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Neamen Microelectronics, 4e Chapter 5-1McGraw-Hill

Microelectronics Circuit Analysis and Design

Donald A. Neamen

Chapter 5

The Bipolar Junction Transistor

Neamen Microelectronics, 4e Chapter 5-2McGraw-Hill

In this chapter, we will:

� Discuss the physical structure and operation of the bipolar junction transistor.

� Understand the dc analysis and design techniques of bipolar transistor circuits.

� Examine three basic applications of bipolar transistor circuits.

Neamen Microelectronics, 4e Chapter 5-3McGraw-Hill

Cross Section of Integrated Circuit npn Transistor

Neamen Microelectronics, 4e Chapter 5-4McGraw-Hill

Modes of Operation

� Forward-Active

�B-E junction is forward biased

�B-C junction is reverse biased

� Saturation

�B-E and B-C junctions are forward biased

� Cut-Off

�B-E and B-C junctions are reverse biased

� Inverse-Active (or Reverse-Active)

�B-E junction is reverse biased

�B-C junction is forward biased

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Neamen Microelectronics, 4e Chapter 5-5McGraw-Hill

npn BJT in Forward-Active

Neamen Microelectronics, 4e Chapter 5-6McGraw-Hill

Electrons and Holes in npn BJT

Neamen Microelectronics, 4e Chapter 5-7McGraw-Hill

Electrons and Holes in npn BJT

With a + potential across the C-E terminals.

If a positive voltage is applied to the base

(>0.6V), the B-E pn junction is forward biased.

The E side electrons cross the pn junction and

many electrons are swept to the positive C side

voltage (since the p base material is thin). This

results in electron flow from E to C.

(Conventional current flow from C to E).

Neamen Microelectronics, 4e Chapter 5-8McGraw-Hill

Circuit Symbols and Current Conventions

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Neamen Microelectronics, 4e Chapter 5-9McGraw-Hill

Current Relationships

α

αβ

α

β

β

−=

=

+=

=

+=

1

)1(

EC

BE

BC

BCE

ii

ii

ii

iii

Neamen Microelectronics, 4e Chapter 5-10McGraw-Hill

Common-Emitter Configurations

Neamen Microelectronics, 4e Chapter 5-11McGraw-Hill

Current-Voltage Characteristics of a Common-Emitter Circuit

Neamen Microelectronics, 4e Chapter 5-12McGraw-Hill

Early Voltage/Finite Output Resistance

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Neamen Microelectronics, 4e Chapter 5-13McGraw-Hill

DC Equivalent Circuit for npn Common Emitter

Neamen Microelectronics, 4e Chapter 5-14McGraw-Hill

DC Equivalent Circuit for pnp Common Emitter

Neamen Microelectronics, 4e Chapter 5-15McGraw-Hill

Load Line

Neamen Microelectronics, 4e Chapter 5-16McGraw-Hill

Problem-Solving Technique:Bipolar DC Analysis

1. Assume that the transistor is biased in forward active mode

a. VBE = VBE(on), IB > 0, & IC = βIB

2. Analyze ‘linear’ circuit.

3. Evaluate the resulting state of transistor.

a. If VCE > VCE(sat), assumption is correct

b. If IB < 0, transistor likely in cutoff

c. If VCE < 0, transistor likely in saturation

4. If initial assumption is incorrect, make new assumption and return to Step 2.

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Neamen Microelectronics, 4e Chapter 5-17McGraw-Hill

Voltage Transfer Characteristic for npn Circuit

Neamen Microelectronics, 4e Chapter 5-18McGraw-Hill

Voltage Transfer Characteristic for pnp Circuit

Neamen Microelectronics, 4e Chapter 5-19McGraw-Hill

Common Emitter with Voltage Divider Biasing and Emitter Resistor

CCTH VRRRV )]/([ 212 +=

Neamen Microelectronics, 4e Chapter 5-20McGraw-Hill

Microelectronics Circuit Analysis and Design

Donald A. Neamen

Chapter 6

Basic BJT Amplifiers

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Neamen Microelectronics, 4e Chapter 5-21McGraw-Hill

In this chapter, we will:

� Understand the concept of an analog signal and the principle of a linear amplifier.

� Investigate how a transistor circuit can amplify a small, time-varying input signal.

� Discuss and compare the three basic transistor amplifier configurations.

