Microelectronic Circuit Design McGraw-Hill Chapter 5 Bipolar Junction Transistors Microelectronic...

Microelectronic Circuit DesignMcGraw-Hill

Chapter 5Bipolar Junction Transistors

Microelectronic Circuit Design

Richard C. Jaeger

Travis N. Blalock


Circuit Representations for the Transport Models

In npn transistor (expressions analogous for pnp transistors), total current traversing base is modeled by a current source given by:

iTi

F i

RI

Sexp v

BEV

T

exp v

BCV

T

iB

IS

F

exp vBE

VT

1

IS

R

exp vBC

VT

1

Diode currents correspond directly to the two components of base current.


Operation Regions of Bipolar Transistors

Base-Emitter Junction

Base-Collector Junction

Reverse Bias Forward Bias

Forward BiasForward-Active

Region

(Good Amplifier)

Saturation Region

(Closed Switch)

Reverse BiasCutoff Region

(Open Switch)

Reverse-Active Region

(Poor Amplifier)

Binary Logic States


i-v Characteristics of Bipolar Transistor: Common-Emitter Output Characteristics

For iB = 0, transistor is cutoff. If iB > 0, iC also increases.

For vCE > vBE, npn transistor is in forward-active region, iC = F iB is independent of vCE.

For vCE < vBE, transistor is in saturation.

For vCE < 0, roles of collector and emitter reverse.


i-v Characteristics of Bipolar Transistor: Common-Emitter Transfer Characteristic

Defines relation between collector current and base-emitter voltage of transistor.

Almost identical to transfer characteristic of pn junction diode

Setting vBC = 0 in the collector-current expression yields

iCI

Sexp v

BEV

T

1

Collector current expression has the same form as that of the diode equation


Simplified Forward-Active Region Model

In forward-active region, emitter-base junction is forward-biased and collector-base junction is reverse-biased. vBE > 0, vBC < 0If we assume that

then the transport model terminal current equations simplify to

vBE

4kTq

0.1V and vBC

4kTq

0.1V

iCI

Sexp v

BEV

T

IS

R

ISexp v

BEV

T

iE

IS

F

exp vBE

VT

IS

F

I

S

F

exp vBE

VT

iB

IS

F

exp vBE

VT

IS

F

I

S

R

I

S

F

exp vBE

VT

iC

FiE

iC

FiB

iE(

F1)i

B

BJT is often considered a current-controlled device, though fundamental forward-active behavior suggests a voltage- controlled current source.


Simplified Forward-Active Region Model(Example 1)

• Problem: Estimate terminal currents and base-emitter voltage• Given data: IS =10-16 A, F = 0.95, VBC = VB - VC = -5 V, IE = 100 A• Assumptions: Simplified transport model assumptions, room temperature operation, VT = 25.0 mV• Analysis: Current source forward-biases base-emitter diode, VBE > 0, VBC < 0, we know that transistor is in forward-active operation region.

IC

FI

E0.95100A95A

F

F

1 F

0.951 0.95

19

IB

IE

F1

100A20

5A

VBE

VTln

FI

E

IS

0.69V


Simplified Forward-Active Region Model (Example 2)

• Problem: Estimate terminal currents, base-emitter and base-collector voltages.• Given data: IS = 10-16 A, F = 0.95, VC = +5 V, IB = 100 A• Assumptions: Simplified transport model assumptions, room temperature operation, VT = 25.0 mV• Analysis: Current source causes base current to forward-bias base-emitter diode, VBE > 0, VBC <0, we know that transistor is in forward-active operation region.

IC

FI

B19100A1.90mA

IE(

F1)I

B20100A2.00mA

VBE

VTln

IC

IS

0.764V

VBC

VB V

CV

BE V

C 4.24V

Jaeger/Blalock4/26/07


Simplified Circuit Model for Forward-Active Region



VBE

8200IE V

EE0

IE 8.3V

82001.01 mA

IB

IE

F1

1.02mA51

19.8 A

IC

FI

B0.990 mA

VCE

VCC

IC

RC

( VBE

)

9 0.99mA(4.3K)0.75.44 V



iC

IS

R

exp vBC

VT

iE I

Sexp v

BCV

T

iB

IS

R

exp vBC

VT

iE

RiC

iE

RiB



IC 0.7V-(-9V)

82001.01 mA

IB

IC

R1

1.01mA2

0.505 mA

IEI

B0.505 mA

Microelectronic Circuit Design McGraw-Hill Chapter 5 Bipolar Junction Transistors Microelectronic...

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