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Analog Electronics Tutorial Series Bipolar Junction Transistors. C. BJTs. B. E. Kristin Ackerson, Virginia Tech EE Spring 2002. Table of Contents. The Bipolar Junction Transistor_______________________________slide 3 BJT Relationships – Equations________________________________slide 4

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slide1

Analog Electronics Tutorial Series

Bipolar Junction Transistors

C

BJTs

B

E

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide2

Table of Contents

The Bipolar Junction Transistor_______________________________slide 3

BJT Relationships – Equations________________________________slide 4

DC and DC  _____________________________________________slides 5

BJT Example_______________________________________________slide 6

BJT Transconductance Curve_________________________________slide 7

Modes of Operation_________________________________________slide 8

Three Types of BJT Biasing__________________________________slide 9

Common Base______________________slide 10-11

Common Emitter_____________________slide 12

Common Collector___________________slide 13

Eber-Moll Model__________________________________________slides 14-15

Small Signal BJT Equivalent Circuit__________________________slides 16

The Early Effect___________________________________________slide 17

Early Effect Example_______________________________________slide 18

Breakdown Voltage________________________________________slide 19

Sources__________________________________________________slide 20

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide3

The BJT – Bipolar Junction Transistor

 Note: It will be very helpful to go through the Analog Electronics Diodes Tutorial to get information on doping, n-type and p-type materials.

The Two Types of BJT Transistors:

npn

pnp

n

p

n

p

n

p

E

C

E

C

C

C

Cross Section

Cross Section

B

B

B

B

Schematic Symbol

Schematic Symbol

E

E

  • Collector doping is usually ~ 106
  • Base doping is slightly higher ~ 107 – 108
  • Emitter doping is much higher ~ 1015

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide4

BJT Relationships - Equations

IE

IC

IE

IC

-

VCE

+

+

VEC

-

E

C

E

C

-

-

+

+

VBE

VBC

IB

VEB

VCB

IB

+

+

-

-

B

B

npn

IE = IB + IC

VCE = -VBC + VBE

pnp

IE = IB + IC

VEC = VEB - VCB

Note: The equations seen above are for the transistor, not the circuit.

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide5

DC and DC 

 = Common-emitter current gain

 = Common-base current gain

 = IC = IC

IB IE

The relationships between the two parameters are:

 =   = 

 + 1 1 - 

Note:  and  are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide6

BJT Example

Using Common-Base NPN Circuit Configuration

C

  • Given: IB = 50  A , IC = 1 mA
  • Find: IE ,  , and 
  • Solution:
  • IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA
  • = IC / IB = 1 mA / 0.05 mA = 20

 = IC / IE = 1 mA / 1.05 mA = 0.95238

 could also be calculated using the value of  with the formula from the previous slide.

 =  = 20 = 0.95238

 + 1 21

IC

VCB

+

_

IB

B

VBE

IE

+

_

E

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide7

BJT Transconductance Curve

Typical NPN Transistor 1

Collector Current:

IC =  IESeVBE/VT

Transconductance:

(slope of the curve)

gm =  IC /  VBE

IES = The reverse saturation current

of the B-E Junction.

VT = kT/q = 26 mV (@ T=300K)

 = the emission coefficient and is

usually ~1

IC

8 mA

6 mA

4 mA

2 mA

VBE

0.7 V

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide8

Modes of Operation

Active:

  • Most important mode of operation
  • Central to amplifier operation
  • The region where current curves are practically flat

Saturation:

  • Barrier potential of the junctions cancel each other out causing a virtual short

Cutoff:

  • Current reduced to zero
  • Ideal transistor behaves like an open switch

* Note: There is also a mode of operation called inverse active, but it is rarely used.

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide9

Three Types of BJT Biasing

Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions.

Common-Base Biasing (CB) : input = VEB & IE

output = VCB & IC

Common-Emitter Biasing (CE): input = VBE & IB

output = VCE & IC

Common-Collector Biasing (CC): input = VBC & IB

output = VEC & IE

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide10

Common-Base

Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of how the BJT works.

