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Microelectronics

Circuit Analysis and Design

Donald A. Neamen

Chapter 6

Basic BJT Amplifiers

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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.

Analyze the common-emitter amplifier. Understand the ac load line & determine the

maximum symmetrical swing of the output.Analyze the emitter-follower amplifier.

Analyze the common-base amplifier. Analyze multitransistor or multistage amplifiers. Understand the concept of signal power gain in an

amplifier circuit.

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Common Emitterwith Time-Varying Input

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IBVersus VBECharacteristic

bB

T

beBQB iI

V

vIi )1(

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ac Equivalent Circuitfor Common Emitter

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Small-Signal Hybrid pModel for npn BJT

p

p

rg

I

Vr

V

Ig

m

CQ

T

T

CQ

m

Phasor signals are shown in parentheses.

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Small-Signal Equivalent Circuit UsingCommon-Emitter Current Gain

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Small-Signal Equivalent Circuit for

npn Common Emitter circuit

))((B

CmvRr

rRgA

p

p

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Problem-Solving Technique:BJT AC Analysis

1. Analyze circuit with only dc sources to find Qpoint.

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

3. Analyze the small-signal equivalent circuit

after setting dc source components to zero.

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Transformation of Elements

Element DC Model AC Model

Resistor R R

Capacitor Open C

Inductor Short L

Diode +Vg, rf rd= VT/ID

Independent Constant

Voltage Source

+ VS - Short

Independent ConstantCurrent Source

IS Open

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Hybrid p Model for npn with Early Effect

CQ

Ao

IVr

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Hybrid p Model for pnp with Early Effect

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Expanded Hybrid pModel for npn

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h-Parameter Model for npn

p

fe

bie

h

rrrh

o

oe

re

rrh

r

rh

11

p

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T-Model of an npn BJT

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4 Equivalent 2-port Networks

Voltage Amplifier

Current Amplifier

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4 Equivalent 2-port Networks

TransconductanceAmplifier

Transresistance

Amplifier

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Common Emitter with Voltage-DividerBias and a Coupling Capacitor

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Small-Signal Equivalent Circuit Coupling Capacitor Assumed a Short

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npn Common Emitterwith Emitter Resistor

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Small-Signal Equivalent Circuit:Common Emitter with RE

)()1(

)1(

21

Si

i

E

C

v

ibi

Eib

RR

R

Rr

RA

RRRR

RrR

p

p

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REand Emitter Bypass Capacitor

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Problem-Solving Technique:Maximum Symmetrical Swing

1. Write dc load line equation that relates ICQand VCEQ.

2. Write ac load line equations that relates icand vce

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

4. In general, vce= VCEQVCE(min), whereVCE(min) is some specified minimum

collector-emitter voltage.

5. Combine above 4 equations to find optimumICQ and VCEQ.

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Common-Collectoror Emitter-Follower Amplifier

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Small-Signal Equivalent Circuit:Emitter Follower

)())(1(

))(1(

))(1(

21

Si

i

Eo

Eo

v

ibi

Eoib

RR

R

Rrr

RrA

RRRR

RrrR

p

p

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Output Resistance:Emitter Follower

oEo rRrRp

1

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Common-Base Amplifier

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Small-Signal Equivalent Circuit:Common Base

]1

)[(

)(

E

LC

Cmi

LCmv

Rr

RR

RgA

RRgA

p

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Input Resistance:Common Base

Rie= rp/(1+)

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Output Resistance:Common Base

RO = RC

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Common Emitter Cascade Amplifier

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Small-Signal Equivalent Circuit:Cascade Amplifier

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Darlington Pair

21

iA

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Cascode Amplifier

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Small-Signal Equivalent Circuit:Cascode Amplifier

)(1 LCmv RRgA