ch03

Robert BoylestadDigital Electronics

Copyright ©2002 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

Chapter 3: Bipolar Junction Transistors




Transistor Construction

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

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




Transistor Operation

With 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

• Active Operating range of the amplifier.

• Cutoff The amplifier is basically off. There is voltage but little current.

• Saturation The 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

constantVIIac

CBE

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 B

CBOCEO




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

constantVIIac CE

B

C




Determining beta () from a Graph

Note: AC = DC

1087.5)(forVCEA25

2.7mADC

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 Collector

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

IE

IB1

IB2

IB3

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 Tracer Provides a graph of the characteristic curves.2. DMM Some DMM’s will measure DC or HFE.3. Ohmmeter




Transistor Terminal Identification

ch03

Documents

pearson education

upper saddle river

new jersey

output current ic

input emitter base

input baseemitter

base characteristics

transistor amplification