BIPOLAR JUNCTION TRANSISTORS (BJTs)

Dr Derek Molloy, DCU

What are BJTs?

• Two PN junctions joined together is a BJT– Simply known as a transistor!

• Bipolar? Current carried by electrons and holes

• Will see FETs (Field Effect Transistors)– BJTs have a higher gain (amplification).– BJTs can supply more current.– FETs are less complex and require less

power.

Production

Example of a bipolar transistor production

Transistor types

• So, they are like diodes:– Semiconductor material– Doped p and n regions

• Unlike a diode:– 3 alternated doped regions p-n-p or n-p-n– Narrow channel between 2 terminals is

controlled by a voltage on a 3rd terminal.

Transistor controlled to operate as a switch or a variable resistorKey element in the design of amplifier

BJT CONFIGURATIONS

BJT PRINCIPLE OF OPERATION

A small current flowing in the base-emitter circuit can control the amount of a much larger current flowing in the collector-emitter circuit

n

p

nVBE

+

+

VCE

Base

Emitter

Collector

bI

CI

A Current Amplifier!

Common Emitter Configuration

-

BE diode is reverse biased.Open circuit, high resistance between the collector and emitter.Small current flowing between C-E, ICEO

BE diode is forward biased.Closed circuit between the collector and emitter.Current flowing between C-E,

BJT PRINCIPLE OF OPERATION

SOME CHARACTERISTICS

I C

VBE

I C

I B 0.7V

Current transfer characteristic(slightly non-linear)

Current-voltage transfer characteristic(highly non-linear)

(mA)

( )A

IF VCE is large IC α IB IC depends on VBE

β=hFE

BASICS BJT

IE=IC+IB

Numbers for illustration of measurements only

Not a worked example!

CURRENT GAIN

• DC or large signal gain: hFE or β

• Small signal gain: hfe

B

CFE I

Ih

B

C

B

Cfe I

IdIdIh

For most practical purposes, hfe and hFE are considered equal

Voltage Gain is the ratio of output voltage, to input voltage.Current Gain is the ratio of output current, to input current.Transconductance is the ratio of output current, to input voltage.Transimpedance is the ratio of output voltage, to input current.

COMMON EMITTER BIPOLAR AMPLIFIER

VCC

RCRB

Vi

VO

BE

C

h = 100FE

R = 910k

R = 4.7k

B

C

V = 10VCC

Input signal between base and emitter.Output signal between collector and emitter.Base needs to be biased.

Base-Emitter is forward biased due to RBRB sets the quiescent (steady-state with no input signal applied) base IBQ, ICQ, VOQ, VCEQVBE=0.7V

Vcc=10VRB=910kRC=4.7k hFE=100

I V VR

ABQCC BE

B

10 0 7

91010 2V V

k. .

I h I A mACQ FE BQ 100 10 2 1 02. .

VRIVV CCQCCOQ 206.5470000102.010

0V

VBE

Coupling Capacitor acts as a high-pass filter, allowing AC signal voltage on to the transistor, while blocking all DC voltage from being

shorted through the AC signal source.

INPUT CHARACTERISTICS

• DC input characteristics

• AC input impedance

re temperaturoomat e1 40 BE

BEV

BSkT

eV

BSB IeII

h dVdI Iie

BE

B B

1

40( )

IC

VBE

IC

IB 0.7V

Current transfer characteristic(slightly non-linear)

Current-voltage transfer characteristic

(highly non-linear)

(mA)

( )A

I B

VBE0.7V

IBS is a constant determined by the base characteristics

OUTPUT CHARACTERISTICS

OUTPUT CHARACTERISTICS

Saturation Region:VCE is not large enough, IC isIndependent of IB and depends on VCE.

Active Region:VCE is large enough, IC isindependent of VCE and depends on IB(and the gain).

VCE(V)

Ic(mA)

IB(µA)

0

10

20

30

40

50

60

0100200

300

400

500

600

Saturation regionActive region

For amplification, BJT operates in active regionFor switch or digital applications, BJT swings between saturation (switch on ) and cut-off (switch off)

OPERATING REGION

VCE

Ic

IBA: Saturation regionHighly non-linear

B: Maximum of IcDamage Transistor

D: Maximum VCEResult in Avalanche Breakdown of the transistor

C: Maximum Power Dissipation VCE and IcP = VCE x Ic

Operating point placed in the white area

A suitable operating point, Icmax/2 to avoid any red zones, close to centre Maximum swing possible with AC input.

AB

C

D

VARIATION AROUND THE QUIESCENT POINT

Ic IB

VCC

ic iB

VCEvC

E

IBVB VRC VCE Vout

Avoid: clamping the wave

Operating Point

DC DESIGN PARAMETERS OF A COMMON EMITTER AMPLIFIER

bmxRV

RV

RVV

Ic

cc

c

ce

c

ceccc

VCC

RCRB

Vi

V0

BE

C

h = 100FE

R = 910k

R = 4.7k

B

C

V = 10VCC

The load line graphical method

VCE(V)

Ic(mA)

IB(µA)

0

10

20

30

40

50

60

0100200

300

400

500

600

Satu

ratio

n re

gion

Active region

VcccR

1

VCE

1) Chose an operating point (IC , VCE) and a power rail VCC . Then draw the load line and calculate the slope. RC is given by -1/slope.2) Chose RC and VCC. Draw a load line through VCE = VCC with a slope of -1/RC. Then select an operating point somewhere along this load-line.

A

B

A: cut-off B: saturation