ANALOGUE

ELECTRONICS

& CIRCUITS Short Course

Dr. Sohiful Anuar

1

OBJECTIVES

To become proficient in analog circuits To understand and analyze the Bipolar Junction

Transistor To analyze the single stage amplifier circuits in

term of their frequency response To understand the AC & DC analysis

2

COURSE CONTENTS

TOPIC 1 Basic Bipolar Junction Transistor (BJT)

TOPIC 2 BJT Operation Mode

TOPIC 3 BJT Single Stage Amplifier

TOPIC 4 Bipolar Transistor Biasing

TOPIC 5 Bipolar Transistor Configurations

TOPIC 6 DC Analysis

TOPIC 7 AC Analysis

TOPIC 8 Frequency Response

TOPIC 9 BJT Design Example

3

BASIC BIPOLAR JUNCTION TRANSISTOR (BJT)

TOPIC 1

4

Has 3 separately doped regions and 2 p-n junctions

Single p-n junctions has 2 modes of operation

Forward bias

Reverse bias

Both electrons & holes participate in the conduction process

Modern bipolar transistors replaced the germanium with Si & replaced the point contacts with two closely coupled p-n

junctions in the form of p-n-p & n-p-n structures

Bipolar junction transistor (BJT) used extensively in high-speed circuits, analog circuits and power applications

Overview of BJT

5

Ideal BJT Structure

6

Perspective view of a silicon p-n-p bipolar transistor.

3-D BJT Stucture

7

Actual BJT Cross Section

8

BJT Layout

9

BJT Schematic Symbol

10

BJT Schematic Symbol

11

BJT Collector Characteristic

12

Collector Characteristic

13

Base-Emitter Voltage Control

14

Transistor Action

15

Diffusion Currents

16

Transistor Breakdown Voltage (VCBO) and Effective Common-BaseCurrent Gain (F)

17

Cont

18

BJT Current

19

TBE Vve/

TBE Vv

SFEFC eIii/

BJT : Avalanche

20

Origin of F

21

Collector Current

Majority of E current is due to injection of electrons into B

(No. of electrons reaching the C per unit time no. of electrons injected into B) function of B-E voltage

IC

IC Independent of the reverse-biased B-C voltage

F 1 (but less than 1)

F : Common-base current gain

TBE Vv

SFEFC eIii/

TBE Vve/

22

Base Current

Holes from B flow across B-E junction into E

Electrons recombine with holes in the base

Total IB:

TBE Vv

B ei/

1

TBE Vv

B ei/

2

TBE Vv

B ei/

23

Base-emitter junction: forward biased

Most cases: vBE>>VT (thermal voltage), (-1) term is valid

IS : multplying constant (contains electrical parameters of the junction)

IS active B-E cross-sectional area

Typical values of IS 10-12 to 10-15

TBETBE VvSVvSE eIeIi // 1

Emitter Current

24

Electron and hole currents in an npn transistor biased in the forward-

active mode

iB

iB2 iB1

iC iE

Electron & Hole Currents

25

Ebers-Moll Equations

The emitter & collector currents in terms of

internal currents at two junction

26

Ebers-Moll Equivalent Circuit

27

Forward Active Region

28

Simplified Ebers-Moll

29

Cont

30

Transconductace, gm

31

Cont

32

BJT Current

33

Comparison with MOSFET

34

BJT Base Currents

35

Small Signal Current Gain

36

Input Resistance r

37

Output Resistance ro

38

Graphical Interpretation of ro

39

The Early Voltage

40

Cont

41

BJT Small-Signal Model

42

BJT Capacitances

43

Complete Small-Signal Model

44

Parasitic Elements in BJT

45

BJT Operation Modes

TOPIC 2

46

Depends on voltage polarity BASE-EMITTER JUNCTION

COLLECTOR-BASE JUNCTION

VBE FB

VCB RB

VCE

n

p

n

c

b

e

npn

VBE +ve FB

VCB +ve RB

VCE +ve

FORWARD ACTIVE MODE

+ VCE

0

+ IC

saturation

cut-off

E F BE T BC T

C F F F B

I I V V V V

FB RB

I I I

RB: Reverse-biased

FB: Forward-biased

BJT Forward Active Mode

47

BJT Saturation Mode

CEsatCEBEonBEsat

TBECBF

VVVV

FB

VVII

VBE FB

VCB FB

VCE(sat)

n

p

n

c

b

e

n-p-n

VBE +ve FB

VCB -ve FB

VCE +ve VCE(sat)

