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.