ECE-312 Electronic Circuits (A) Lecture #3 - Bu Shoubra/Electrical... · ECE-312 Electronic...

Lecture #3 BJT Biasing Circuits Instructor: Dr. Ahmad El-Banna

Benha University Faculty of Engineering at Shoubra

ECE-312 Electronic Circuits (A)

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

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:

Base–emitter junction forward-biased Base–collector junction reverse-biased

2. Cutoff-region operation: Base–emitter junction reverse-biased Base–collector junction reverse-biased

3.Saturation-region operation: Base–emitter junction forward-biased Base–collector junction forward-biased

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.

• Base–emitter loop. • Collector–emitter loop.

Fixed-Bias Configuration Example

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Fixed-Bias Configuration ...

• Transistor Saturation

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• Determining ICsat for the fixed-bias configuration. • Determining ICsat

• Saturation regions: (a) Actual (b) approximate.

Fixed-Bias Configuration ...

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• Load Line Analysis

Emitter-Bias Configuration

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• Base-Emitter Loop

• DC equivalent ct • BJT bias circuit with emitter resistor.

Emitter-Bias Configuration

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Collector-Emitter Loop

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 as temperature and transistor beta, change.

• Saturation Level

• Load Line Analysis

Voltage-Divider Configuration

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• Exact Analysis • Voltage-divider bias configuration.

• DC components of the voltage-divider configuration.

Voltage-Divider Configuration

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• Approximate Analysis • Transistor Saturation

• Load-Line Analysis

Voltage-Divider Configuration Example

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Collector Feedback Configuration

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• DC bias circuit with voltage feedback.

• Base–Emitter Loop

• Collector–Emitter Loop

Collector Feedback Configuration

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• Saturation Conditions

Using the approximation I’C = IC

• Load-Line Analysis

Continuing with the approximation I’C = 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.

Emitter-Follower Configuration

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i/p ct

o/p ct

• dc equivalent ct

• Common-collecter (emitter-follower) configuration.

Common-Base Configuration

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• i/p ct

• Determining VCB & VCE

• Common-base configuration

MISCELLANEOUS BIAS CONFIGURATIONS

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Summary Table

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Summary Table..

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DESIGN OPERATION 23

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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, the nearest standard commercial values are normally chosen and any variations due to not using the exact resistance values are accepted as part of the design.

Design Operations Example

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Design Operations Example..

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• Design of a Current-Gain-Stabilized (Beta-Independent) Circuit

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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• R–C coupling

• Darlington configuration

MULTIPLE BJT NETWORKS..

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MULTIPLE BJT NETWORKS…

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• Feedback Pair

MULTIPLE BJT NETWORKS….

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• Direct Coupled

CURRENT MIRRORS

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CURRENT SOURCE CIRCUITS

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• Bipolar Transistor Constant-Current Source

• Transistor/Zener Constant-Current Source

TRANSISTORS

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TRANSISTOR SWITCHING NETWORKS

TRANSISTOR SWITCHING NETWORKS..

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TROUBLESHOOTING TECHNIQUES 36

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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 value—a 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

BIAS STABILIZATION .. S(VBE)& S(β)

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For fixed-bias

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

Practical Application..

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• Maintaining a Fixed Load Current

• Light Control

Practical Application…

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• Alarm System with a CCS

• Voltage Level Indicator

Practical Application….

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• Logic Gates

• For more details, refer to:

• Chapter 4 at R. Boylestad, Electronic Devices and Circuit Theory, 11th edition, Prentice Hall.

• The lecture is available online at:

• https://speakerdeck.com/ahmad_elbanna

• For inquires, send to:

• ahmad.elbanna@fes.bu.edu.eg

• ahmad.elbanna@ejust.edu.eg

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ECE-312 Electronic Circuits (A) Lecture #3 - Bu Shoubra/Electrical... · ECE-312 Electronic...

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