+ VCE 0

+ IC

saturation mode

cut-off

depends on voltage polarity BASE-EMITTER JUNCTION

COLLECTOR-BASE JUNCTION

FB: Forward-

biased

48

BJT Cut-off Mode

VBE RB

VCB RB

VCE

n

p

n

c

b

e

depends on voltage polarity BASE-EMITTER JUNCTION

COLLECTOR-BASE JUNCTION

ESFCSC

CSRESE

III

III

npn

VBE -ve RB

VCB +ve RB

VCE +ve

+ VCE 0

+ IC

cut-off mode

cut-off

basically leakage currents

49

Mode of Operation (npn Transistor)

-VBE

SWITCH ON

FORWARD ACTIVE REGION IB > 0 IC = F IB

VBE > VBEon

VCE > VCEsat

VBEon ~ 0.7V

b

e

c F IB

VBEon

VBE FB VCB RB

AMPLIFIER

CUT OFF REGION

IC = IB = IE ~ 0 VBE < VBEon (RB)

VCB < VCBon (RB)

b

e

c

VCE = ? V

NOT used very often

SWITCH OFF

SATURATION REGION

IB > 0 IC > 0 IC < F IB VBE > VBEon VCEsat ~ 0.8V

b

e

C

VCEsat

VBEsat

VBE FB VCB FB

INVERSE ACTIVE REGION

IB > 0 IE = R IB VBC > VBEon

VEC > VCEsat

VBEon ~ 0.7V

B

E

C

R IB VBEon

VBE RB VCB FB

+VBC

+VBE

-VBC

-VBE

50

BJT Single Stage Amplifiers

TOPIC 3

51

VCC DC voltage

powers the amplifier

sets the DC operating point

provides the energy for the output ac signal

DC bias potential divider sets the Q point

emitter resistor stabilises Qpoint

VCC

RC

RE RB2

RB1

LOAD ?

Amplifier Basic BJT Amplifier

52

RC

R

E

VCC

RB2

RB1 CC2

ensures generator does not affect the bias (Q point of transistor) transistor DC voltage does not affect the source

ensures the load does not affect transistor bias to provide only ac output to the load

RL vout(ac)

vin(ac)

RS

CC1

Signal & Load Coupling

53

VCC

RC

RE RB2

RB1

RL

VCEQ

RS

DC analysis : replace capacitors with open circuit

DC analysis : replace ac source with internal impedance

CC2 CC1

Vin(ac)

Basic DC Analysis

54

(a) Common-emitter circuit with an npn transistor

and (b) dc equivalent circuit, with piecewise linear

parameters

Assume B-E junction: forward biased V drop is the cut-in /

turn-on V [VBE (on)]

IC represented as a dependent I

source (function of IB)

Neglect reverse-biased junction leakage current & Early effect

B

BEBBB

R

onVVI

)(

BC II

CCCCCE RIVV

CECCCC VRIV

VBB>VBE(on) IB>0

VBB

Fig. 3.22: (a) base-emitter junction characteristics and the input load line and (b)

common- emitter transistor characteristics and the collector-emitter load line

-help us visualize the characteristic

of a transistor

Load Line

56

Fig. 3.19: Common-emitter circuit

Cont

57

Based on fig. 3.19 above,

Kirchoffs voltage law equation (around B-E loop):

B

BE

B

BBB

R

V

R

VI Both load line & quiescent IB change as either

or both VBB & RB change

Kirchoffs voltage law equation (around C-E loop):

CCCCCE RIVV

)(2

5 mAV

R

V

R

VI CE

C

CE

C

CC

C IC & VCE relationship represents DC load line

Cont

58

59

Bipolar Transistor Biasing

TOPIC 4

60

i. Single Base Resistor Biasing

ii. Emitter Biasing

iii.Voltage-divider Biasing

iv.Collector-feedback Biasing

BJT Bias Circuits

61

RC

RB

+VCC

VBE

+

-

B

BECC

BR

VVI

B

BECC

DCCR

VVI

CCCCCE RIVV

Single Base Resistor Biasing

62

CC (Coupling capacitor): acts as open circuit to DC- isolating signal source from DC IB

