Bipolar Junction Transistors (BJT)

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• The Two Types of BJT TransistorsThe Two Types of BJT Transistors

npnnpn pnppnp

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BJT Relationships - EquationsBJT Relationships - Equations

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= Common-emitter current gain= Common-emitter current gain = Common-base current gain= Common-base current gain = IC= IC = IC = IC IBIB IE IE

The relationships between the two parameters are:The relationships between the two parameters are: = = = = + 1+ 1 1 - 1 -

Note: Note: and and are sometimes referred to as are sometimes referred to as dc and dc and dc dc because the relationships being dealt with in because the relationships being dealt with in the BJT the BJT are DC.are DC.

DC DC and DC and DC

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Modes of OperationModes of Operation

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Three Types of BJT BiasingThree Types of BJT Biasing

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Electrostatic variables for a pnp BJT at equilibrium

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Common-BaseCommon-Base

Circuit Diagram: NPN TransistorCircuit Diagram: NPN Transistor

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Common-BaseCommon-Base

Although the Common-Base configuration is not the most Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of common biasing type, it is often helpful in the understanding of

how the BJT works. how the BJT works.

Emitter-Current CurvesEmitter-Current Curves

Sa

tura

tio

n R

egio

nS

atu

rati

on

Reg

ion

IIEE

IICC

VVCBCB

Active Active RegionRegion

CutoffCutoff

IIEE = 0 = 0

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Common-EmitterCommon-Emitter

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Common-CollectorCommon-Collector

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Eber-Moll BJT ModelEber-Moll BJT Model

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Common-emitter transistor

• Common-emitter transistor ampliers are so-called because the input and output voltage points share the emitter lead of the transistor in common with each other, not considering any power supplies.

• ² Transistors are essentially DC devices: they cannot directly handle voltages or currents that reverse direction. In order to make them work for amplifying AC signals, the input signal must be offset with a DC voltage to keep the transistor in its active mode throughout the entire cycle of the wave. This is called biasing.

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Common-collector transistor

• Common-collector transistor ampliers are so-called because the input and output voltage points share the collector lead of the transistor in common with each other, not considering any power supplies.

• ² The output voltage on a common-collector amplier will be in phase with the input voltage, making the common-collector a non-inverting amplier circuit.

• ² The current gain of a common-collector amplier is equal to Ø plus 1. The voltage gain is approximately equal to 1 (in practice, just a little bit less).

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Common-base transistor

• Common-base transistor ampliers are so-called because the input and output voltage points share the base lead of the transistor in common with each other, not considering any power supplies.

• ² The current gain of a common-base amplier is always less than 1. The voltage gain is a function of input and output resistances, and also the internal resistance of the emitterbase junction, which is subject to change with variations in DC bias voltage.

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TRANSISTOR MODEL

• Common-Base Configuration

Common-base BJT transistorre modelre equivalent cct.

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E

BB

C

Common-base BJT transistor - pnp

Ic Ie

e

b b

c

ec I αI

IcIe

re model for the pnp common-baseconfiguration

e

b b

c

ec I αI

IcIe

common-base re equivalent cct

re

Therefore, the input impedance, Zi = rethat less than 50Ω.For the output impedance, it will be as follows;

e

b b

c

A0Ic

IcIe=0A

Determining Zo for common-base

reVs=0V

e EE(dc)

26mVr I is the DC level of

I

emitter current

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The hybrid parameters: hie, hre, hfe, hoe are developed and used to model the transistor. These parameters can be found in a specification sheet for a transistor.

Hybrid Equivalent Model

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Determination of parameter

0VVo

i12

0VVi

i11

o12i11i

o

o

VV

h

IV

h

VhIhV

0AIo

o22

0VVo

i21

o

o22i21O

o

o

VI

h

II

h

, 0VV Solving

VhIhI

0VVo

i12

0VVi

i11

o12i11i

o

o

VV

h

IV

h

VhIhV

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General h-Parameters for any Transistor Configuration

hi = input resistancehr = reverse transfer voltage ratio (Vi/Vo)hf = forward transfer current ratio (Io/Ii)ho = output conductance

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Common emitter hybrid equivalent circuit

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Common base hybrid equivalent circuit

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Transistor as a Switch

B BEB

B

V VI

R

CE CB BE CB CE BEV V V or V V V

, 0,CC CE CC BECM CB CM

C C

V V V VI for V I

R R

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Power Transistor Structure

Em itterBase

n + 1019 cm - 3

p 1 0 16 cm - 3

n – 1014 cm - 3

n + 1019 cm - 3

Co llector

250 m

50- 200 m

10 m

5- 20 m

(C ollector driftregion)

Base Th ickness

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Switching Characteristics of BJT

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