Bjt Basics 1

7/27/2019 Bjt Basics 1

http://slidepdf.com/reader/full/bjt-basics-1 1/69

Dr. D G Borse

B

C

E



Dr. D G Borse

The BJT – Bipolar Junction Transistor

Note: Normally Emitter layer is heavily doped, Base layer is lightly

doped and Collector layer has Moderate doping.

The Two Types of BJT Transistors:npn pnp

n p nE

B

C p n pE

B

C

Cross Section Cross Section

B

C

E

Schematic

Symbol

B

C

E

Schematic

Symbol

•

Collector doping is usually ~ 109

• Base doping is slightly higher ~ 1010 – 1011

• Emitter doping is much higher ~ 1017



Dr. D G Borse

BJT Current & Voltage - Equations

B

CE

IE IC

IB

-

+

VBE VBC

+

-

+- VCE

B

CE

IE IC

IB-

+

VEB VCB

+

-

+ -VEC

n p n

IE = IB + IC

VCE = -VBC + VBE

p n p

IE = IB + IC

VEC = VEB - VCB



Dr. D G Borse

Figure : Current flow (components) for an n-p-n BJT in the active region.

NOTE: Most of the current is due to electrons moving from the emitter through base to the

collector. Base current consists of holes crossing from the base into the emitter and of holes

that recombine with electrons in the base.

- Electrons

+ Holes

VBE

VCB

+

-

+

-

n+

n

p-

IneIpe

-I co

Bulk-recombination

Current

Inc



Dr. D G Borse

Physical Structure• Consists of 3 alternate layers of n- and p-

type semiconductor called emitter ( E ),base ( B) and collector (C ).

• Majority of current enters collector,crosses base region and exits throughemitter. A small current also enters baseterminal, crosses base-emitter junctionand exits through emitter.

• Carrier transport in the active baseregion directly beneath the heavilydoped (n+) emitter dominates i-v

characteristics of BJT.



Dr. D G Borse

- - - - - -

- - -

- - - - - - - -

- - - - - - -- - --- - - - - -

- - - -- -

- - - - - - - - -

- -

- - - - -

- + - - + - -

Recombination

- Electrons

+ Holes

+

_

+

_

C

B

E

n

p

n

+

IB

Ic

IE

VBE

VCB


http://slidepdf.com/reader/full/bjt-basics-1 7/69Dr. D G Borse

Figure: An npn transistor with variable biasing sources (common-emitter configuration).

Inc

IneIpe

For CB Transistor IE= Ine+ Ipe

Ic= Inc- Ico

And Ic= - αIE + ICo

CB Current Gain, α ═ (Ic- Ico) .(IE- 0)

For CE Trans., IC = βIb + (1+β) Ico

where β ═ α ,1- α is CE Gain

ICO

Bulk-

recombinationcurrent



Common-Emitter Circuit Diagram

+ _ VC

C

ICVCE

IB

Collector-Current Curves

VCE

IC

Active

Region

IB

Saturation Region

Cutoff Region

IB = 0

Region of

Operation

Description

Active Small base current

controls a large

collector current

Saturation VCE(sat) ~ 0.2V, VCE increases with IC

Cutoff Achieved by reducing IB

to 0, Ideally, IC will also

be equal to 0.



BJT’s have three regions of operation:

1) Active - BJT acts like an amplifier (most common use)

2) Saturation - BJT acts like a short circuit

3) Cutoff - BJT acts like an open circuitBJT is used as a switch by switching

between these two regions.

r sat

Vo

_ +

C

B

E

Saturation Region Model

Vo

_ +

C

B

E

Active Region Model #1

bdc IB

IB

R o

Vo

_ +

C

B

E

Active Region Model #2

bdc IB ICEO

R BB

VCE (V)

IC(mA)

IB = 50 mA

IB = 0

30

5 10 15 200

0

IB = 100 mA

IB = 150 mA

IB = 200 mA

22.5

15

7.5

Saturation Region

Active Region

Cutoff Region

C

E

B

When analyzing a DC

BJT circuit, the BJT

is replaced by one of

the DC circuit models

shown below.