If input signal freq & CC : signal can be coupled thru CC to B with little attenuation

Fig. 3.50: (a) Common emitter circuit with a single bias resistor in the base, (b)

dc equivalent circuit

*See example

3.13 page 138

Cont

63

RC

RB

RE

+VCC

-VEE

ECCEECCCE RRIVVV

CCCCC RIVV

DCBE

BEEEC

RR

VVI

Emitter Biasing

64

+VCC

RE

RC R1

R2

CCB VRR

RV

21

2

E

BEBC

R

VVI

ECCCCCE RRIVV

Voltage-Divider Biasing & Bias Stability

65

(a) A common-emitter circuit with an emitter resistor and

voltage divider bias circuit in the base; (b) the dc circuit

with a Thevenin equivalent base circuit

RB

is replaced

added

CCTH VRRRV )]/([ 212

21 || RRRTH

EEQBETHBQTH RIonVRIV )(

BQEQ II )1(

ETH

BETHBQ

RR

onVVI

)1(

)(

ETH

BETHBQCQ

RR

onVVII

)1(

)((

Around B-E loop:

Parallel resistors

Forward active-mode:

*See example 3.15 page 142

Current mirror

Cont

66

RC RB

+VCC

B

BEC

BR

VVI

DCBC

BECC

CRR

VVI

CCCCCE RIVV

Collector-feedback Biasing

67

Bipolar Transistor Configurations

TOPIC 5

68

69

3 basic single-transistor amplifier configurations can be formed:

COMMON EMITTER (C-E configuration)

COMMON COLLECTOR / EMITTER FOLOWER (C-C configuration)

COMMON BASE (C-B configuration)

Each configuration has its own advantage in the form of

INPUT IMPEDANCE

OUTPUT IMPEDANCE

CURRENT / VOLTAGE AMPLIFICATION

BJT Amplifiers Configuration

70

COMMON EMITTER AMPLIFIER

RC

RE RB2

RB1

RL

CC1 CC2

vin

RS vout

CC2

RE RB2 RB1

RL

CC1

vin

RS vout RC

e

b

c

input base-emitter

output collector-emitter

emitter common (to input & output)

COMMON EMITTER MODE

Prof. R T Kennedy

Common-Emitter Amp. (CE)

Basic CE Amplifier

71

Cont

72

73

RS

R1

R2 RE

RC

vs

vO

CC

VCC

(a)

CE amplifier with emitter resistor

(b)

Small-signal equivalent circuit

RC

RS

RE

R1 || R2

Vo

Vs

r Ib

Ri Ro

+

V_

Rib

+

Vin

_

Ib

C-E Amplifier with Emitter Resistor

74

Assume Early voltage is infinite, ro is neglected

Cbo RIV

Ebbbin RIIrIV

Eb

inib Rr

I

VR 1 ibi RRRR 21

s

Si

iin V

RR

RV

Si

i

E

Cv

sib

inC

s

Cb

s

ov

RR

R

Rr

RA

VR

VR

V

RI

V

VA

1

1

Si RR

rRE 1

EC

E

Cv

R

R

R

RA

1

1. ac output voltage

2. To find the small-signal voltage gain

3. Combining equations in (1) and (2)

If

and if

Cont

75

RS

R1

R2 RE

RC

vs

vO

CC

VCC

CE

B C

E

Vo

Vs RC

RS

r ro R1|| R2 gmV

CE provides a short circuit to

ground for the ac signals

C-E Amplifier with Emitter Bypass Capacitor

76

COMMON COLLECTOR AMPLIFIER

CC2

RC RB2 RB1

RL

CC1

vin

RS vout RE

collector common (to input & output)

COMMON COLLECTOR MODE

c

b

e

input base-collector

output emitter - collector

Prof. R T Kennedy

Common-Collector Amp. (CC)