Emitter-Current Curves

IC

Active Region

IE

Saturation Region

Cutoff

IE = 0

VCB

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide11

Common-Base

VCE

IC

IE

C

E

VCB

VBE

IB

+

_

+

_

B

VCB

VBE

Circuit Diagram: NPN Transistor

The Table Below lists assumptions that can be made for the attributes of the common-base biased circuit in the different regions of operation. Given for a Silicon NPN transistor.

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide12

Common-Emitter

Circuit Diagram

Collector-Current Curves

VCE

IC

IC

+

_

Active Region

VCC

IB

IB

VCE

Saturation Region

Cutoff Region

IB = 0

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide13

Common-Collector

Emitter-Current Curves

IE

The Common-Collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at IC as a function of VCE and IB we are looking at IE.

Also, since  ~ 1, and  = IC/IE that means IC~IE

Active Region

IB

VCE

Saturation Region

Cutoff Region

IB = 0

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide14

Eber-Moll BJT Model

The Eber-Moll Model for BJTs is fairly complex, but it is valid in all regions of BJT operation. The circuit diagram below shows all the components of the Eber-Moll Model:

IE

IC

E

C

RIC

RIE

IF

IR

IB

B

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide15

Eber-Moll BJT Model

R = Common-base current gain (in forward active mode)

F = Common-base current gain (in inverse active mode)

IES = Reverse-Saturation Current of B-E Junction

ICS = Reverse-Saturation Current of B-C Junction

IC = FIF – IR IB = IE - IC

IE = IF - RIR

IF = IES [exp(qVBE/kT) – 1] IR = IC [exp(qVBC/kT) – 1]

 If IES & ICS are not given, they can be determined using various

BJT parameters.

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide16

Small Signal BJT Equivalent Circuit

The small-signal model can be used when the BJT is in the active region. The small-signal active-region model for a CB circuit is shown below:

iB

iC

B

C

r

iB

r = ( + 1) * VT

IE

iE

E

@  = 1 and T = 25C

r = ( + 1) * 0.026

IE

Recall:

 = IC / IB

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide17

The Early Effect (Early Voltage)

IC

Note: Common-Emitter Configuration

IB

-VA

VCE

Green = Ideal IC

Orange = Actual IC (IC’)

IC’ = IC VCE + 1

VA

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide18

Early Effect Example

Given: The common-emitter circuit below with IB = 25A, VCC = 15V,  = 100 and VA = 80.

Find: a) The ideal collector current

b) The actual collector current

Circuit Diagram

VCE

IC

  • = 100 = IC/IB

a)

IC = 100 * IB = 100 * (25x10-6 A)

IC = 2.5 mA

+

_

VCC

IB

b) IC’ = IC VCE + 1 = 2.5x10-3 15 + 1 = 2.96 mA

VA 80

IC’ = 2.96 mA

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide19

Breakdown Voltage

The maximum voltage that the BJT can withstand.

BVCEO = The breakdown voltage for a common-emitter biased circuit. This breakdown voltage usually ranges from ~20-1000 Volts.

BVCBO = The breakdown voltage for a common-base biased circuit. This breakdown voltage is usually much higher than BVCEO and has a minimum value of ~60 Volts.

  • Breakdown Voltage is Determined By:
  • The Base Width
  • Material Being Used
  • Doping Levels
  • Biasing Voltage

Kristin Ackerson, Virginia Tech EE

Spring 2002

slide20

Sources

Dailey, Denton. Electronic Devices and Circuits, Discrete and Integrated. Prentice Hall, New Jersey: 2001. (pp 84-153)

1 Figure 3.7, Transconductance curve for a typical npn transistor, pg 90.

Liou, J.J. and Yuan, J.S. Semiconductor Device Physics and Simulation. Plenum Press, New York: 1998.

Neamen, Donald. Semiconductor Physics & Devices. Basic Principles. McGraw-Hill, Boston: 1997. (pp 351-409)

Web Sites

http://www.infoplease.com/ce6/sci/A0861609.html

Kristin Ackerson, Virginia Tech EE

Spring 2002