DC Models for a BJT:



DC b and DC

b = Common-emitter current gain

= Common-base current gainb = IC = IC

IB IE

The relationships between the two parameters are:

= b b =

b + 1 1 -

Note: and b are sometimes referred to as dc and bdc

because the relationships being dealt with in the BJT

are DC.



Output characteristics: npn BJT (typical)

VCE (V)

IC(mA)

IB = 50 mA

IB = 0

30

5 10 15 200

0

IB = 100 mA

IB = 150 mA

IB = 200 mA

22.5

15

7.5

Cdc FE

B

I= = h

I b

Note: Two key specifications for the BJT are

Bdc and Vo (or assume Vo is about 0.7 V)

Note: The PE review text

sometimes uses dc instead of bdc.

They are related as follows:

Input characteristics: npn BJT (typical)

VBE (V)

IB(mA)

200

0.5 1.00

0

VCE = 0

150

100

50

VCE = 0.5 V

VCE

> 1 V

The input characteristics look like the characteristics of a

forward-biased diode. Note that VBE varies only slightly,

so we often ignore these characteristics and assume:Common approximation: VBE = Vo = 0.65 to 0.7V

dcdc

dc

=+ 1

b

b

• Find the approximate values

of bdc and dc from the graph.

dc

dc

-1

b dc



Figure: Common-emitter characteristics displaying exaggerated secondary effects.



Figure: Common-emitter characteristics displaying exaggerated secondary effects.



Various Regions (Modes) of Operation of BJT

• Most important mode of operation

• Central to amplifier operation

• The region where current curves are practically flat

Active:

Saturation: • Barrier potential of the junctions cancel each other outcausing a virtual short (behaves as on state Switch)

Cutoff: • Current reduced to zero

• Ideal transistor behaves like an open switch

* Note: There is also a mode of operation called

inverse active mode, but it is rarely used.



BJT Trans-conductance CurveFor Typical NPN Transistor 1

VBE

IC

2 mA

4 mA

6 mA

8 mA

0.7 V

Collector Current:

IC = IES eVBE /VT

Transconductance:

(slope of the curve)

gm = IC / VBE

IES = The reverse saturation current

of the B-E Junction.

VT

= kT/q = 26 mV (@ T=300oK)

= the emission coefficient and is

usually ~1



Three Possible Configurations of BJT

Biasing the transistor refers to applying voltages to thetransistor to achieve certain operating conditions.

1. Common-Base Configuration (CB) : input = VEB & IE

output = VCB & IC

2. Common-Emitter Configuration (CE): input = VBE & IB

output= VCE & IC

3. Common-Collector Configuration (CC) :input = VBC & IB

(Also known as Emitter follower) output = VEC & IE



Common-Base BJT Configuration

Circuit Diagram: NPN Transistor

+_ +_

IC IE

IB

VCB VBE

E C

B

VCE

VBEVCB

Region of

OperationIC VCE VBE VCB

C-B

Bias

E-B

Bias

Active bIB

=VBE

+VCE

~0.7V 0V Rev. Fwd.

Saturation Max ~0V ~0.7V -0.7V<VCE<0 Fwd. Fwd.

Cutoff ~0 =VBE+VCE 0V 0V Rev.None

/Rev.

The Table Below lists assumptions

that can be made for the attributes

of the common-base BJT circuit in

the different regions of operation.

Given for a Silicon NPN transistor .



Common-Base (CB) Characteristics

Although the Common-Base configuration is not the most

common configuration, it is often helpful in the understanding

operation of BJTVc- Ic (output) Characteristic Curves

S a t u r

a t i o n R e g i o n

IE

IC

VCB

ActiveRegion

Cutoff

IE = 0

0.8V 2V 4V 6V 8V

mA

2

4

6

IE=1mA

IE=2mA

Breakdown Reg.

C C ll BJT Ch i i



Common-Collector BJT Characteristics

Emitter-Current Curves

VCE

IE

Active

Region

IB

Saturation Region

Cutoff Region

IB = 0

The Common-

Collector biasingcircuit is basically

equivalent to the

common-emitter

biased circuit except

instead of looking at

IC as a function of VCE

and IB we are looking

at IE.

