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Bipolar Junction Transistors

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Introduction

The analysis or design of a transistor amplifier requires

a knowledge of both the dc and ac response of the

system

The improved output ac power level is the result of atransfer of energy from the applied dc supplies

The analysis or design of any electronic amplifier

therefore has two components: the dc portion and theac portion

the superposition theorem is applicable and the

investigation of the dc conditions can be totally

separated from the ac response

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DC Level of Operation

The dc level of operation of a transistor is

controlled by a number of factors, including:

1. the range of possible operating points on the

device characteristics. Once the desired dccurrent and voltage levels have been defined, anetwork must be constructed that will establish

the desired operating point

2. Each design will also determine the stability of the system, that is, how sensitive the system is

to temperature variations

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Important Basic Relationships for a

Transistor

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BJT DC Operating Point

The term biasing is the term for the application of dc voltages toestablish a fixed level of current and voltage.

For transistor amplifiers the resulting dc current and voltageestablish an oper ating point on the characteristics that define theregion that will be employed for amplification of the applied signal.

Since the operating point is a fixed point on the characteristics, it is

also called the qui esc ent  point (abbrev iat ed Q- point  ). It meansquiet, still, inactive.

The biasing circuit can be designed to set the device operation atany of these points or others within the acti ve regi on. Themaximum ratings are indicated on the characteristics by ahorizontal line for the maximum collector current ICmax and a

vertical line at the maximum collector-to-emitter voltage VCEmax.The maximum power constraint is defined by the curve PCmax inthe same figure. At the lower end of the scales are the cutoff region, defined by IBµ0 , and the saturation region, defined byVCE VCEsat.

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Operating Point Using Load Line

The BJT device could be biased to operate outside

these maximum limits, but the result of such operation

would be either a considerable shortening of thelifetime of the device or destruction of the device.

Confining ourselves to the acti ve regi on , one can select

many different operating areas or points.

The chosen Q- point of t en depend s on the intendeduse of the circuit. Still, we can consider some

differences among the various points shown in Fig. 4.1

to present some basic ideas about the operating point

and, thereby, the bias circuit.

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DC Load Line

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Linear, Cut-off, Saturation

Operation in the cut-off, saturation, and linear regions of 

the BJT characteristic are provided as follows:

1. Linear -regi on oper ati on:

Baseemitter junction forward biasedBasecollector junction reverse biased

2. C ut off -regi on oper ati on:

Baseemitter junction reverse biased

3. Sat ur ati on-regi on oper ati on:

Baseemitter junction forward biased

Basecollector junction forward biased

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Cut-Off 

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Saturation

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DC Bias

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Cut-off and Saturation

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BJT Biasing

For the BJT to be biased in its linear or active

operating region the following must be true:

1. The baseemitter junction must be forward-

biased (p-region voltage more positive ) ,with aresulting forward-bias voltage of about 0.6 to 0.7V.

2. The basecollector junction must be reverse-

biased (n-region more positive ) , with the reverse-bias voltage being any value within the maximum

limits of the device.

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Graphical Analysis

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DC Load Line

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Linear Operation

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LINEAR OPERATION

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Troubleshooting Transistors

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DMM Test Of a Properly Functioning

Transistor

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FIXED-BIAS NETWORK

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Emitter Stabilized

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Summary of Transistor Bias Circuits

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