RC

RE RB2

RB1

RL

CC1 CC2

vin

RS

vout

Basic CC Amplifier

77

vO

VCC

vS

CC

RE

RS

R1

R2

B C

E

RS

RE

R1||R2

ro r Ib

Vo

Vs Vin

V

+ +

+ -

-

-

Emitter-follower circuit Small-signal equivalent circuit

CC Amplifier Anlysis

78

bo II 1 Eobo RrIV 1

Eobin RrrIV 1

Eob

inib Rrr

I

VR 1

s

Si

iin V

RR

RV

Si

i

Eo

Eo

s

ov

RR

R

Rrr

Rr

V

VA

1

1

oE

S

o rRRRRr

R

1

21

C-C Amplifier

Another small-signal equivalent circuit

B

C

ERS

RE R1||R2 ro

r

Ib

Vo

Vs Vin

V + +

-

-

Ri Rib Ro

Ii

Ib

Io Ie

ibi RRRR 21

Cont

79

iei

I

IA

iib

bo IRRR

RRII

21

2111

o

Eo

oe I

Rr

rI

Eo

o

ibi

ei

Rr

r

RRR

RR

I

IA

21

211

If R1R2 Rib and ro RE then Ai (1+)

Current gain where

Cont

80

81

COMMON BASE AMPLIFIER

base common (to input & output)

COMMON BASE MODE

RC

RE RB2

RB1

RL CC1

CC2

RS vout

vin

CC2

RE

RB2 RB1 RL CC1

vin

vout RC RS

RS

RB2 RB1

CC2

RL RC

vout vin

CC1

Prof. R T Kennedy

Common-Base Amp. (CB)

vout

e

b

c

input emitter-base

output collector-base

Basic CB Amplifier

82

BCE

Vs

Vo

RS

RE RC RL r gmV

V

Ii Io

Ib

-

+

RE

RS

RB

RC RL

VCC VEE

CC2 CC1

vO

vS

CB

Common-base

circuit

Small-signal

equivalent

circuit

Ri = re Ro = RC Input resistance Output resistance

CB Amplifier Analysis

83

11

rgA mio

SE

S

LC

m

s

ov RR

r

R

RRg

V

VA

1

If RS approaches zero, then Av = gm(RC||RL)

Voltage gain

Current gain

If RE approaches infinity and RL approaches zero, then

E

LC

Cm

i

oi R

r

RR

Rg

I

IA

1

short-circuit

current gain

Cont

84

Summary of Two-Port Parameters

85

DC Analysis

TOPIC 6

86

Amplifier DC Equations

87

VCC

RC

RE RB2

RB1

VCC

RC

RE RB2

RB1

VCC

VCC

RC

RE

RBB

VBB

CCVBRBR

BRBBV

21

2

21

21

BRBR

BRBRBBR

Prof. R T Kennedy

88

GUSTAV ROBERT KIRCHOFF

1824-1887

VCC

RC

RE

RBB

VBB input loop

output loop

IB

IE

IE

VRE

VRC

IC

IB VBE

VRBB

VCE

IC

Prof. R T Kennedy

Cont

89

VCE

EBQCEQCCQCC

EECECCCC

RIVRIV

RIVRIV

)1(

CBE III

CCCEQCEsat VVV

VCC

RC

RE

RBB

VBB input loop

output loop

IB

IE

IE

VRE

VRC

IC

IB VBE

VRBB

IC

EBQBEonBBBQBB

EEBEBBBBB

RIVRIV

RIVRIV

)1(

Prof. R T Kennedy

Cont

90

Q POINT

FORWARD ACTIVE MODE

+ VCE 0

+ IC saturation

cut-off

ICQ

VCEQ

Prof. R T Kennedy

Q-Point

91

Q POINT LOCUS

FORWARD ACTIVE MODE

+ VCE 0

+ IC saturation

cut-off

ICQ

VCEQ

Q

CCV

LOCUS:

STRAIGHT LINE

DC LOAD LINE EC

CC

RR

V

EC RR

slope

1

Cmxy

VRR

VRR

I

RIVRIV

RIVRIV

CCEC

CEQEC

CQ

ECQCEQCCQCC

EECECCCC

11

assume ICQ=IE

Prof. R T Kennedy

Q-Point Locus

92

FORWARD ACTIVE MODE

+ VCE 0

+ IC saturation

cut-off

VCC

VCC

VCC

EC RRslopetcons

1tan

Prof. R T Kennedy

DC Load Line: Vcc Change

93

FORWARD ACTIVE MODE

+ VCE 0

+ IC saturation

cut-off

VCC

RC

RC

SLOPE CHANGES

Prof. R T Kennedy

DC Load Line: Rc Change

AC Analysis

TOPIC 7

94

4.1 Analog Signals & Linear Amplifiers

Signal- contains information -eg:sound waves Analog signal electrical signals are in the form of timevarying current & voltage such as o/p signal from compact disc, signal from microphone & ect. Analog circuits electronic circuits that process analog signal - example: linear amplifier magnifies an i/p signal to produce large o/p signal