Also, since ~ 1, and = IC /IE that means

IC~IE



n p n Transistor: Forward Active Mode Currents

Forward Collector current is

I co is reverse saturation current

1exp T V BE V

co I C I

A910A1810 co I

V T = kT/q =25 mV at room temperature

Base current is given by

1expco

T V BE V

F F

C I

B I I

b b

50020 F

b

Emitter current is given by

1exp

T V BE V

F

co I

B I

C I

E I

0.1

1

95.0

F

F

F b

b

is forward common-emitter

current gain

is forward common-

base current gain

In this forward active operation region,

F B I

C I

b F

E I

C I

VBE

IE=

IC=

IB=



Various Biasing Circuits used for BJT

• Fixed Bias Circuit

• Collector to Base Bias Circuit

• Potential Divider Bias Circuit



Dr. D G Borse

The Thermal Stability of Operating Point SIco

The Thermal Stability Factor : SIco

SIco = ∂Ic

∂Ico

This equation signifies that Ic Changes SIco times as fast as Ico

Differentiating the equation of Collector Current IC & rearrangingthe terms we can write

SIco ═ 1+β

1- β (∂Ib /∂IC)

It may be noted that Lower is the value of SIco better is the stability

Vbe, β



Dr. D G Borse

The Fixed Bias Circuit

15 V

C

E

B

15 V

200 k 1 k

The Thermal Stability Factor : SIco

SIco = ∂Ic

∂Ico

General Equation of SIco Comes out to be

SIco ═ 1 + β

1- β (∂Ib /∂IC)

Vbe, β

Applying KVL through Base Circuit we

can write, Ib Rb+ Vbe= Vcc

Diff w. r. t. IC, we get (∂Ib / ∂Ic) = 0

SIco= (1+β) is very large

Indicating high un-stability

Ib

Rb

RC

RC



Dr. D G Borse

The Collector to Base Bias Circuit

The General Equation for Thermal

Stability Factor,SIco = ∂Ic

∂Ico

Comes out to be

SIco ═ 1 + β

1- β (∂Ib /∂I

C)

Vbe, β

Applying KVL through base circuit

we can write (Ib+ IC) RC + Ib Rb+ Vbe= Vcc

Diff. w. r. t. IC we get

(∂Ib / ∂Ic) = - RC / (Rb + RC)Therefore, SIco ═ (1+ β)

1+ [βRC /(RC+ Rb)]

Which is less than (1+β), signifying better

thermal stability

VCC

R C

C

E

B

R F

Ic

Ib

VBE+- IE

The Potential Devider Bias Circuit



Dr. D G Borse

The Potential Devider Bias Circuit

VCC

R C

C

E

B

VCC

R 1

R ER 2

The General Equation for Thermal Stability

Factor, SIco ═ 1 + β

1- β (∂Ib /∂IC)

Applying KVL through input base circuit

we can write IbRTh + IE RE+ Vbe= VTh

Therefore, IbRTh + (IC+ Ib) RE+ VBE= VTh

Diff. w. r. t. IC & rearranging we get

(∂Ib / ∂Ic) = - RE / (RTh + RE)

Therefore,

This shows that SIco is inversely proportionalto RE and

It is less than (1+β), signifying better thermalstability

VCC

R C

C

E

B

R E

R Th

VTh _ +

Thevenin

Equivalent Ckt

IC

Ib

IC

Ib

IC

Thevenins

Equivalent Voltage

Self-bias Resistor

Rth = R1*R2 & Vth = Vcc R2

R1

+R2

R1

+R2

Th R R

R

E

E

IcoS b

b

1

1



Dr. D G Borse

A Practical C E Amplifier Circuit

VCC

R C

C

E

B

VCC

R 1

R ER 2

R s Ci

R L

Co

CEvi

vo

+

+

vs

+

_ _

_

io

ii

Common Emitter (CE) Amplifier

Input Signal Source



Dr. D G Borse

BJT Amplifier (continued)

An 8 mV peak change in v BE gives a 5mA change in i B and a 0.5 mA change in

iC .

The 0.5 mA change in iC gives a 1.65 V

change in vCE .

If changes in operating currents and

voltages are small enough, then I C

and V CE waveforms are undistorted

replicas of the input signal.

A small voltage change at the basecauses a large voltage change at the

collector. The voltage gain is given

by:

The minus sign indicates a 1800

phase shift between input and

output signals.

