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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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Base-Emitter Loop

DC equivalent ct

BJT bias circuit with emitter resistor.

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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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DC bias circuit with

voltage feedback.

BaseEmitter Loop

CollectorEmitter Loop

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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:

[email protected]

[email protected]

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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

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