Analysis of BJT circuits

7/29/2019 Analysis of BJT circuits

1/14

EE2603-01 1

Analysis of BJT circuits (8.1 8.8)

o BJT amplifier parameter r e

o CE amplifierso CE with R E amplifiers

o EF amplifiers

o CB amplifiers


2/14

EE2603-01 2

BJT amplifier parameter r e

RE

R2

RC

VCC

0.7

I C

+

-

+-

R1

VB+

-I C RE

b

BJT amplifier

CeC

EB

CECBC

BB

21B21

1CCBB

ImV26rfindto)2(fromIFind

)2(7.0RR

IRI7.0RI

V

)1(R//RR,RR

RVV

=

++=++=

=+

=


3/14

EE2603-01 3

Example: Find I C and r e

0.2K

15k

VCC=12V

I C8.2k

b=100

47k

=====

++=++==

===

+=

+

=

7.3mA03.7

mV26ImV26rmA03.7

k313.0V2.2)mA(I

7.0k2.0100k37.11

)mA(I7.0RR

I9.2V

k37.1115//47R,V9.24715

1512RR

RVV

CeC

CEB

CBB

B21

1CCBB


4/14

EE2603-01 4

Types of BJT amplifiersVCC

RC

RL

R i CCCB

v i R2

R1

RE

vo

VCC

RL

R i

CC

CB

v i R2

R1

RE

vo

(a) common emitter (CE)

VCC

RC

RL

CE

R i CCCB

v i R2

R1

RE

vo

(c) emitter follower (EF)

VCC

RC

RLR i

CC

CEv i

R2

R1

RE

vo

CB

(d) common base (CB)

(b) common emitter (CE) with R E

(a) Common Emitter (CE)High AV and low Ri

(Voltage amplifications)

(b) Common Emitter (CE) with R ELow AV and high R i

(Stability applications)

(c) Emitter Follower (EF)Very low Ro and high R i

( Buffer applications)

(d) Common Base (CB)Very low Ri and high A V

(High Frequency applications)


5/14

EE2603-01 5

1. CE amplifiers

VCC

RC

RL

CE

R i

CC

CB

v i R2

R1

RE

vo

R in

Rov in

iiniLB

C

E

io

vo

When the input to the amplier is at Base and the output of the amplier is at Collector, Emitter becomes common to both input

and output and the amplier is Common Emitter ( CE) amplier


6/14

EE2603-01 6

CE amplier analysis

RC RL

Ri

vi R2R1RE

vo

Rin Rovin

iiniL

io

vo

RC RL

Ri

vi br eRB

vo

Rin Ro

vin

iin iLio

vo

ibC

B

E

bib

e

L C

e b

L C b

in

o v r

R // R

r i

R // R i

v

v

A =

==

Voltage gain

e B in

in in r // R i

v R ==Input resistance

C o

o o R i v R ==Output resistance

L

in v

L

in in

o

in in

L o

in

L i R

R A

R

R v v

R v

R v

i

i A =

===

Current gain

VCC

RC

RL

CE

R i

CC

CB

v i R2

R1

RE

vo

R in

Rov in

iin iLB

C

E

io

vol First replace VCC with ac ground and all capacitors short , replace BJT with its equivalent circuit, Calculate Av ,Ri, Ro , Ai, from the equivalent circuit l CE amplier has the following ac circuit properties


7/14

EE2603-01 7

Example:

0.2K

15k

VCC=12V

8.2k

b=100

47k VOVin

ROR in

10k

I in I L

=====

++=++

==

===

+=

+

=

7.3mA03.7

mV26ImV26rmA03.7

k313.0V2.2)mA(I

7.0k2.0100

k37.11)mA(I7.0RR

I9.2V

k37.1115//47R,V9.24715

1512RR

RVV

CeC

CEB

CBB

B21

1CCBB

) 8.121k037.0

k10//k2.8r

R//Rri

R//RivvA

e

LC

eb

LCb

in

ov ===

==Voltage gain

k36.07.3100//k37.11r//Ri

vR eB

in

inin ==== Input resistance

k2.8Ri

vR C

o

oo ===Output resistance

38.4k10

k36.08.121

R

RA

R

Rvv

Rv

Rv

i

iA

L

inv

L

inin

o

in

in

L

o

in

Li =

==

===Current gain

.amplifierCEfolowingtheofII

A,vvA,

iv

R,RFindin

Li

in

ov

in

inino

===


8/14

EE2603-01 8

2. CE with R E amplifiers

The amplier is Common Emitter ( CE) amplier but the Emitter capacitor CE is removed .

