Dr. D G Borse

B

C

E

Dr. D G Borse

The BJT – Bipolar Junction TransistorNote: 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 Relationships - 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

Note: The equations seen above are for the transistor, not the circuit.

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 alternating 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 through emitter. A small current also enters base terminal, crosses base-emitter junction and exits through emitter.

• Carrier transport in the active base region directly beneath the heavily doped (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

Dr. 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-recombination

current

Dr. D G Borse

Common-Emitter Circuit Diagram

+_VCC

IC

VCE

IB

Collector-Current Curves

VCE

IC

Active Region

IB

Saturation RegionCutoff 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.

Dr. D G Borse

BJT’s have three regions of operation:1) Active - BJT acts like an amplifier (most common use)2) Saturation - BJT acts like a short circuit3) Cutoff - BJT acts like an open circuit

BJT is used as a switch by switchingbetween these two regions.

rsat

Vo

_ +

C

B

E

Saturat ion Region Model

Vo

_ +

C

B

E

Active Region Model #1

dc IB

IB

Ro

Vo

_ +

C

B

E

Active Region Model #2

dc IB ICEO

RBB

VCE (V)

IC(mA)

IB = 50 A

IB = 0

30

5 10 15 20 0

0

IB = 100 A

IB = 150 A

IB = 200 A

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:

Dr. D G Borse

DC and DC

= Common-emitter current gain

= Common-base current gain

= IC = IC

IB IE

The relationships between the two parameters are:

= =

+ 1 1 -

Note: and are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.

Dr. D G Borse

Output characteristics: npn BJT (typical)

VCE (V)

IC(mA)

IB = 50 A

IB = 0

30

5 10 15 20 0

0

IB = 100 A

IB = 150 A

IB = 200 A

22.5

15

7.5

Cdc FE

B

I = = h

I

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 dc.

They are related as follows:

Input characteristics: npn BJT (typical)

VBE (V)

IB(A)

200

0.5 1.0 0

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

• Find the approximate values

of bdc and adc from the graph.

dc

dc

- 1

dc

Dr. D G Borse

Figure: Common-emitter characteristics displaying exaggerated secondary effects.

Dr. D G Borse

Figure: Common-emitter characteristics displaying exaggerated secondary effects.

Dr. D G Borse

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 out causing 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.

Dr. D G Borse

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

Dr. D G Borse

Three Possible Configurations of BJT

Biasing the transistor refers to applying voltages to the transistor 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

Dr. D G Borse

Common-Base BJT Configuration

Circuit Diagram: NPN Transistor

+ _ + _

IC IE

IB

VCB VBE

EC

B

VCE

VBEVCB

Region of Operation

IC VCE VBE VCBC-B Bias

E-B Bias

Active IB =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.

Dr. D G Borse

Common-Base (CB) Characteristics

Although the Common-Base configuration is not the most common configuration, it is often helpful in the understanding

operation of BJT

Vc- Ic (output) Characteristic Curves

Sa

tura

tio

n R

egio

n

IE

IC

VCB

Active Region

Cutoff

IE = 0

0.8V 2V 4V 6V 8V

mA

2

4

6

IE=1mA

IE=2mA

Breakdown Reg.

Dr. D G Borse

Common-Collector BJT Characteristics

Emitter-Current Curves

VCE

IE

Active Region

IB

Saturation Region

Cutoff RegionIB = 0

The Common-Collector biasing circuit 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

Dr. D G Borse

n p n Transistor: Forward Active Mode Currents

Forward Collector current is

Ico is reverse saturation current

1expT

VBE

VcoI

CI

A910A1810 coI

VT = kT/q =25 mV at room temperature

Base current is given by

1expco

TVBE

V

FF

CI

BI I

50020 F

Emitter current is given by

1expT

VBE

V

F

coIB

IC

IE

I

0.11

95.0

F

FF

is forward common-emitter current gain

is forward common- base current gain

In this forward active operation region,

FB

IC

I

FE

IC

I

VBE

IE=

IC=

IB=

Dr. D G Borse

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 & rearranging the 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/∂IC)

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

RC

C

E

B

RF

Ic

Ib

VBE+

- IE

Dr. D G Borse

The Potential Devider Bias Circuit

VCC

RC

C

E

B

VCC

R1

RE R2

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 proportional

to RE and It is less than (1+β), signifying better thermal stability

VCC

RC

C

E

B

RE

RTh

VTh _ +

Thevenin Equivalent Ckt

IC

Ib

IC

Ib

IC

Thevenins Equivalent Voltage

Self-bias ResistorRth = R1*R2 & Vth = Vcc R2

R1+R2 R1+R2

ThRR

R

E

EIcoS

1

1

Dr. D G Borse

A Practical C E Amplifier Circuit

VCC

RC

C

E

B

VCC

R1

RE R2

Rs Ci

RL

Co

CE vi

vo

+

+

vs

+

_ _

_

io

ii

Common Emitter (CE) Amplifier

Input Signal Source

Dr. D G Borse

BJT Amplifier (continued)

An 8 mV peak change in vBE gives a 5 A change in iB 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 IC and VCE waveforms are undistorted replicas of the input signal.