2061802060008.0

18065.1

~

~~

be

v

cev

v A



Dr. D G Borse

A Practical BJT Amplifier using Coupling and Bypass

Capacitors

• AC coupling through capacitors is used to inject an ac input signal

and extract the ac output signal without disturbing the DC Q-point

• Capacitors provide negligible impedance at frequencies of interest

and provide open circuits at dc.

In a practical amplifier design, C 1 and C 3 are

large coupling capacitors or dc blockingcapacitors, their reactance (XC = |ZC| = 1/wC ) at

signal frequency is negligible. They are effective

open circuits for the circuit when DC bias isconsidered.

C 2 is a bypass capacitor. It provides a low

impedance path for ac current from emitter to

ground. It effectively removes RE (required for good Q-point stability) from the circuit when ac

signals are considered.



Dr. D G Borse

D C Equivalent for the BJT Amplifier (Step1)

• All capacitors in the original amplifier circuit are replaced by opencircuits, disconnecting v I , R I , and R3 from the circuit and leaving R E

intact. The the transistor Q will be replaced by its DC model.

DC Equivalent Circuit



Dr. D G Borse

A C Equivalent for the BJT Amplifier (Step 2)

• Coupling capacitor CC and Emitter bypass capacitor CE are replaced by short

circuits.

• DC voltage supply is replaced with short circuits, which in this case is connected

to ground.

R1IIR2=RB

Rin

Ro



Dr. D G Borse

A C Equivalent for the BJT Amplifier(continued)

100k Ω4.3k Ω3R CR R

30k Ω10k Ω2

R 1

R B

R

• By combining parallel resistors into equivalent R B and R, the equivalent AC

circuit above is constructed. Here, the transistor will be replaced by itsequivalent small-signal AC model (to be developed).

All externally connected capacitors are assumedas short circuited elements for ac signal

A C Analysis of CE Amplifier



Dr. D G Borse

A C Analysis of CE Amplifier1) Determine DC operating point and

calculate small signal parameters

2) Draw the AC equivalent circuit of Amp.

• DC Voltage sources are shorted to ground

• DC Current sources are open circuited

• Large capacitors are short circuits

• Large inductors are open circuits

3) Use a Thevenin circuit (sometimes a

Norton) where necessary. Ideally the

base should be a single resistor + a single

source. Do not confuse this with the DC

Thevenin you did in step 1.

4) Replace transistor with small signal model

5) Simplify the circuit as much as necessary.

Steps to Analyze a Transistor Amplifier

6) Calculate the small signal parameters and

gain etc.

Step 1

Step2

Step3

Step4

Step5 π-model

used



Dr. D G Borse

Hybrid-Pi Model for the BJT

• The hybrid-pi small-signal model is the

intrinsic low-frequency representation of

the BJT.

• The small-signal parameters arecontrolled by the Q-point and are

independent of the geometry of the BJT.

Transconductance:

q KT

T V

C I

m g T V ,

Input resistance: Rin

m g

o

C I

T V o

r b b

Output resistance:

C I

CE V

AV

or

Where, V A is Early Voltage

(V A=100V for npn)



Dr. D G Borse

Hybrid Parameter Model

hi

hr V

oh

oh

f I

iV

i

I i

2

2'

I o

V o

1

1'

11 12

21 22

i i o i i r o

o i o f i o o

V h I h V h I h V

I h I h V h I h V

Linear Twoport DeviceVi

Ii Io

Vo



Dr. D G Borse

11 12

21 22

0 0

0 0

i i

o ii o

o o

o ii o

V V h hV I I V

I I

h hV I I V

h -Parameters

h11 = hi = Input Resistance

h12 = hr = Reverse Transfer Voltage Ratioh21 = h f = Forward Transfer Current Ratioh22 = ho = Output Admittance

Th S ll i l M d l f CE T i t



Dr. D G Borse

The Mid-frequency small-signal models

b

e

hoe

hie

hrevce hfei b v be

i b ic

vce

c

e

+ _

+ +

_ _

h-parameter model

b

e

r dgmv v be

i b ic

vce

c

e

+ +

_ _

hybrid- model

r v

+

_

b

e

bi b v be

i b ic

vce

c

e

+ +

_ _

r e model

br e

fe ac o

Alternate names:

h = = = b b b

m C C

o fe d

oe

ore ie

m

38.92g = I (Note: Uses DC value of I )

n

where n = 1 (typical, Si BJT)

