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Chapter 3

BJT (Bipolar Junction Transistor)

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Bipolar Junction Transistor

There are two types oftransistors: pnp npn

The terminals are labeled: E - Emitter B - Base C - Collector

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Transistor operation

With the external sources, VEEand VCC, connected asshown below:

The emitter-base junction is forwardbiased

The base-collector junction is reverse

biased

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Currents in transistor

The collector current is comprised oftwo currents:

BI

CI

EI

COminoritICmajorityICI

Emitter current is the sum of thecollector and base currents:

Leakage

current

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COMMON-BASE configuration

The base is common to both input (emitterbase)and output (collectorbase) of the transistor.

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Operating regionsThis graph demonstrates the output

current (IC)to an output voltage (VCB)for various levels of input current (IE).

Output/collector Characteristics

Operating range

of the amplifier

The amplifier is

basically OFF.There is voltage,

but little current

The

amplifier is

full ON.

There is

current,

but little

voltage.

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Transistor amplification

Voltage Gain:

250

200mV

50V

iV

LV

vA

V50)k5)(ma10(RL

IL

V

mA10i

IL

I

EI

CI

10mA20

200mV

iR

iV

iIEI

Currents and Voltages:

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Common-emitter configuration

The emitter is common to both input (base-emitter) andoutput (collector-emitter).

The input is on the base and the output is on the collector.

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Collector characteristics

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Currents in C-E amplifier

Ideal Currents

IE= IC+ IB IC= IE

Actual Currents

IC= IE+ ICBO

When IB= 0 A the transistor is in cutoff, but there is

some minority current flowing called ICEO.

A0ICBO

CEO B1

II

where ICBO= minority collector current

This is usually so small that it can beignored, except in high power transistorsand in high temperature environments.

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Amplification factor

In DC mode:

In AC mode:

represents the amplification factor of a transistor. ( issometimes referred to as hfe, a term used in transistormodeling calculations)

B

Cdc

I

I

ttanconsVB

Cac CEI

I

Relationship betweenamplification factors and 1

1

Relationship BetweenCurrents

BC II BE 1)I(I

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Determining from a graph

Note: AC= DC

108

A25

mA2.7 7.5VDC CE

100

A10

mA1

A)20A(30

mA)2.2mA(3.2

7.5V

AC

CE

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Common-collector configuration

The input is on the base and the output is on the emitter.

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Limits of operation

VCEis at maximum and ICis

at minimum (ICmax= ICEO) in

the cutoff region.

ICis at maximum and VCEis

at minimum (VCE max= VCEsat

= VCEO) in the saturation

region.

The transistor operates in theactive regionbetween

saturation and cutoff.

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Power dissipation

Common-collector:

CCBCmax IVP

CCECmax IVP

ECECmax IVP

Common-base:

Common-emitter:

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Transistor specification sheet