Introdução ao TBJ

Robert BoylestadDigital Electronics 2004 by Pearson Education

Chapter 3:Bipolar Junction Transistors


Transistor ConstructionThere are two types of transistors: pnp and npn-type.

Note: the labeling of the transistor: E - EmitterB - BaseC - Collector


Transistor OperationWith the external sources (VEE and VCC) in the polarities as shown:

The E-B junction is forward-biased and the B-C junction is reverse biased.


Currents in a Transistor

Note that IC is comprised of two currents:

[Formula 3.1]

[Formula 3.2]

BCE III +=

COminorityCmajorityC III +=


Common Base Configuration

The base is common to both input (emitter base) and output (collector base) of the transistor.


Input Characteristics for a Common-Base Amplifier

This demonstrates the input current IE to input voltage VBE for various levels of output voltage VCB.


Output Characteristics for a Common-Base Amplifier

This demonstrates the output current IC to an output voltage VCB for various levels of input current IE.


3 Regions of Operation

ActiveOperating range of the amplifier.

CutoffThe amplifier is basically off. There is voltage but little current.

SaturationThe amplifier is full on. There is little voltage but lots of current.


Approximations

[Formula 3.3]

[Formula 3.4]

EC II

0.7VBE =


Alpha ()

Ideally = 1, but in reality it is between 0.9 and 0.998.

Alpha () in AC mode:

Alpha () relates the DC currents IC to IE :

[Formula 3.5]

[Formula 3.6]

E

C

IIdc =

constantVII

acCBE

C

=

=


Transistor AmplificationThe AC input is amplified.

[Fig. 3.12]

IC IE so IL Ii = 10mA

VL = IL * R = (10mA)(5k) = 50V

Voltage Gain (AV):

10mA20

200mVRiViIiIE ====

250200mV

50VViVAv L ===


Common Emitter Configuration

The Emitter is common to both input (base-emitter) and output (collector-emitter).The input is on the Base and the output is on the Collector.


Characteristics of Common-Emitter

Collector characteristics = output characteristics.Base characteristics = input characteristics.


Amplifier Currents

IE = IC + IB

IC = IE


Actual Amplifier Currents

IC = IE + ICBO

ICBO = minority collector current. This is usually so small that it can be ignored, except in high power transistors and in high temperature environments.

[Formula 3.9]

When IB = 0A the transistor is in cutoff, but there is some minority current flowing called ICEO.

A 0I1II BCBOCEO

=

=


Beta ()

In DC mode: [Formula 3.10]

In AC mode: [Formula 3.11]

indicates the amplification factor of a transistor. ( is sometimes referred to as hfe, a term used in transistor modeling calculations)

B

C

IIdc =

constantVII

ac CEB

C=

=


Determining beta () from a Graph

Note: AC = DC1087.5)(forVCE

A252.7mADC ===

1007.5)(forVCE010

1mA)020330(

2.2mA)(3.2mAAC ===

=


Relationship between and Both indicate an amplification factor.

[Formula 3.12a]

[Formula 3.12b]

1+

=

1=


provides a Relationship between Currents

[Formula 3.14]

[Formula 3.15]

BC II =

BE 1)I(I +=


Common Collector Configuration

The input on the Base and the output is on the Emitter.


Characteristics of Common CollectorThe Characteristics are similar to those of the Common-Emitter. Except the vertical axis is IE.

IE

IB1IB2IB3

VCE


Limitations of Operation for Each Configuration

Note: VCE is at maximum and IC is at minimum (ICmax=ICEO) in the cutoff region. IC is at maximum and VCE is at minimum (VCE max = VCEsat = VCEO) in the saturation region.The transistor operates in the active region between saturation and cutoff.


Power of Dissipation

Common Base: [Formula 3.18]

Common Emitter: [Formula 3.16]

Common Collector: a

CCBC IVmaxP =

CCEC IVmaxP =

ECEC IVmaxP =


Transistor Specification Sheet


Transistor Testing

1. Curve TracerProvides a graph of the characteristic curves.

2. DMMSome DMMs will measure DC or HFE.

3. Ohmmeter


Transistor Terminal Identification

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Introdução ao TBJ