1= h r =

h

h = 0 r = h =g

b

b

e B

B

o fe

o e ie

re

oe d

oe

26 mVr = (Note: uses DC value of I )

I

= h

r = h

h = 0

1h = 0, or use r =

h

b b

Three Small signal Models of CE Transistor

BJT Mid f A l i i th h b id d l



Dr. D G Borse

BJT Mid-frequency Analysis using the hybrid- model:

b

e

r dgmv vi

ii

io

vo

c

e

+ +

_ _

mid-frequency CE amplifier circuit

r v

+

_

R C R LR Thvs

+

_

is

R S

A common emitter (CE) amplifier VCC

R C

C

E

B

VCC

R 1

R ER 2

R s Ci

R L

Co

CEvi

vo

+

+

vs

+

_ _

_

io

ii

The mid-frequency circuit is drawn as follows:

• the coupling capacitors (Ci and Co) and thebypass capacitor (CE) are short circuits

• short the DC supply voltage (superposition)

• replace the BJT with the hybrid- model

The resulting mid-frequency circuit is shown belo

si

iv

s

i

i

o

s

o

svC Lo L Lmi

ov R Z

Z A

v

v

v

v

v

v A R Rr R R g

v

v A where, , ,''

Rwhere,21

R Rr R I

v Z

ThTh

i

i

i ,

C o

ovo

o

o Rr

i

v

Z

i

i

o

i i

i A

An a c Equivalent Circuitr o

Details of Small-Signal Analysis for Gain Av (Using Π-model)



Dr. D G Borse

Details of Small Signal Analysis for Gain Av (Using Π model)

3

3 RC R

C R

or

L R R ,

iv

bev

bev

ov

iv

ov

v A

Lbem

Rv g v

L

R

o

I

o

Rs

Rs

L R

or R

C R

bev

m g

ov

3

r B

RS

R

r B

R

L R

m g

v A

r B

RS

R

r B

Riv

bev

From input circuit



Dr. D G Borse

C-E Amplifier Input Resistance

• The input resistance, the total resistance

looking into the amplifier at couplingcapacitor C 1, represents the total

resistance presented to the AC source.

r R Rr B

R R

r

B

R

21xixv

in

)(xixv





Dr. D G Borse

High-Frequency Response – BJT Amplifiers

Capacitances that will affect the high-frequency response:

• Cbe, Cbc, Cce – internal capacitances

• Cwi, Cwo – wiring capacitances

• CS, CC – coupling capacitors

• CE – bypass capacitor

Frequency Response of Amplifiers



Dr. D G Borse

Frequency Response of Amplifiers

The voltage gain of an amplifier is typically flat over the mid-frequency

range, but drops drastically for low or high frequencies. A typical

frequency response is shown below.

LM(Avi) = 20log(vo/vi) [in dB]

BW

3dB

20log(Avi(mid))

f f LOW f HIGH

LM Response for a General Amplifier

For a CE BJT: (shown on lower right)• low-frequency drop-off is due to CE, Ci and Co • high-frequency drop-off is due to device capacitances Cp and Cm

(combined to form Ctotal)• Each capacitor forms a break point (simple pole or zero) with a break

frequency of the form f=1/(2pR EqC), where R Eq is the resistance seen bythe capacitor

• CE usually yields the highest low-frequency breakwhich establishes f Low.



Dr. D G Borse

Amplifier Power Dissipation

• Static power dissipation in amplifiers is determined from their DCequivalent circuits.

P D

V CE

I C

V BE

I B

Total power dissipated in C-B

and E-B junctions is:

where

Total power supplied is:

B I I I I

C I

CC V

S P

12 where ,

2

BE V CBV CE V

E R

F EQ R

BE V

EQV

B I

R R

CC V

I

1

and

211

b

The difference is the power dissipated by the bias resistors.



Dr. D G Borse



Dr. D G Borse

Figure 4.36a Emitter follower.