Dc voltage

source

Signal

source Amp LOAD

Figure 4.1 Block diagram of a compact disc player system

* transistor is heart

of an amplifier

Analog Signal

95

DC biased transistor @ Q-pt ==> transistor biased in forward active region.

A time-varying signal (eg. Sinusoidal) is added / superimposed on dc input voltage (bottom ac)

Output = ac on left.

Linear amplifier - output follows input shape but with much larger amplitude; if output shape is different = distortion (measured as Total Harmonic Distortion, THD)

Circuit above functions as inverter - output is amplified but inverted by 1800

Traditionally-BJT is used as linear amplifier

Bipolar Linear Amplifier

96

Variable Meaning

iB, vBE Total instantaneous

values

IB, VBE DV values

ib, vbe Instantaneous ac

value

Ib, Vbe Phasor values

Summary of notation

Cont

97

98

Definition

Small signal : ac input signal voltages and

currents are in the order of 10 percent of Q-point voltages and currents.

e.g. If dc current is 10 mA, the ac current (peak-to-

peak) < 0.1 mA.

99

Rules for ac analysis

Replacing all capacitors by short circuits

Replacing all inductors by open circuits

Replacing dc voltage sources by ground connections

Replacing dc current sources by open circuits

RC

RE RB2

RB1

RL

CC1

CC2

vin(ac)

RS vout(ac)

VCC

ac analysis : replace DC source with internal impedance

Basic AC Analysis

100

101

RC

RB

vs

vO

vce

vbe

ic

ib +

+

-

-

AC equivalent circuit of C-E with npn transistor

AC Equivalent Circuit

102

beBbs vRiv

0 ceCc vRi

Input loop:

Output loop:

be

T

BQ

b vV

Ii

bc ii

0.026 V

Cont

103

Transconductance parameter

gm=ICQ/VT

r=VT/ICQ

Small-signal hybrid- equivalent circuit

104

ib(Ib )

Current gain parameter

Cont

105

RS

R1

R2 RE

RC

RL vs

vO

CC1

CC2

VCC

vO

vs R1 R2

RS

RE

RC RL

(a)

CE amplifier with emitter resistor

(b)

AC equivalent circuit

AC Operation

106

+ VCE 0

+ IC

ICQ

VCEQ

Q

CCV

EC

CC

RR

V

i

v

LC

CEQ

RR

Vi

)( LCCQ RRIv )( LCCQCEQoffcut RRIVv

LC

CEQCQcsat

RR

VIi

AC Load Line

4.2.1 Graphical Analysis & AC Equivalent Circuit

Fig 4.4 C-E transistor characteristics, dc load line, & Q-point

Graphical Analysis & AC Equavalent Circuit

107

Cont

108

Cont

IBQ IS

1 F

exp

VBEQ

VT

VBEQ VBEon

vBE VBEQ vbe

iB IS

1 Fexp

vBE

VT

This base current cannot written as an ac current superimposed on dc

quiescent value, unless.if then the exponential term can be expanded via Taylor series and keeping only LINEAR TERM which leads to

the SMALL SIGNAL approximation

vbe VT

iB IBQ 1vbe

VT

quiescent base current

109

Using the appropriate substitutions for the various voltages and currents,

and making the assumption that the ac signal source, vs=0 then we get a

term for the base-emitter loop when all dc terms are set to zero.

Similar re-arrangements will lead to an equation for the collector-emitter

loop, with all dc terms set to zero.

BOTH EQUATION RELATE THE AC PARAMETERS OF THE CIRCUIT

AND

HAVE BEEN OBTAINED BY SETTING ALL DC CURRENTS AND VOLTAGES

TO ZERO OR IN OTHER WORDS

DC VOLTAGE SOURCES = SHORT CIRCUITS

DC CURRENT SOURCES = OPEN CIRCUITS

THIS IS A DIRECT CONSEQUENCE OF SUPERPOSITION

Cont

110

Below is the ac equivalent circuit, due to the equations derived

previously.

