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7/23/2019 ECE312_lec03
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Lecture #3
BJT Biasing CircuitsInstructor:
Dr. Ahmad El-Banna
Benha University
Faculty of Engineering at Shoubra
Octo
ber
2014
ECE-312
Electronic Circuits (A)
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Agenda
Operating Point
Transistor DC Bias Configurations
Design Operations
Various BJT Circuits
Troubleshooting Techniques & Bias Stabilization
Practical Applications 2
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Introduction
Any increase in ac voltage, current, or power is the result of a
transfer of energy from the applied dc supplies.
The analysis or design of any electronic amplifier therefore has two
components: a dc and an ac portion.
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Basic Relationships/formulas for a transistor:
Biasing means applying of dc voltages to establish a fixed level of
current and voltage. >>> Q-Point
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Operating Point
For transistor amplifiers the resulting dc current and voltage
establish an operating point on the characteristics that define the
region that will be employed for amplification of the applied signal.
Because the operating point is a fixed point on the characteristics, it
is also called the quiescent point (abbreviated Q-point).
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Transistor Regions Operation:
1. Linear-region operation:Baseemitter junction forward-biased
Basecollector junction reverse-biased
2. Cutoff-region operation:Baseemitter junction reverse-biased
Basecollector junction reverse-biased
3.Saturation-region operation:Baseemitter junction forward-biased
Basecollector junction forward-biased
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TRANSISTOR DC BIAS CONFIGURATIONS
Fixed-Bias Configuration
Emitter-Bias Configuration
Voltage-Divider Bias Configuration
Collector Feedback Configuration
Emitter-Follower Configuration Common-Base Configuration
Miscellaneous Bias Configurations
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Fixed-Bias Configuration
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Fixed-bias circuit.
DC equivalent ct.
Baseemitter loop. Collectoremitter loop.
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Fixed-Bias Configuration Example
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Fixed-Bias Configuration ...
Transistor Saturation
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Determining ICsatfor the fixed-bias configuration. Determining ICsat
Saturation regions:
(a) Actual(b) approximate.
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Fixed-Bias Configuration ...
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Load Line Analysis
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Emitter-Bias Configuration
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Base-Emitter Loop
DC equivalent ct
BJT bias circuit with emitter resistor.
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Emitter-Bias Configuration
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Collector-Emitter Loop
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Emitter-Bias Configuration Improved bias stability (check example 4.5)
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The addition of the emitter resistor to
the dc bias of the BJT provides
improved stability, that is, the dc bias
currents and voltages remain closer to
where they were set by the circuit
when outside conditions, such astemperature and transistor beta,
change.
Saturation Level
Load Line Analysis
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Voltage-Divider Configuration
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Exact Analysis
Voltage-dividerbias configuration.
DC components of the
voltage-divider configuration.
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Voltage-Divider Configuration
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Approximate Analysis Transistor Saturation
Load-Line Analysis
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Voltage-Divider Configuration Example
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Collector Feedback Configuration
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DC bias circuit with
voltage feedback.
BaseEmitter Loop
CollectorEmitter Loop
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Collector Feedback Configuration
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Saturation Conditions
Using the approximation IC = IC
Load-Line Analysis
Continuing with the approximation
IC = IC results in the same load
line defined for the voltage-divider
and emitter-biased configurations.
The level of IBQ is defined by the
chosen bias configuration.
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Emitter-Follower Configuration
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i/p ct
o/p ct
dc equivalent ct
Common-collecter
(emitter-follower) configuration.
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Common-Base Configuration
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i/p ct
Determining VCB & VCE
Common-base configuration
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MISCELLANEOUS BIAS CONFIGURATIONS
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Summary Table
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Summary Table..
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DESIGN OPERATION 23ECE-3
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Design Operations
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Discussions thus far have focused on the analysis of existing networks.
All the elements are in place, and it is simply a matter of solving for the
current and voltage levels of the configuration.
The design process is one where a current and/or voltage may be
specified and the elements required to establish the designated levels
must be determined.
The design sequence is obviously sensitive to the components that are
already specified and the elements to be determined. If the transistor
and supplies are specified, the design process will simply determine
the required resistors for a particular design.
Once the theoretical values of the resistors are determined, thenearest standard commercial values are normally chosen and any
variations due to not using the exact resistance values are accepted as
part of the design.
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Design Operations Example
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Design Operations Example..
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Design of a Current-Gain-Stabilized (Beta-Independent) Circuit
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VARIOUS BJT CIRCUITS
MULTIPLE BJT NETWORKS
CURRENT MIRRORS
CURRENT SOURCE CIRCUITS
Bipolar Transistor Constant-Current Source
Transistor/Zener Constant-Current Source
PNP TRANSISTORS
TRANSISTOR SWITCHING NETWORKS
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MULTIPLE BJT NETWORKS
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RC coupling
Darlington configuration
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MULTIPLE BJT NETWORKS..
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MULTIPLE BJTNETWORKS
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Feedback Pair
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MULTIPLE BJT NETWORKS.
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Direct Coupled
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CURRENT MIRRORS
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CURRENT SOURCE CIRCUITS
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Bipolar Transistor
Constant-Current Source
Transistor/Zener
Constant-Current Source
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pnpTRANSISTORS
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TRANSISTOR SWITCHING
NETWORKS
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TRANSISTOR SWITCHING NETWORKS..
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TROUBLESHOOTING TECHNIQUES 36ECE-3
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TROUBLESHOOTING TECHNIQUES For an on transistor, the voltage VBE should be in the neighborhood of 0.7 V.
For the typical transistor amplifier in the active region, VCE is usually about 25% to
75% of VCC .
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BIAS STABILIZATION
The stability of a system is a measure of the sensitivity of a network
to variations in its parameters.
In any amplifier employing a transistor the collector current IC is
sensitive to each of the following parameters:
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BIAS STABILIZATION .. S(Ico)
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fixed-bias configuration
the level of IC would continue
to rise with temperature, with
IB maintaining a fairly constant
valuea very unstable situation.
emitter-bias configuration
there is a reaction to an increase
in IC that will tend to oppose the
change in bias conditions.
feedback configuration
a stabilizing effect as described for
the emitter-bias configuration.
voltage-divider bias
The most stable of the
configurations
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BIAS STABILIZATION .. S(VBE)& S()
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For fixed-bias
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PRACTICAL APPLICATION
BJT Diode Usage and Protective Capabilities
Relay Driver
Light Control
Maintaining a Fixed Load Current
Alarm System with a CCS
Voltage Level Indicator
Logic Gates
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Practical Application
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BJT Diode Usage and Protective Capabilities
Relay Driver
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Practical Application..
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Maintaining a Fixed Load Current
Light Control
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PracticalApplication
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Alarm System with a CCS
Voltage Level Indicator
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PracticalApplication.
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Logic Gates
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For more details, refer to:
Chapter 4 at R. Boylestad, Electronic Devices and Circuit Theory,
11thedition, Prentice Hall.
The lecture is available online at:
https://speakerdeck.com/ahmad_elbanna
For inquires, send to:
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https://speakerdeck.com/ahmad_elbannamailto:[email protected]:[email protected]:[email protected]:[email protected]://speakerdeck.com/ahmad_elbannahttps://speakerdeck.com/ahmad_elbanna