Chapter 06 BJTAmplifiers 1

Microelectronics Circuit Analysis and DesignCircuit Analysis and Design

Donald A. Neamen

Chapter 6Chapter 6

Basic BJT Amplifiers

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

In this chapter, we will:p ,

Understand the concept of an analog signal and the principle of a linear amplifier.e p p e o a ea a p e Investigate how a transistor circuit can amplify a

small, time-varying input signal. Discuss and compare the three basic transistor Discuss and compare the three basic transistor

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

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

Analyze multitransistor or multistage amplifiers. Analyze multitransistor or multistage amplifiers. circuit. Understand the concept of signal power

gain in an amplifier


Common Emitter Common Emitter with Time-Varying Input


IB Versus VBEIB Versus VBECharacteristic

be iIvIi )1( bBT

beBQB iI

VIi )1(


ac Equivalent Circuit qfor Common Emitter


Small-Signal Hybrid Model for npn BJT Small Signal Hybrid Model for npn BJT

VI

gT

CQm

I

Vr T

T

rg

I

m

CQ

gm

Phasor signals are shown in parentheses.


Small-Signal Equivalent Circuit Using Small Signal Equivalent Circuit Using Common-Emitter Current Gain


S ll Si l E i l t Ci it f Small-Signal Equivalent Circuit for npn Common Emitter circuit

))((B

Cmv RrrRgA


B

Problem-Solving Technique: Problem Solving Technique: BJT AC Analysis

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

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

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


Transformation of ElementsElement DC Model AC Model

Resistor R R

Capacitor Open C

Inductor Short L

Diode +V, rf – rd = VT/ID, f d T/ D

Independent Constant Voltage Source

+ VS - Short

Independent Constant Current Source

IS Open


Hybrid Model for npn with Early EffectHybrid Model for npn with Early Effect

Ao I

Vr CQI


Hybrid p Model for pnp with Early EffectHybrid p Model for pnp with Early Effect


Expanded Hybrid Model for npnExpanded Hybrid Model for npn


h-Parameter Model for npnh Parameter Model for npn

bie

h

rrrh re rrh

feh

ooe rr

h 11


T-Model of an npn BJTT Model of an npn BJT


4 Equivalent 2-port Networks4 Equivalent 2 port Networks

Voltage Amplifier

Current Amplifier


4 Equivalent 2-port Networks4 Equivalent 2 port Networks

Transconductance Amplifier

Transresistance Amplifier


Common Emitter with Voltage-Divider gBias and a Coupling Capacitor


Small-Signal Equivalent Circuit –Small Signal Equivalent Circuit Coupling Capacitor Assumed a Short


npn Common Emitter pwith Emitter Resistor


Small-Signal Equivalent Circuit:Small Signal Equivalent Circuit:Common Emitter with RE

)( )1(

21 ibi

Eib

RRRRRrR

)()1( Si

i

E

Cv RR

RRr

RA


RE and Emitter Bypass CapacitorRE and Emitter Bypass Capacitor


Current Source Biasing and Current Source Biasing and Nonlinear Load


Small-Signal Equivalent Circuit with Small Signal Equivalent Circuit with Current Biasing and Nonlinear Load

)( comv rrgA


dc AND ac Load Lines: dc AND ac Load Lines: RE and Emitter Bypass Capacitor


Problem-Solving Technique:g qMaximum Symmetrical Swing

1. Write dc load line equation that relates ICQ and VCEQ.

2 W it l d li ti th t l t i 2. Write ac load line equations that relates ic and vce

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

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

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


Common-Collector Common Collector or Emitter-Follower Amplifier


Small-Signal Equivalent Circuit:Small Signal Equivalent Circuit:Emitter Follower

))(1(

21 ibi

Eoib

RRRR

RrrR

)())(1(

))(1(

Si

i

Eo

Eov

ib

RRR

RrrRr

A


))(( SiEo

Output Resistance:Output Resistance:Emitter Follower

oEo rRrR oEo 1


Common-Base AmplifierCommon Base Amplifier


Small-Signal Equivalent Circuit:Small Signal Equivalent Circuit:Common Base

)( LCmv RRgA

]1

)[( ELC

Cmi Rr

RRRgA


Input Resistance:Input Resistance:Common Base

R /(1 )Rie = r/(1+)


Output Resistance:Output Resistance:Common Base

R = RRO = RC


Common Emitter Cascade AmplifierCommon Emitter Cascade Amplifier


Small-Signal Equivalent Circuit:Small Signal Equivalent Circuit:Cascade Amplifier


Darlington PairDarlington Pair

21iA


Cascode AmplifierCascode Amplifier


Small-Signal Equivalent Circuit:Small Signal Equivalent Circuit:Cascode Amplifier

)(1 LCmv RRgA


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Chapter 06 BJTAmplifiers 1