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BJT Transistor circuits analysis:

There is an underlying similarity between the analysis of each configuration due to the recurring

use of the following important basic relationships for a transistor:

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 a

resulting forward-bias voltage of about 0.6 to 0.7 V.

2. The basecollector junction mustbe reverse-biased (n-region morepositive), with the reverse-

bias voltage being any value within the maximum limits of the device.

[Note that for forward bias the voltage across the p-n junction is p-positive, while for reverse

bias it is opposite (reverse) with n-positive. This emphasis on the initial letter should provide a

means of helping memorize the necessary voltage polarity.]

Operation in the cutoff, saturation, and linear regions of the BJT characteristic are provided as

follows:

1.Linear-region operation:

Baseemitter junction forward biased

Basecollector junction reverse biased

2. Cutoff-region operation:

Baseemitter junction reverse biased

3. Saturation-region operation:

Baseemitter junction forward biased

Basecollector junction forward biased

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Fig1: fixed biased circuit fig2: dc equivalent of fig1

F ixed biased circuit:

Forward Bias of BaseEmitter:

BaseEmitter loop

CollectorEmitter Loop;

Applying Kirchhoffs voltage law in theclockwise direction around the indicatedclosed loop of next figure will result in

the following:

The dc bias network of the following Fig containsan emitter resistor to improve the stability level

over that of the fixed-bias configuration.

BJT bias Circuit with emitter resistor

BaseEmitter Loop;

Baseemitter loop

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CollectorEmitter loop

As a brief review of single- and double-subscript

notation recall that:

Where VCE is the voltage from collector to

emitter and VC and VE are the voltages from

collector and emitter to ground respectively. But

in this case, since VE =0 V,we have:

Ic Saturation

Load Line Analysis:

Writing Kirchhoffs voltage law around the

indicated loop in the clockwise direction will resultin the following equation:

Recall:

Substituting :

The collectoremitter loop is redrawn in Fig

Writing Kirchhoffs voltage law for the indicated

loop in the clockwise direction will result in:

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The single-subscript voltage VE is the voltage from emitter

to ground and is determined by:

Saturation level:

Load line analysis: