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BJT, Bipolar Junction Transisor

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Base CurrentBase Current

ControlsControls

Output currentOutput current

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AGENDA

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BJT transistormanTransistor typesBipolar Junction TransistorBJT modelsparameterswater modelNPN and PNPoperation modesswitch openswitch closed

BJTlinear, controlled current sourceactive operationcharacteristicsDC input characteristicsac input characteristicsBJT DC biasing circuitsbase biasbase bias + collector feedbackbase bias + emitter feedbackvoltage divider

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BJT, transistor man

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TransistorTypes

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Output currentOutput current controlled controlled

by by input currentinput current

Output currentOutput current controlledcontrolled

by by input voltageinput voltage

BJT ==

Bipolar Junction Bipolar Junction TransistorTransistor

FET ==

Field Effect TransistorField Effect Transistor

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BJT, Bipolar Junction Transisor

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BE ForwardBE Forward bias, bias, BC ReverseBC Reverse bias bias

So So lowlow ohmic ohmic highhigh ohmic ohmic

TransistorTransistor == TranTransfersfer ReResistorsistor

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BJT, Bipolar Junction Transisor

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Emitter Emitter = Sent = Sent electronselectrons

BaseBase = Base = Base

CollectorCollector = Get = Get electrons electrons

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BJT, Models

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BJT, parameters

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BJT, Water model

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BJT, Water model

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BJT, NPN and PNP

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BJT, Operation modes

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Cut-off and Cut-off and saturation;saturation;

BJT is used as a BJT is used as a switchswitch

Active operationActive operation

Quiecent Point;Quiecent Point;

BJT is used as a BJT is used as a

controlled controlled

current source,current source,

or or analog amplifieranalog amplifier

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BJT, Switch open

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BJT, Switch closed

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BJT, Lineair, controlled current source

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BJT, active operation

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BJT, characteristics

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DCDC model model acac model model

DCDC model; Vbe = 0V7 Ube, Uce, Ic, Ib, Ie model; Vbe = 0V7 Ube, Uce, Ic, Ib, Ie CapitalsCapitals

acac model; re = 26mV/Ie ube, uce, ic, ib, ie model; re = 26mV/Ie ube, uce, ic, ib, ie Low Low casescases

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BJT, DC input characteristics

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Vbe = 0V7

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BJT, AC input characteristics

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re = 26mV/Ic

The dynamic resistor can be calculated by the DC current

Ic

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BJT, characteristics

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BJT, DC biasing circuits

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A base biasA base bias

B base bias + emitter feedbackB base bias + emitter feedback

C base bias + collector feedbackC base bias + collector feedback

D voltage dividerD voltage divider

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BJT, base bias, introduction

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Base current determined by Vcc, Rb and Vbe

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BJT, base bias

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cc Rb beV U U

cc b b beV I R U

c bI I Calculate Ib and then Ic

Ic directly dependent on ß variation

So, for stability a “bad” circuit

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BJT, base bias load line

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Load line is the loading of the transistor seen from Uce (>0V7)

Vcc and Rc determines the; “open voltage” and the “short circuit current”

Q-point = Quiecient

point= Working point

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BJT, base bias load line

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Load line is the loading of the transistor seen from Uce (>0V7)

Vcc and Rc determines the; “open voltage” and the “short circuit current”

Reliable circuit= Q-point

stability

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BJT, base bias load line

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Vce always > 0V7BC junction

REVERSE

If Rc too big, transistor in saturation; then;

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BJT, base bias load line

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Vce always > 0V7BC junction

REVERSE

If Vcc too small, transistor in saturation; then;

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BJT, base bias example

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Calculate;

Ib, IcURc, Uc, Uce

Draw output caracteristic

Calculate now;

Uce if ß = 40How many % did Uce

ChangeIb = 47 uA, Ic = 2,35 mA, URc = 5,17 V, Uc = 6,83 V, Uce = 6,83 VUce (for ß = 40) = 7,86 Ξ 15 %

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BJT, base bias example

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Ib = 33 uA, Ic = 2,9 mA URc = 7,9 V, Uc = 8,1 V

Rb = 282,5 kΩ, Ic = 3,2 mA,

Rc = 1,855 kΩ

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BJT, base bias example

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ß = 200, VRc = 8,8 VVcc = 16 VRb = 765 kΩ

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BJT, base bias + emitter feedback

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Base current determined by Vcc, Rb, Vbe and Ve

More stable for ß variations, than base bias.

