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Neamen Microelectronics, 4e Chapter 6-1McGraw-Hill

Microelectronics Circuit Analysis and Design

Donald A. Neamen

Chapter 6

Basic BJT Amplifiers

Neamen Microelectronics, 4e Chapter 6-2McGraw-Hill

In this chapter, we will:

� Understand the principle of a linear amplifier.

� Discuss and compare the three basic transistor amplifier configurations.� the common-emitter amplifier.� the emitter-follower amplifier.� the common-base amplifier.

� Analyze multi-transistor amplifiers.

Neamen Microelectronics, 4e Chapter 6-3McGraw-Hill

Common Emitter with Time-Varying Input

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

ac Equivalent Circuit for Common Emitter

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

IB Versus VBE

Characteristic

bB

T

beBQB iI

V

vIi +=+≅ )1(

�� � ���exp��

��� �

� � ≅ 1 + x +..

��� � ��� � ��

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

ac Equivalent Circuit

Neamen Microelectronics, 4e Chapter 6-7McGraw-Hill

ac Equivalent Circuit

Neamen Microelectronics, 4e Chapter 6-8McGraw-Hill

ac Equivalent Circuit

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Neamen Microelectronics, 4e Chapter 6-9McGraw-Hill

ac Equivalent Circuit

Neamen Microelectronics, 4e Chapter 6-10McGraw-Hill

Small-Signal Equivalent Circuit Using Common-Emitter Current Gain

Neamen Microelectronics, 4e Chapter 6-11McGraw-Hill

ac Equivalent Circuit

Neamen Microelectronics, 4e Chapter 6-12McGraw-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.

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Neamen Microelectronics, 4e Chapter 6-13McGraw-Hill

Small-Signal Equivalent Circuit for npn Common Emitter circuit

))((B

CmvRr

rRgA

+−=

π

π

Neamen Microelectronics, 4e Chapter 6-14McGraw-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.

Neamen Microelectronics, 4e Chapter 6-15McGraw-Hill

Transformation of Elements

Element DC Model AC Model

Resistor R R

Capacitor Open C

Inductor Short L

Diode +Vγ, rf – rd = VT/ID

Independent Constant Voltage Source

+ VS - Short

Independent Constant Current Source

IS Open

Neamen Microelectronics, 4e Chapter 6-16McGraw-Hill

Hybrid π Model for npn with Early Effect

CQ

Ao

I

Vr =

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Neamen Microelectronics, 4e Chapter 6-17McGraw-Hill

Hybrid p Model for pnp with Early Effect

All voltage and current polarities are reversed.Gain equation are the SAME.

)||( oCmo rRvgv π=

)||( oCmo rRgv =

)(B

sRr

rvv

+−=

π

ππ

)||( oCbo rRBiv =

)(B

sb

Rr

vi

+

−=

π

Neamen Microelectronics, 4e Chapter 6-18McGraw-Hill

h-Parameter Model for npn

loopoutputatKCLVhIhI

loopinputatKVLVhIhV

ceoebfec

cerebiebe

+=

+=

Neamen Microelectronics, 4e Chapter 6-19McGraw-Hill

h-Parameter Model for npn

β

µπ

=

+=

fe

bie

h

rrrh

o

oe

re

rrh

r

rh

11+

+=

≅

µ

µ

π

β

Neamen Microelectronics, 4e Chapter 6-20McGraw-Hill

Common Emitter with Voltage-Divider Bias and a Coupling Capacitor

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Neamen Microelectronics, 4e Chapter 6-21McGraw-Hill

Small-Signal Equivalent Circuit – Coupling Capacitor assumed a Short

)(

21

Si

iomv

coo

i

RR

RRgA

RrR

rRRR

+−=

=

= π

Neamen Microelectronics, 4e Chapter 6-22McGraw-Hill

npn Common Emitter with Emitter Resistor

Neamen Microelectronics, 4e Chapter 6-23McGraw-Hill

Small-Signal Equivalent Circuit:Common Emitter with RE

)()1(

)1(/

21

Si

i

E

Cv

ibi

Ebib

RR

R

Rr

RA

RRRR

RrIVinR

+++

−=

=

++==

β

β

β

π

π

�� � −�����

��� � �� �� � ���������

Neamen Microelectronics, 4e Chapter 6-24McGraw-Hill

RE and Emitter Bypass Capacitor

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Neamen Microelectronics, 4e Chapter 6-25McGraw-Hill

DC and AC Load Lines: RE and Emitter Bypass Capacitor

Neamen Microelectronics, 4e Chapter 6-26McGraw-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 6-27McGraw-Hill

Common-Collector or Emitter-Follower Amplifier

Neamen Microelectronics, 4e Chapter 6-28McGraw-Hill

Small-Signal Equivalent Circuit:Emitter Follower

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Neamen Microelectronics, 4e Chapter 6-29McGraw-Hill

Small-Signal Equivalent Circuit:Emitter Follower

Typically a large value due to the reflected impedance

Neamen Microelectronics, 4e Chapter 6-30McGraw-Hill

S-S Equivalent :Emitter Follower

1)())(1(

))(1(≅

+++

+=

Si

i

Eo

Eo

vRR

R

Rrr

RrA

β

β

π

Neamen Microelectronics, 4e Chapter 6-31McGraw-Hill

Output Resistance: Emitter Follower

oEo rRr

Rβ

π

+=

1Typically a low value due to the reflected impedance

x

xo

I

VR =

Neamen Microelectronics, 4e Chapter 6-32McGraw-Hill

Emitter Follower Current Gain

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Neamen Microelectronics, 4e Chapter 6-33McGraw-Hill

Emitter Follower Current Gain

Neamen Microelectronics, 4e Chapter 6-34McGraw-Hill

Common-Base Amplifier

Neamen Microelectronics, 4e Chapter 6-35McGraw-Hill

Small-Signal Equivalent Circuit:Common Base

]1

)[(

)(

E

LC

Cmi

LCmv

Rr

RR

RgA

RRgA

βπ

++=

≅

Neamen Microelectronics, 4e Chapter 6-36McGraw-Hill

Small-Signal Equivalent Circuit:Common Base

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Neamen Microelectronics, 4e Chapter 6-37McGraw-Hill

Small-Signal Equivalent Circuit:Common Base

Neamen Microelectronics, 4e Chapter 6-38McGraw-Hill

Common Base

Neamen Microelectronics, 4e Chapter 6-39McGraw-Hill

Common Base

Neamen Microelectronics, 4e Chapter 6-40McGraw-Hill

Common Base

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Neamen Microelectronics, 4e Chapter 6-41McGraw-Hill

Neamen Microelectronics, 4e Chapter 6-42McGraw-Hill

Common Emitter Cascade Amplifier

Neamen Microelectronics, 4e Chapter 6-43McGraw-Hill

Small-Signal Equivalent Circuit:Cascade Amplifier

Neamen Microelectronics, 4e Chapter 6-44McGraw-Hill

Darlington Pair

21ββ≅iA

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Neamen Microelectronics, 4e Chapter 6-45McGraw-Hill

Cascode Amplifier

Neamen Microelectronics, 4e Chapter 6-46McGraw-Hill

Small-Signal Equivalent Circuit:Cascode Amplifier

)(1 LCmv RRgA −≅