Lecture no 2 to 5 THE BASIC BJT AMPLIFIER CONFIGURATIONS

Lecture no 2 to 5

THE BASIC BJT AMPLIFIER CONFIGURATIONS

Prepared by Engr:Sarfaraz Khan Turk

Lecturer at IBT LUMHS Jamshoro

Basic BJT Amplifiers Circuits

Single-Stage BJT Amplifiers

Key Words: Common-Emitter Amplifier Graphical Analysis Small-Signal Models Analysis Common-Collector Amplifier Common-Base Amplifier

Basic BJT Amplifiers CircuitsSingle-Stage BJT Amplifiers

C-E AmplifiersTo operate as an amplifier, the BJT must be biased to operate in active mode and then superimpose a small voltage signal vbe to the base.

oC

CER

cii

BBEC

i vviivv CBC 12

DC + small signal

OC vi Bi iv CB ii

coupling capacitor (only passes ac signals)


C-E Amplifiers

iV

iV

Vi

+

iV


C-E Amplifiers

vBE=vi+VBE

bBB iIi

Apply a small signal input voltage and see ib


Single-Stage BJT AmplifiersC-E Amplifiers

• vi = 0 IB 、 IC 、 VCE

ceCECE

CCC

bBBi

vVviIi

iIiv 0

)()( ioiMoM ffVV •

• vo out of phase with vi

iC=ic+IC

vCE=vce+VCE

See how ib translates into vce.

BJT Amplifiers CircuitsSingle-Stage BJT Amplifiers

C-E Amplifiers Considering (all the capacitors are replaced by open circuits)

CV

Considering (all the capacitors are replaced by short circuits)

iV


Single-Stage BJT AmplifiersC-E Amplifiers

Considering (all the capacitors are replaced by open circuits)

CV

Considering (all the capacitors are replaced by short circuits)

iV


Single-Stage BJT AmplifiersGraphical Analysis

VCC

• Can be useful to understand the operation of BJT circuits.• First, establish DC conditions by finding IB (or VBE)• Second, figure out the DC operating point for IC

Can get a feel for whether the BJT will stay in active region of operation – What happens if RC is larger or smaller?


Graphical Analysis

VCC

')//( LcLCcce RiRRiv



Q-point is centered on the ac load line:

VCC

VCC



Clipped at cutoff(cutoff distortion)

Q-point closer to cutoff:

VCC



Clipped at cutoff(saturation distortion)

Q-point closer to saturation:


Graphical Analysis


Small-Signal Models Analysis

Steps for using small-signal models1. Determine the DC operating point of the BJT － in particular, the collector current2. Calculate small-signal model parameters: rbe

3. Eliminate DC sources – replace voltage sources with short circuits and current sources with open circuits4. Replace BJT with equivalent small-signal models5. Analysis



IC ≈ βIB, IE = IC + IB = (1+β)IB

eEBEbBCBC RIVRIR)II(V

))(1( eb

BECB RRR

VVI

)( eECCCCE RRIRIVV

Example 1



Example 2

vs

CCbb

bB V

RRRV

21

2

eBe

BEBEC RV

RVVII /

C

BII

)RR(IVV eCCCCCE


Single-Stage BJT AmplifiersSmall-Signal Models Analysis

There are three basic configurations for single-stage BJT amplifiers: – Common-Emitter – Common-Base – Common-Collector

VBB VCCRC

NN P ce

b

(a)

VBB VCC

RcN

NP

c

e

b

(b)

VBB

VCC

Re

N

NP

c

e

b

(c)

E B CV V V E B CV V V E B CV V V


Common-Collector Amplifier

eEBEbBCC RIVRIV eBBEbB RIVRI )1(

eb

CC

eb

BECCB RR

VRR

VVI

1)1(

BC II

eCCEeECECC RIVRIVV

eCCCCE RIVV

(a) 共集电极电路

Note : is slightly less than due to the voltage drop introduced by

oV iV BEV1VA




The last basic configuration is to tie the collector to a fixed voltage, drive an input signal into the base and observe the output at the emitter.



)//()]//)(1([ LeebebLebebi RRIrIRRrIV

)//)(1()//( LebLeeo RRIRRIV

1)//)(1(

)//()//)(1(

)//)(1(

Lebe

Le

Lebe

Le

i

OV RRr

RRRRr

RR

V

VA

Let’s find Av ， Ai ：



bbiLebeb RIIRRrI )())//)(1((

b

bLeb

b

bLebebi R

RRRIR

RRRrII

)//)(1()//)(1(

L

Lebo R

RRII )//)(1(

bLe

b

L

Lei RRR

RR

RRA

)//)(1(

)//)(1(

L

Le

RRR )//)(1(

)//)(1( Le RR << Rb

iAL

Le

RRR )//)(1(

>>1

Let’s find Av ， Ai ： )//()1()//( LebLeeLo RRIRRIRI

oI

iI



Ri

)//)(1()//( LebebLeebebi RRriRRiriv

b

ii i

vR )//)(1( Lebe RRr

)//(////)]//)(1([// LebbLebebii RRRRRRrRRR

Let’s find Ri ：



Ro

(b)

I

IRe

Let’s find Ro ：

bb IIII Re

)1()//(11

1

bsbee

o

RRrRivR

Re eI I I Re eI I I

1Re e Re bI I I I I

bsbee RRrv

Rv

//)1(

1

)//(// bsbee

RRrR

IeI

ReI



Ro

(b)

I

IRe

Ri (a)

bLebei RRRrR //)]//)(1([

1)//(

// bsbeeo

RRrRR



bLebei RRRrR //)]//)(1([

1

)//(// bsbe

eoRRr

RR

C-C amp characteristics:• Voltage gain is less than unity, but close (to unity) since β is large and rbe is small.• Also called an emitter follower since the emitter follows the input signal.• Input resistance is higher, output resistance is lower. - Used for connecting a source with a large Rs to a load with low resistance.