An Emitter Follower (CC Amplifier) Amplifier



Dr. D G Borse

Figure Emitter follower.

Very high input Resistance

Very low out put Resistance

Unity Voltage gain with no phase shiftHigh current gain

Can be used for impedance matching or acircuit for providing electrical isolation

( p ) p



Dr. D G Borse

Figure 4.36b Emitter follower.





Dr. D G Borse

Capacitor Selection for the CE Amplifier

Z c 1 jw C

Capacitive Reactance X c Z c 1w C

where w 2 f

X c1

R Br

Make X c1

0.01 R B

r

for < 1% gain error

X

c2

0 Make X

c2

1 for <1% gain error.

X c3

R3 Make X

c30.01 R

3

for <1% gain error

The key objective in design is to make the capacitive reactance

much smaller at the operating frequency f than the associated

resistance that must be coupled or bypassed.



Dr. D G Borse

Summary of Two-Port Parameters for

CE/CS, CB/CG, CC/CD



Dr. D G Borse

A Small Signal h-parameter Model of C E - Transistor

= h11

Vce*h12



Dr. D G Borse

A Simple MOSFET Amplifier

The MOSFET is biased in the saturation region by dc voltage sources V GS and

V DS = 10 V. The DC Q-point is set at (V DS , I DS ) = (4.8 V, 1.56 mA) with V GS =

3.5 V.

Total gate-source voltage is: gsvGS

V GS

v

A 1 V p-p change in vGS gives a 1.25 mA p-p change in i DS and a 4 V p-p change

in v DS . Notice the characteristic non-linear I/O relationship compared to the BJT.

Eber-Moll BJT Model



Dr. D G Borse

Eber Moll BJT Model

The Eber-Moll Model for BJTs is fairly complex, but it is

valid in all regions of BJT operation. The circuit diagram

below shows all the components of the Eber-Moll Model:

E C

B

IRIF

IE IC

IB

RIERIC

Eber-Moll BJT Model



Dr. D G Borse

be o J ode

R = Common-base current gain (in forward active mode)

F = Common-base current gain (in inverse active mode)IES = Reverse-Saturation Current of B-E Junction

ICS = Reverse-Saturation Current of B-C Junction

IC = FIF – IR IB = IE - IC

IE = IF - RIR

IF = IES [exp(qVBE /kT) – 1] IR = IC [exp (qVBC /kT) – 1]

If IES & ICS are not given, they can be determined using various

BJT parameters.

Small Signal BJT Equivalent Circuit



Dr. D G Borse

g qThe small-signal model can be used when the BJT is in the active region.

The small-signal active-region model for a CB circuit is shown below:

biBr

iE

iCiBB C

E

r = (b + 1) * VT

IE@ = 1 and T = 25C

r = (b + 1) * 0.026

IE

Recall:

b = IC / IB

The Early Effect (Early Voltage)



Dr. D G Borse

y ( y g )

VCE

ICNote: Common-Emitter

Configuration

-VA

IB

Green = Ideal IC

Orange = Actual IC (IC’)

IC’ = IC VCE + 1

VA

Early Effect Example



Dr. D G Borse

y p

Given: The common-emitter circuit below with IB = 25mA,

VCC = 15V, b = 100 and VA = 80.

Find: a) The ideal collector current

b) The actual collector current

Circuit Diagram

+ _ VCC

ICVCE

IB

b = 100 = IC /I

B

a)

IC = 100 * IB = 100 * (25x10-6 A)

IC = 2.5 mA

b) IC’ = IC VCE + 1 = 2.5x10-3 15 + 1 = 2.96 mA

VA 80

IC’ = 2.96 mA

Breakdown Voltage



Dr. D G Borse

g

The maximum voltage that the BJT can withstand.

BVCEO = The breakdown voltage for a common-emitter biased circuit. This breakdown voltage usually

ranges from ~20-1000 Volts.

BVCBO = The breakdown voltage for a common-base biased

circuit. This breakdown voltage is usually much

higher than BVCEO and has a minimum value of ~60

Volts.

Breakdown Voltage is Determined By:

• The Base Width

• Material Being Used

• Doping Levels

• Biasing Voltage



Dr. D G Borse

Potential-Divider Bias Circuit with Emitter Feedback Most popular biasing circuit.