A small voltage change at the base causes 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~

~~

bevcev

vA

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, C1 and C3 are large coupling capacitors or dc blocking capacitors, their reactance (XC = |ZC| = 1/wC) at signal frequency is negligible. They are effective open circuits for the circuit when DC bias is considered.

C2 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 open circuits, disconnecting vI, RI, and R3 from the circuit and leaving RE 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)

100kΩ4.3kΩ3

R C

RR

30kΩ10kΩ2

R 1

RB

R

• By combining parallel resistors into equivalent RB and R, the equivalent AC circuit above is constructed. Here, the transistor will be replaced by its equivalent small-signal AC model (to be developed).

All externally connected capacitors are assumed as short circuited elements for ac

signal

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

Step 2

Step 3

Step 4

Step 5 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 are controlled by the Q-point and are independent of the geometry of the BJT.

Transconductance:

qKT

TV

CI

mg TV ,

Input resistance: Rin

mgo

CI

TVor

Output resistance:

CI

CEV

AV

or

Where, VA is Early Voltage (VA=100V for npn)

Dr. D G Borse

Hybrid Parameter Model

hi

hrVohohfIiVi

Ii 2

2'

Io

Vo

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 Two port 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 Vh h

V II V

I Ih h

V II V

h-Parameters

h11 = hi = Input Resistanceh12 = hr = Reverse Transfer Voltage Ratioh21 = hf = Forward Transfer Current Ratioh22 = ho = Output Admittance

Dr. D G Borse

The Mid-frequency small-signal models

b

e

hoe

hie

hrevce hfeib vbe

ib ic

vce

c

e

+ _

+ +

_ _

h-parameter model

b

e

rd gmv vbe

ib ic

vce

c

e

+ +

_ _

hybrid- model

r v

+

_

b

e

ib vbe

ib ic

vce

c

e

+ +

_ _

re model

re

fe ac o

Alternate names:

h = = =

m C C

o fe doe

ore ie

m

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

nwhere n = 1 (typical, Si BJT)

1 = h r =

h

h = 0 r = h = g

e BB

o fe

o e ie

re

oe doe

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

I

= h

r = h

h = 0

1h = 0, or use r =

h

Three Small signal Models of CE Transistor

Dr. D G Borse

BJT Mid-frequency Analysis using the hybrid-p model:

b

e

rd gmv vi

ii io

vo

c

e

+ +

_ _

mid-frequency CE amplifier circuit

r v

+

_

RC RL RTh vs

+

_

is

RS

A common emitter (CE) amplifier VCC

RC

C

E

B

VCC

R1

RE R2

Rs Ci

RL

Co

CE vi

vo

+

+

vs

+

_ _

_

io

ii

The mid-frequency circuit is drawn as follows:

• the coupling capacitors (Ci and Co) and the

bypass capacitor (CE) are short circuits

• short the DC supply voltage (superposition)• replace the BJT with the hybrid-p model

The resulting mid-frequency circuit is shown below.

si

iv

s

i

i

o

s

o

svCLoLLmi

ov RZ

ZA

v

v

v

v

v

vARRrRRg

v

vA where, , ,''

R where, 21

RRrRI

vZ

ThThi

ii

, Co

ovo

oo

Rri

vZ

i

i

oi i

iA

An a c Equivalent Circuitro

Dr. D G Borse

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

33

RCRC

Ro

rL

R R ,

ivbe

v

bev

ov

ivo

v

vA

LbemRvgv

LR

oI

o

Rs

Rs

LR

orR

CR

bev

mg

ov

3

rB

RS

R

rB

R

LR

mg

vA

rB

RS

R

rB

Ri

v

bev

From input circuit

Dr. D G Borse

C-E Amplifier Input Resistance

• The input resistance, the total resistance looking into the amplifier at coupling capacitor C1, represents the total resistance presented to the AC source.

rRRrBRR

rBR

21xixv

in

)(xixv

Dr. D G Borse

C-E Amplifier Output Resistance

• The output resistance is the total equivalent resistance looking into the output of the amplifier at coupling capacitor C3. The input source is set to 0 and a test source is applied at the output.

CRorC

RR

mgorC

R

xixv

out

bevxvxv

xi

But vbe=0.

since ro is usually >> RC.