1)//)(1(

)//(

Lebe

Le

i

OV RRr

RR

V

VA

iAL

Le

RRR )//)(1(

>>1


Single-Stage BJT AmplifiersCommon-Base Amplifier

(a) 共基极电路

Rc

2b2b1b

CCB R

RRVV

eEBEB RIVV e

B

e

BEBEC R

VR

VVII

C

BI

I )( eCCCCeECCCCCE RRIVRIRIVV

Ground the base and drive the input signal into the emitter



(a) 共基极电路 Ri

Ro

be

Lc

beb

Lccv r

RRri

RRiA

)//()//(

i

oi I

IA

1

)1()(

//)1(

)(

C

ELC

C

ebe

be

LC

C

IIRR

R

Rrr

RRR

For RL<<RC, CEi IIA

since1)1(

e

be Rr

//)1(

Ri=

Ro≈RC



(a) 共基极电路

be

Lcv r

RRA )//(

i

oi I

IA

LC

CLC

C

RRRRR

R

)1(

)(

For RL<<RC, 1)1(

iA

)1(//

)1(

be

ebe rRr

Ri=

Ro≈R

C CB amp characteristics:• current gain has little dependence on β• is non-inverting• most commonly used as a unity-gain current amplifier or current buffer and not as a voltage amplifier: accepts an input signal current with low input resistance and delivers a nearly equal current with high output impedance• most significant advantage is its excellent frequency response

C-C C-E C-B

Input

Output

Functions

Summary for three types of diodes:

BI BIBI

EI CI CI


Zout < ZinVout > Vin

Zout > ZinVout > VinVout ≈ Vin

Zout > Zin


Frequency Response

Key Words: Basic Concepts High-Frequency BJT Model Frequency Response of the CE Amplifier


Frequency ResponseBasic Concepts

Time

0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0msV(1) V(2)

-1.0V

-0.5V

0V

0.5V

1.0V


Frequency ResponseBasic Concepts

Frequency0Hz 2KHz 4KHz 6KHz 8KHz 10KHz 12KHz 14KHz 16KHz 18KHz 20KHzV(2) V(1)

0V

200mV

400mV

600mV

800mV

Frequency

10Hz 100Hz 1.0KHz 10KHz 100KHz 1.0MHzV(2)

0V

0.5V

1.0V

Basic BJT Amplifiers CircuitsFrequency Response

Basic Concepts

Lower cut off frequency Upper cut off frequency

)()()()( vvv AAorffAA

The drops of voltage gain (output/input) is mainly due to:1 、 Increasing reactance of (at low f)2 、 Parasitic capacitive elements of the network (at high f)3 、 Dissappearance of changing current (for transformer coupled amp.)

ecs CCC ,,


Frequency Response

C

C

In BJTs, the PN junctions (EBJ and CBJ) also have capacitances associated with them

rbe

C

C

C'rbe C'

High-Frequency BJT Model


vs

Frequency ResponseFrequency Response of the CE Amplifier

C'rbe C'

There are three capacitors in the circuit.

At the mid frequency band, these are considered to be short circuits and internal capacitors and are considered to be open circuits.

C',C'


vs


At low frequencies, C1, C2 are an open circuit and the gain is zero. Thus C1 has a high pass effect on thegain, i.e. it affects the lower cutoff frequency of the amplifier.

)////( 2111 bebbs rRRRC

2 is the time constant for C2. 12 ---is neglected

11 2

1

Lf


vs


)////( 2111 bebbs rRRRC

12 ---is neglected

Capacitor Ce is an open circuit. The pole time constant is given by the resistance multiplied by Ce.

eebesb

e CRrRR

//1

)//(

222

211.1 LeLLL ffff

eLef

21

C'rbe C'


Frequency Response of the CE Amplifier

vs

At high frequencies, C1, C2 Ce are all short circuit. The frequency that dominates is thelowest pole frequency.

The time constant is neglected for)1( '

CjRL C'

CrRR besbC )////(

C

Hf

2

1

In summary:the lower cut off frequency is determined by network capacitence. e.g. The higher cut off frequency is determined by the parasitic ferquency of the BJT. e.g.

C

eCCC ,21

C'rbe C'



vs )1)(1(HL

Lvmv

ffj

ffj

ffj

AA

frequency-mid0,, —vmv

HLHL AA

ff

fffffFor

frequency-low1

,0),( —

L

Lvmv

HHL

ffj

ffj

AAffffffFor

frequencyHigh

ffj

AAffffffFor

H

vmvL

LH

—1

1,0)(


vs

)1)(1(HL

Lvmv

ffj

ffj

ffj

AA

H

HH

L

LL ff

21

221

2

C'rbe C'



decadedecade

0


For student References.

Chapter 9 amplifier fundamentals (9.1) (9.2) from the book Electronic devices, circuit and systems (Micheal M cirovic)

Chapter 8 introduction to amplifiers (8.1) (8.2) from the book introductory electronic devices and circuits by author (Robert T .paynter).

Wikipedia and world wide web

Lecture no 2 to 5 THE BASIC BJT AMPLIFIER CONFIGURATIONS

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