• BJT Circuits & Limitations• LTspice

6.101 Spring 2017 Lecture 4 1

Acnowledgements:Neamen, Donald: Microelectronics Circuit Analysis and Design, 3rd Edition

General Configuration

6.101 Spring 2017 Lecture 4 2

CommonEmitter

CommonCollector

CommonBase

Transistor Configurations

6.101 Spring 2017 Lecture 4 3

+15V

+

Vin

-

+

VOUT

-

RL

R1

+

+

R2

[a] Common Emitter Amplifier [b] Common Collector [Emitter Follower] Amplifier

RE RE

+15V

+Vin

-

+

VOUT

-

RL

R1

+

+

R2

+

[c] Common Base Amplifier

TRANSISTOR AMPLIFIER CONFIGURATIONS

R2

+15V

R 1

+

Vin

-

+VOUT

-

RE

+

+

Base Current – Resistor Divider

6.101 Spring 2017 Lecture 4 4

68K

33K

IC F

3.7 mA 50

4.0 mA 100

4.2 mA 200

4.3 mA 300

IC=0.6 mAib

Make small compared to the current through R2

ib

Commom Emitter – Hybrid π

6.101 Spring 2017 Lecture 4 5

RB

+15V

2N3904

ICRL

C +

vout

_

IB

+

TRANSISTOR AMPLIFIER CONFIGURATIONS WITH HYBRID- EQUIVALENT CIRCUITS

Rs

+vin

_

Rs

r

RL

ib

+

vout

_

c

e

b

+vin

_

RB

COMMON EMITTER AMPLIFER

Lm

m

o

Lov

Lo

b

Lbo

in

outv

Rg

g

RAthen

rR

riRi

vvA

1

1

mvVVI

g

rg

THTH

CQm

m

26

0

vgv

m

outv1

inv1

Common Emitter with Emitter Degeneration

6.101 Spring 2017 Lecture 4 6

ELvEo

Eo

Lo

Eob

Lbo

in

outv

RRAthenRrif

RrR

RriRi

vvA

/;1

;111

1

• Input resistance (β+1)RE• Voltage gain reduced by (1+gm RE)• Voltage gain less dependent on β

(linearity)

outv1

inv1

AC Coupled vs DC Coupled Amplifiers• AC Coupling

– Advantage: easy cascading with DC blocking capacitor, bias stability and stage independent

– Disadvantage: lot’s of R’s and C’s, no DC gain, need large C for low freqency

• DC coupling– Some gain at DC– Fewer R’s C’s

6.101 Spring 2017 Lecture 4 7

Gain vs Frequency

6.101 Spring 2017 Lecture 4 8

Cutoff Frequency Analysis

6.101 Spring 2017 Lecture 4 9

Low Pass Filter LPF

6.101 Spring 2017 10

RV1 V2C

RCfcutofffrequencyHigh

sRCA

RCjCj

R

CjXjR

XjVVA

v

C

Cv

21

11

11

1

1

1

2

log f

AV (dB)

-3dB

fHI or f-3dB

slope = -6 dB / octaveslope = -20 dB / decade

0

log f

Degrees

-45o

fHI or f-3dB

0o

-90o

PHASE LAG

Lecture 2

log scale

Cutoff Frequency Analysis

6.101 Spring 2017 Lecture 4 11

3db f3db f 1

2RC 1

2 r (C C )

but 0 g m r or f g m

2 r (C C )

ib vbe

r vbe j (C C )

hfe gmvbe

ib gmr

1 j r (C C )

1 j r (C C )

h fe

1 j( ff

)ft hfe 1 or ft

g m

2 r (C C )

Cutoff Frequency Parameters

6.101 Spring 2017 Lecture 4 12

g mq

kT

IC

0 hfe (datasheet)C Cob (datasheet)

g m

2 (C C ) fT (transit frequency datasheet)

C g m

2 fTrC

6.101 Spring 2017 Lecture 4 13

β

Use max for worst case cu

Miller Effect* – Common Emitter

6.101 Spring 2017 Lecture 4 14

)](1[ LCmM RRgCC • Neamen, Microlectronics 3rd Edition p 514

Miller Effect

6.101 Spring 2017 Lecture 4 15

RC RC 4k r 2.6k RB 200kC 4pF C 0.2pF gm 38.5ma /V

f3db f 1

2 r ||RB (C C )15.5MHz

withMiller EffectCM C[1 gm (RC RL )]

f3db f 1

2 r ||RB (C CM )3.16MHz

*Neamen, Microlectronics 3rd Edition p 515

6.101 Spring 2017 Lecture 4 16

2N3904CE configuration, VCC +15v

Common Base Configuration

6.101 Spring 2017 Lecture 4 17

Common Collector (Emitter Follower)

6.101 Spring 2017 Lecture 4 18

• Buffer with unity gain• High input resistance driving low

output resistance (current gain).

mvVVI

g

rg

THTH

CQm

m

26

0

outv1inv1

1;1

;1'

11'

11

1

vEo

Eos

Eo

Eosb

Ebo

in

outv

AthenRrif

RrRR

RrRiRi

vvA

Common Collector – Emitter Follower Biasing

• Β = 100, iB = 7.5ma/100 =‐ 75µa• Using Thevenin equivalent,

RB = R1||R2, VB =

6.101 Spring 2017 Lecture 4 19

+15V

R 1

2N3904

7.5 mA

1.0 k7.5 mA

R2

A

B

7.5 V

2N3904

7.5 mA

+15V

VB

RB

IB

21

115RR

R

VB = IBRB + 0.6V + 7.5VVB = [75 µA x 10k] + 0.6V + 7.5VVB = 750 mV + 0.6V + 7.5VVB = 8.9V

[15 R1] ÷ [R1 + R2] = 8.9V15 R1 = 8.9 x [R1 + R2][15−8.9] R1 = 8.9 R2R1 = 1.44 R2[R1 x R2] ÷ [R1 + R2] = 10 kΩ

[1.44R2 x R2] ÷ [1.44 R2 + R2] = 10kΩR2 = 16.9 kΩ (use 16 kΩ)R1 = 1.44 R2 = 24.4 kΩ (use 24 kΩ)

Common Collector – Emitter Follower Biasing

• With R1 = 24kΩ, R2 = 16 kΩ, the current through the voltage divider is 15 ÷ [40 kΩ] = 375 µA.

• The 75 µA base current is 20% of 375 µA.

• With R1 = 2 kΩ, will need a divider current that is ~ 4.1 mA. (75 µA is only ~2% of 4.1 mA, which is negligible)

• The voltage drop across R2 will be [15 V –8.1 V] = 6.9 V; R2 = 1.7 kΩ

• But input impedance will be low = ~890Ω

• Use bootstrapping configuration

6.101 Spring 2017 Lecture 4 20

= 24.4 kΩ (use 24 kΩ)

+15V

R 1

2N3904

7.5 mA

8.1 V

1.0 k7.5 mA

R2

A

B

IDivider

Low Frequency Hybrid‐ Equation Chart

6.101 Spring 2017 Lecture 4 21

High gain, better high frequency responseLow input resistance

Unity gain, low output resistanceHigh input resist.

High gain applicationsModerate input resistance

High output resistance

Introduction to LTspice

6.101 Spring 2017 Lecture 4 22

Acknowledgment: LTspice material based in part by Devon Rosner (6.101 TA), Engineer, Linear Technology