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BJT, base bias + emitter feedback

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Always calculate in the smallest current Ib !!

Recc Rb beV U U U

Re1cc b b be bV I R U I U

Rc c cV I R

c cc c cV V I R

e e eV I R

ce c eV V V

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BJT, base bias + emitter feedback

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Load line is the loading of the transistor seen from Uce (>0V7)

Vcc, Rc and Re determines the; “open voltage” and the “short circuit current”

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BJT, base bias + emitter feedback example

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Calculate;

Ib, IcURc, Uc, Ue, Uce

Draw output caracteristic

Ib = 6,2 uA, Ic = 0,74 mA, URc = 8,9 V, Uc = 7,1 V, Ue =-0,9 V, Uce = 8,0 V

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BJT, base bias + emitter feedback example

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Calculate;

Ib, IeURe, Ue, Uce

Draw output caracteristic

Ib = 24 uA, Ie = 2,9 mA, URc = 3,5 V, Ue = -2,5 V, Uce = 2,5 V

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BJT, base bias + collector/emitter feedback

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If Ic > then Uc < then Ib <

If Ic > then Uc <and Ue > then Ib <

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BJT, base bias + collector feedback

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cc Rc Rb beV U U U

1cc b c b b beV I R I R U

Always calculate in the smallest current Ib !!

The current through Rc is not Ic but Ic + Ib,

so (β+1)Ib !!!

If Ic rises for any reason, then Uc falls and

also Ib decreases, so then Ic decreases

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BJT, base bias collector feedback example

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Calculate;

Ib, ß, Ic

Draw output caracteristic

Ib = 13 uA, ß = 196, Ic = 2,5 mA

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BJT, base bias collector/emitter feedback

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Recc Rc Rb beV U U U U

1

1

cc b c

b b

be

b e

V I R

I R

U

I R

Always calculate in the smallest current Ib !!

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BJT, base bias collector/emitter feedback ex

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Calculate;

Ib, IeURc, Uc, Ue, Uce

Draw output caracteristic

Ib = 11,8 uA, Ie = 1,1 mA

URc = 5,2 V, Uc = 4,8 V

Ue = 1,3 V, Uce = 3,5 V

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BJT, voltage divider

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Vb is a stable voltage - 0,7 V =

so Ve is a stable voltageIe is determined by Ve/ Re

Ic = Ie . ß/(ß+1)

Ic is very stable and nearly independent to ß

variation, as long as ß is BIG in value

2 methods of calculating Ic - neglegting Ib, use voltage divider - not neglecting Ib and use thevenin

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BJT, voltage divider, neglect Ib

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2

1 2b cc

RV V

R R

0 7e bV V V

ee

e

VI

R

1c eI I

So neglegt Ib to R2, or in general Ri >> R2In practice 10 times bigger

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BJT, voltage divider, exact, thevenin

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Thevenin resistance

R1 // R2 62k // 9k1= 7k9

Thevenin voltage 2

1 2th cc

RV V

R R

9 116 2 0

62 9 1th

kV V

k k

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BJT, voltage divider, exact, thevenin

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2V0

7k9

1th b th be b eV I R V I R

2,0 7 9 0,7 80 1 0,68b bI k I k Ib = 20 uA

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BJT, voltage divider, example

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Thevenin resistance = 6k8

Thevenin voltage = 3V1

Ib = 18,8 uAIc = 2,25 mAre = 11,5 ΩURc = 7V4Uc = 10V6Ue = 2V3Uce = 5V1

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BJT, voltage divider, example

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Thevenin resistance = 255k

Thevenin voltage = 0V0

Ib = 14,3 uAIc = 1,9 mAre = 14 ΩURc = 17V3Uc = 0V7Ue = -3V7Uce = 4V4

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BJT

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BJT

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