Problem: bdc can vary over a wide range for BJT’s (even with the same part number)

Solution: Adding the feedback resistor R E. How large should R E be? Let’s see. Substituting the active region model into

the circuit to the left and analyzing the

circuit yields the following well known

equation:

VCC

R C

C

E

B

VCC

R 1

R ER 2

VCC

R C

C

E

B

R E

R Th

VTh _ +

2Th CC Th 1 2

1 2

R V = V and R = R R R + R

dc Th o CEO Th E

C

Th dc E

CEO dc CBO

V - V + I R + R

I = R + + 1 R

where I = + 1 I

b

b

b

ICEO has little effect and is often

neglected yielding the simpler

relationship:

dc Th o

C

Th dc E

V - VI =R + + 1 R

b b

Test for stability: For a stable Q-point w.r.t. variations in bdc choose:

Th dc ER << + 1 R b Why? Because then

dc Th o dc Th o Th o

C dc

Th dc E dc E E

V - V V - V V - VI = (independent of )

R + + 1 R + 1 R R

b b b

b b

Voltage divider biasing

circuit with emitter

feedback

Replacing the input circuit by a

Thevenin equivalent circuit yields:

PE-Electrical Review Course - Class 4 (Transistors)



Dr. D G Borse

( )

Example : Find the Q-point for the biasing circuit shown below.

The BJT has the following specifications:

bdc = 100, rsat = 100 (Vo not specified, so assume Vo = 0.7 V)15 V

C

E

B

15 V

200 k 1 k

Example : Repeat Example 3 if R C is changed from 1k to 2.2k.




Dr. D G Borse

Example Determine the Q-point for the biasing circuit shown.

The BJT has the following specifications:

bdc varies from 50 to 400, Vo = 0.7 V, ICBO = 10 nA

Solution:

Case 1: bdc = 50C

E

B

18 V

30 k

15 k

10 k

8 k

18 V

Case 2: bdc = 400 Similar to Case 1 above. Results are: IC = 0.659 mA, VCE =

6.14 V Summary:

bdc IC VCE50

400




Dr. D G Borse

BJT Amplifier Configurations

and Relationships:

Using the hybrid- model.

VCC

R C

C

E

B

VCC

R 1

R ER 2

R sCi

R L

Co

CEvi

vo+

+

vs

+

_ _

_

io

ii

Common Emitter (CE) Amplifier

'

o L' '

vi m L m L '

o L

'

L d C L d C L E L

'

i Th E Th o L

m

Th S

o d C d C E

o

i i ivs vi vi vi

s i s i s i

CE CB CC

1 + R A -g R g R

r + 1 + R

R r R R r R R R R

1Z R r R r R r + 1 + R

g

r + R R Z r R r R R

1 +

Z Z ZA A A A

R + Z R + Z R + Z

b

b

b

b

i i iI vi vi vi

L L L

P vi I vi I vi I

Th 1 2

Z Z ZA A A A

R R R

A A A A A A A

where R = R R

VCC

R C

E

R 2

R E

R s Ci

R L

Co

C2

vi vo

+

+

vs

+

_ _ _

ioii

Common Base (CB) Amplifier

R 1

C

B

VCC

C

E

B

VCC

R 1

R ER 2

R s Ci

vi

+

vs

+

_

_

R L

Co

vo

+

_

io

ii

Common Collector (CC) Amplifier (also called “emitter -follower”)

Note: The biasing circuit is

the same for each amplifier.



Dr. D G Borse

Figure 4.16 The pnp BJT.



Dr. D G Borse

Figure 4.17 Common-emitter characteristics for a pnp BJT.



Dr. D G Borse

Figure 4.18 Common-emitter amplifier for Exercise 4.8.



Dr. D G Borse

Figure 4.19a BJT large-signal models. ( Note: Values shown are appropriate for typical small-signal silicon devices at

a temperature of 300K.



Dr. D G Borse

Figure 4.19b BJT large-signal models. ( Note: Values shown are appropriate for typical small-signal silicon devices at




Dr. D G Borse

Figure 4.19c BJT large-signal models. ( Note: Values shown are appropriate for typical small-signal silicon devices at




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Bjt Basics 1