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

Dr. D G Borse

Frequency Response of AmplifiersThe 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

fLOW fHIGH

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/(2pREqC), where REq is the resistance seen by the capacitor

• CE usually yields the highest low-frequency break which establishes fLow.

Dr. D G Borse

Amplifier Power Dissipation

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

PDV

CEICV

BEIB

Total power dissipated in C-B and E-B junctions is:

where

Total power supplied is:

BIIII

CI

CCV

SP

12 where ,

2

BEVCB

VCE

V

ER

FEQR

BEV

EQV

BI

RRCC

VI

1 and

211

The difference is the power dissipated by the bias resistors.

Dr. D G Borse

Dr. D G Borse

Figure An Emitter follower.

Dr. D G Borse

Figure Emitter follower.

Very high input Resistance

Very low out put Resistance

Unity Voltage gain with no phase shift

High current gain

Can be used for impedance matching or a circuit for providing electrical isolation

An Emitter Follower (CC) Amplifier

Dr. D G Borse

Figure An Emitter follower.

Dr. D G Borse

Figure: An Emitter follower.

Dr. D G Borse

Capacitor Selection for the CE Amplifier

Zc1

jwC Capacitive Reactance XcZc

1wC

where w2f

Xc1R

Br Make X

c10.01 R

Br

for < 1% gain error.

Xc2 0 Make X

c21 for <1% gain error.

Xc3R

3 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 forCE/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 VGS and VDS = 10 V. The DC Q-point is set at (VDS, IDS) = (4.8 V, 1.56 mA) with VGS = 3.5 V.

Total gate-source voltage is: gsvGS

VGS

v

A 1 V p-p change in vGS gives a 1.25 mA p-p change in iDS and a 4 V p-p changein vDS. Notice the characteristic non-linear I/O relationship compared to the BJT.

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

Dr. D G Borse

Eber-Moll BJT Model

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.

Dr. D G Borse

Small Signal BJT Equivalent CircuitThe 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:

iBr

iE

iCiB

B C

E

r = ( + 1) * VT

IE

@ = 1 and T = 25C

r = ( + 1) * 0.026

IE

Recall:

= IC / IB

Dr. D G Borse

The Early Effect (Early Voltage)

VCE

ICNote: Common-Emitter Configuration

-VA

IB

Green = Ideal IC

Orange = Actual IC (IC’)

IC’ = IC VCE + 1

VA

Dr. D G Borse

Early Effect Example

Given: The common-emitter circuit below with IB = 25A, VCC = 15V, = 100 and VA = 80.

Find: a) The ideal collector current

b) The actual collector current

Circuit Diagram

+_VCC

IC

VCE

IB

b = 100 = IC/IB

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

Dr. D G Borse

Breakdown VoltageThe 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 FeedbackMost 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 RE. How large should RE 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

RC

C

E

B

VCC

R1

RE R2

VCC

RC

C

E

B

RE

RTh

VTh _ +

2Th CC Th 1 2

1 2

RV = V and R = R R

R + R

dc Th o CEO Th EC

Th dc E

CEO dc CBO

V - V + I R + R I =

R + + 1 R

where I = + 1 I

ICEO has little effect and is often

neglected yielding the simpler relationship:

dc Th oC

Th dc E

V - V I =

R + + 1 R

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

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

dc Th o dc Th o Th oC dc

Th dc E dc E E

V - V V - V V - V I = (independent of )

R + + 1 R + 1 R R

Voltage divider biasing circuit with emitter feedback

Replacing the input circuit by a Thevenin equivalent circuit yields:

Dr. D G Borse

PE-Electrical Review Course - Class 4 (Transistors)

Example : Find the Q-point for the biasing circuit shown below.The BJT has the following specifications:

bdc = 100, rsat = 100 W (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 RC is changed from 1k to 2.2k.

Dr. D G Borse

PE-Electrical Review Course - Class 4 (Transistors)

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 = 50 C

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:

dc IC VCE

50400

Dr. D G Borse

BJT Amplifier Configurations

and Relationships:

Using the hybrid-p model.

VCC

RC

C

E

B

VCC

R1

RE R2

Rs Ci

RL

Co

CE vi

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 + RA -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 RZ r R r R R

1 +

Z Z ZA A A A

R + Z R + Z R + Z

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

RC

E

R2

RE

Rs Ci

RL

Co

C2

vi vo

+

+

vs

+

_

_ _

io ii

Common Base (CB) Amplifier

R1

C

B

VCC

C

E

B

VCC

R1

RE R2

Rs C i

vi

+

vs

+

_

_

RL

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 : 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 : BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices ata 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 ata temperature of 300K.

Dr. D G Borse

Figure: BJT large-signal models. (Note: Values shown are appropriate for typical small-signal silicon devices ata temperature of 300K.

Dr. D G Borse

Figure : Bias circuit Examples