MALVINO

ElectronicPRINCIPLES

SIXTH EDITION

Chapter 6

Bipolar Transistors

COLLECTOR (medium doping)

BASE (light doping)

EMITTER (heavy doping)

N

P

N

The bipolar junction transistor has 3 doped regions.

N

P

N

VCE

VCC

RC

RB

VBB

VBE

In a properly biased NPN transistor, the emitter electrons diffuse into the base and then go on to the collector.

Conventional flow Electron flow

IC

IB

IE

IC

IB

IE

IE = IC + IB IC IE IB << IC

dc = IC

IE

dc = IC

IB

VCE

VCC

RC

RB

VBB

VBE

The common emitter connection has two loops:the base loop and the collector loop.

Subscript notation

• When the subscripts are the same, the voltage represents a source (VCC).

• When the subscripts are different, the voltage is between two points (VCE).

• Single subscripts are used for node voltages with ground serving as the reference (VC).

VCE

VCC

RC

RB

VBB

VBE

The base circuit is usually analyzed with the same approximation used for diodes.

IB =VBB - VBE

RB

0 2 4 6 8 10 12 14 16 18

2468

101214

VCE in Volts

IC in mA

A graph of IC versus VCE

20 A

0 A

100 A

80 A

60 A

40 A

(Note that each new value of IB presents a new curve.)

This set of curves is also called a family of curves.

Regions of operation

• Cutoff - - - used in switching applications

• Active - - - used for linear amplification

• Saturation - - - used in switching applications

• Breakdown - - - can destroy the transistor

Transistor circuit approximations

• First: treat the base-emitter diode as ideal and use IB to determine IC.

• Second: correct for VBE and use IB to determine IC.

• Third (and higher): correct for bulk resistance and other effects. Usually accomplished by computer simulation.

dcIB VCEVBE = 0.7 V

The second approximation:

VCC

RC

RB

VBB

VBE = 0.7 V

IB =VBB - VBE

RB

IB =5 V - 0.7 V

100 k

5 V

100 k

= 43 A

VCC

RC

RB

VBB 5 V

100 k

IB = 43 A

dc = 100

IC = dc IB

IC = 100 x 43 A = 4.3 mA

VCC

RC

RB

VBB 5 V

100 k

IB = 43 A

IC = 4.3 mA

1 k

12 V

VRC = IC x RC

VRC = 4.3 mA x 1 k= 4.3 V

VCC

RC

RB

VBB 5 V

100 k

IB = 43 A

IC = 4.3 mA

1 k

12 V

VCE = VCC - VRC

VCE

VCE = 12 V - 4.3 V = 7.7 V

Typical Breakdown Ratings

• VCB = 60 V

• VCEO = 40 V

• VEB = 6 V

• Note: these are reverse breakdown ratings

0

2468

101214

VCE in Volts

IC in mA

50

A graphic view of collector breakdown

Typical Maximum Ratings

• IC = 200 mA dc

• PD = 250 mW (for TA = 60 oC)

• PD = 350 mW (for TA = 25 oC)

• PD = 1 W (for TC = 60 oC)

Typical “On Characteristics”

IC in mA hFE(min) hFE(max)

0.1 40 ___

1 70 ___

10 100 300

50 60 ___

100 30 ___

Troubleshooting

• Look for gross voltage errors.

• First approximation and mental estimates will usually suffice.

• Resistors don’t short but circuit boards can.

• Circuit boards can and do open.

• Junctions can and do short.

• Junctions can and do open.