All currents & voltages shown are time-varying signals.

Although this is an ac equivalent circuit = = > the implicit

assumption is that the transistor is appropriately forward-biased

vs

ib

ic

+

vbe

-

+

vce

-

RC

RB

+

-

Cont

111

Frequency Response

TOPIC 8

112

Midband

Gain falls of due to the

effects of CC and CE

Gain falls of due to the

effects of C and C

Amplifier gain vs frequency

113

Frequency response of an amplifier is the graph of its gain versus the frequency.

Cutoff frequencies : the frequencies at which the voltage gain equals 0.707 of its maximum value.

Midband : the band of frequencies between 10f1 and 0.1f2. The voltage gain is maximum.

Bandwidth : the band between upper and lower cutoff frequencies

Outside the midband, the voltage gain can be determined by these equations:

21 /1 ff

AA mid

22/1 ff

AA mid

Below midband Above midband

Definition

114

Gain-bandwidth product : constant value of the product of the voltage gain and the bandwidth.

Unity-gain frequency : the frequency at which the amplifiers gain is 1

BWAf midT

Cont

115

At low frequency range, the gain falloff due to coupling capacitors and bypass capacitors.

As signal frequency , the XC - no longer behave as short circuits.

Low Frequency

116

The gain falls off at high frequency end due to the internal capacitances of the transistor.

Transistors exhibit charge-storage phenomena that limit the speed and frequency of their operation.

Small capacitances exist between the

base and collector and between the

base and emitter. These effect the

frequency characteristics of the circuit.

C = Cbe ------ 2 pF ~ 50 pF

C = Cbc ------ 0.1 pF ~ 5 pF

High Frequency

117

Cob = Cbc Cib = Cbe

Output capacitance Input capacitance

Basic data sheet for the 2N2222 bipolar transistor

118

This theorem simplifies the analysis of feedback amplifiers.

The theorem states that if an impedance is connected between the input side and the output side of a voltage amplifier, this impedance can be replaced by two equivalent impedances, i.e. one connected across the input and the other connected across the output terminals.

Millers Theorem

119

Miller equivalent circuit Z

- A

I2 I1

V1 V2

A

Z

V

Z

AVI

VAV

Z

VVI

1

)1( 111

12

211

A

Z

VI

Z

AV

I

VAV

Z

VVI

11

11

22

2

2

12

122

Millers Theorem

120

- A

ZM2

ZM1V1 V2

A

ZZ

A

Z

I

V

A

Z

VI

M 11

11

11

2

2

2

22

A

ZZ

A

Z

I

V

A

Z

VI

M1

1

1

1

1

1

11

Cont

121

)1(

)1(

11

1

1

1

1

1

1

ACC

ACC

A

XX

A

ZZ

M

M

CCM

M

)1

1(

)1

1(

11

11

11

2

2

2

2

ACC

AC

C

A

XX

A

ZZ

M

M

CCM

M

- A

I2 I1

V1 V2

C

- A

CM2

CM1V1 V2

Miller Capacitance Effect

122

B C

E

r ro

C

V gmV C

-

+

C = Cbe C = Cbc

High-frequency hybrid- model

123

ACACC bcMi 11

AC

ACC bcMo

11

11

Miin CCC Moout CC

B C

E

r ro CMi gmV

C CMo

A : midband gain

High-frequency hybrid- model with Miller effect

124

BJT Design Example

TOPIC 9

125

C-E Amplifier - Design Example

126

Cont

127

128

Cont

129

Cont

130

Cont

131

Cont

132

Cont

REFERENCES

Donald A. Neamen, Electronic Circuit Analysis & Design, 2nd Ed., McGraw Hill International Edition, 2001 (ISBN 0-07-118176-8)

Adel S. Sedra, Kenneth C. Smith, Microelectronic Circuits, 5th Ed., Oxford University Press (ISBN 0-19-514252-7)

Thomas L. Floyd, Electronic devices: Conventional Current Version, 7th Ed., Prentice Hall (ISBN 0-13-127827-4)

133

ACKNOWLEDGEMENT

Prof. R.T Kaneddy, UniMAP

Assoc. Prof. Basir Saibon, UniKL

134