Neamen Microelectronics, 4e Chapter 5-22McGraw-Hill

Common Emitter with Time-Varying Input

Neamen Microelectronics, 4e Chapter 5-23McGraw-Hill

IB Versus VBE

Characteristic

bB

T

beBQB iI

V

vIi +=+≅ )1(

Neamen Microelectronics, 4e Chapter 5-24McGraw-Hill

ac Equivalent Circuit for Common Emitter

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Neamen Microelectronics, 4e Chapter 5-25McGraw-Hill

Small-Signal Hybrid π Model for npn BJT

β

β

π

π

=

=

=

rg

I

Vr

V

Ig

m

CQ

T

T

CQ

m

Phasor signals are shown in parentheses.

Neamen Microelectronics, 4e Chapter 5-26McGraw-Hill

Small-Signal Equivalent Circuit Using Common-Emitter Current Gain

Neamen Microelectronics, 4e Chapter 5-27McGraw-Hill

Small-Signal Equivalent Circuit for npn Common Emitter circuit

))((B

CmvRr

rRgA

+−=

π

π

Neamen Microelectronics, 4e Chapter 5-28McGraw-Hill

Problem-Solving Technique: BJT AC Analysis

1. Analyze circuit with only dc sources to find Q point.

2. Replace each element in circuit with small-signal model, including the hybrid π model for the transistor.

3. Analyze the small-signal equivalent circuit after setting dc source components to zero.

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Neamen Microelectronics, 4e Chapter 5-29McGraw-Hill

Hybrid π Model for npn with Early Effect

CQ

Ao

I

Vr =

Neamen Microelectronics, 4e Chapter 5-30McGraw-Hill

Hybrid p Model for pnp with Early Effect

Neamen Microelectronics, 4e Chapter 5-31McGraw-Hill

Expanded Hybrid π Model for npn

Neamen Microelectronics, 4e Chapter 5-32McGraw-Hill

h-Parameter Model for npn

β

µπ

=

+=

fe

bie

h

rrrh

o

oe

re

rrh

r

rh

11+

+=

≅

µ

µ

π

β

��� = ℎ���� + ℎ���� = ℎ���� + ℎ����

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Neamen Microelectronics, 4e Chapter 5-33McGraw-Hill

Common Emitter with Voltage-Divider Bias and a Coupling Capacitor

Neamen Microelectronics, 4e Chapter 5-34McGraw-Hill

Small-Signal Equivalent Circuit –Coupling Capacitor Assumed a Short

Neamen Microelectronics, 4e Chapter 5-35McGraw-Hill

npn Common Emitter with Emitter Resistor

Neamen Microelectronics, 4e Chapter 5-36McGraw-Hill

Small-Signal Equivalent Circuit:Common Emitter with RE

)()1(

)1(

21

Si

i

E

Cv

ibi

Eib

RR

R

Rr

RA

RRRR

RrR

+++

−=

=

++=

β

β

β

π

π

�� = −������ = ��� (�� + � + 1 ��)�

��� = ��(��

�� + ��)

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Neamen Microelectronics, 4e Chapter 5-37McGraw-Hill

RE and Emitter Bypass Capacitor

Neamen Microelectronics, 4e Chapter 5-38McGraw-Hill

dc AND ac Load Lines: RE and Emitter Bypass Capacitor

Neamen Microelectronics, 4e Chapter 5-39McGraw-Hill

Problem-Solving Technique:Maximum Symmetrical Swing

1. Write dc load line equation that relates ICQ

and VCEQ.

2. Write ac load line equations that relates ic and vce

3. In general, ic = ICQ – IC(min), where IC(min) is zero or other minimum collector current.

4. In general, vce = VCEQ – VCE(min), where VCE(min) is some specified minimum collector-emitter voltage.

5. Combine above 4 equations to find optimum ICQ and VCEQ.

Neamen Microelectronics, 4e Chapter 5-40McGraw-Hill

Common-Collector or Emitter-Follower Amplifier

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Neamen Microelectronics, 4e Chapter 5-41McGraw-Hill

Small-Signal Equivalent Circuit:Emitter Follower

Neamen Microelectronics, 4e Chapter 5-42McGraw-Hill

Output Resistance:Emitter Follower

oEo rRr

Rβ

π

+=

1