB
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American International University-Bangladesh
INTRODUCTION
• Biasing:
Applying DC voltages to establish a fixed level of current and voltage.
• The applied DC establishes an operating point (Q-point) that define the region
for the signal amplification.
• For a BJT to be biased in active operating region, the following must be true:
1) BE junction = forward biased
2) BC junction = reverse biased
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BIASING AND THE 3 STATES OF OPERATION
Saturation Region
Active Region
Cut-off Region
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OPERATING POINT (Q-POINT)
• Operating Point: Quiescent point or Q-point (static point)
• The biasing circuit can be designed to set the device operation at any of these
points or others within the active region.
• The BJT device could be biased to operate outside the max limits, but the
result of such operation would be shortening of the lifetime of the device or
destruction of the device.
• The chosen Q-point often depends on the intended use of the circuit.
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VARIOUS Q-POINTS WITHIN THE
LIMITS OF OPERATION
Q-point B:
• The best operating point Q-point D:
for linear gain & largest • Near the maximum voltage &
possible voltage & power level.
current.
• Desired condition for a
small signal analysis.
Q-point C:
• Concern on nonlinearities due
Q-point A: to IB curves is rapidly changes
• I =0A, V= 0V in this region.
• Not suitable for transistor
to operate
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BJT BIAS CONFIGURATIONS
1. Fixed-Bias Configuration
2. Emitter-Bias Configuration
3. Voltage-Divider Bias Configuration
4. Collector Feedback Configuration
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FIXED-BIAS CONFIGURATION
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FORWARD BIAS OF BASE–EMITTER
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COLLECTOR–EMITTER LOOP
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FIXED-BIAS CONFIGURATION
EXAMPLE
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Transistor Saturation
• For a transistor operating in the saturation region, the current is a maximum value for
the particular design
• The highest saturation level is defined by the maximum collector current as provided by
the specification sheet.
• Saturation conditions are normally avoided because the base–collector junction is no
longer reverse-biased and the output amplified signal will be distorted.
• It is in a region where the characteristic curves join, and the collector-to-emitter voltage
is at or below 𝑉𝐶𝐸𝑠𝑎𝑡 .
The resulting saturation current for the fixed-bias
configuration is
See Example 4.2
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Load-Line Analysis
• The characteristics of the BJT are superimposed on a plot of the network equation
defined by the same axis parameters
• The load resistor 𝑅𝐶 for the fixed-bias configuration will define the slope of the
network equa- tion and the resulting intersection between the two plots.
The network of Fig. 4.11(a) establishes an
output equation that relates the variables 𝐼𝐶 and
𝑉𝐶𝐸 in the following manner:
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American International University-Bangladesh
Load-Line Analysis
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EXAMPLE
EXAMPLE 4.3 Given the load line of Fig. 4.16 and the defined Q-point, determine the
required values of 𝑉𝐶𝐶 , 𝑅𝐶 , and 𝑅𝐵 for a fixed-bias configuration.
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Thank You
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American International University-Bangladesh