VCC

RC

RL

R i

CC

CB

v i R2

R1

RE

vo

R in

Ro

v in

iiniLB

C

E

io

vo


9/14

EE2603-01 9

( ) E R e r L R // C R

E R e r b i L R // C R b i

in v o v

v A +

=

+

==

Voltage gain

CE with RE amplier analysis

RC RL

R i

v i R2R1RE

vo

R in Rov in

iiniL

io

vo

RC RL

Ri

vibr e

RB

vo

Rin Ro

vin

iin iLio

vo

ibC

B

Ebib

RE

( ( )E R e r // B R b i

E R b i e r b i // B R in i in v

in R +=+

==

Input resistance

C o

o o R

i

v R ==Output resistance

L

in v

L

in in

o

in in

L o

in

L i R

R A

R

R v v

R v

R v

i

i A =

===Current gain

l First replace VCC with ac ground and all capacitors short , replace BJT with its equivalent circuit, Calculate Av ,Ri, Ro , Ai, from the equivalent circuit l CE with RE amplier has the following ac circuit properties


10/14

EE2603-01 10

3. EF amplifiers

When the input to the amplier is at Base and the output of the amplier is at Emitter, then Collector becomes common to both input and output and the amplier is Common Collector ( CC)amplier or Emitter Follower ( EF) amplier

VCC

RL

R i

CC

CB

v i R2

R1

RE

vo

RoR in

v in

iin

iL


11/14

EE2603-01 11

EF amplier analysis

RL

R i

v i R2R1RE

vo

R in Ro

v in

iin

iL

( ) [ ]+=

+=

==

) R // R ( r // R

i ) R // R ( r i

// R

i v

// R i v

// R R

i B e E

b

i B e b E

b

o E

o

o E o

[ ] 1 ) R // R ( r R // R

) R // R ( r i R // R i

v v

A L E e

L E

L E e b

L E b

in

o v =

+

=

+

==

First replace VCC with ac ground and all capacitors short ,replace BJT with its equivalent circuit, Calculate Av ,Ri, R

o, A

i, from the equivalent circuit

EF amplier has the following ac circuit properties

[ ]( )L R // E R e r // B R b i

L R // E R b i e r b i // B R

in i in v

in R

+=

+

==

L

in v

L

in in

o

in in

L o

in

L i R

R A

R

R v v

R v

R v

i

i A =

===

RE

R i

v ibr eRB

vo

R in Ro

v in

i in

iL

ib CB

E

bib

RL

io

RE

R i br eRB

vo

Ro

ib

E

bib

io +

-

+

-

-

+

Voltage gain

Input resistance

Output resistance

Current gain


12/14

EE2603-01 12

4. CB amplifiers

When the input to the amplier is at Emitter and the output of the

amplier is at Collector, then Base becomes common to both input and output and the amplier is Common Base ( CB) amplier

VCC

RC

RLR i

CC

v iR2

R1

RE

vo

R in Ro

v in

ie

iin

iL

CB amplier without CB


13/14

EE2603-01 13

RCR i

v iR2

R1RE

vo

R in Ro

v in

ie RL

CB

Eiin

iL

CB amplier analysis

RCRB

RE

vo

R in Ro

v in

ie RL

ib bibbr e

B C

E

iL

iin

[ ] +=

+

==

e B

L C

e B b

L C b

in

o

v r R

R // R

r R i

R // R i

v

v A

Voltage gain

+=

+

==

e B

E

b

e B b E

e

in E in

r R

// R

i

r R i // R

i

v // R R

Input resistance

C o R R = Output resistance

L

inv

inin

Lo

in

Li R

RA

R

v

Rv

i

iA ===Current gain

l First replace VCC with ac ground and all capacitors short , replace BJT with its equivalent circuit, Calculate Av ,Ri, Ro , Ai, from the equivalent

circuit l CB amplier has the following ac circuit properties


14/14

EE2603-01 14

VCC

RC

RLR i

CC

v i

R2

R1

RE

vo

CB

R in Ro

v in

ieiin

iL

Ef ect of Base capacitor CB

L

inv

inin

Lo

in

Li R

RA

Rv

Rv

i

iA ===

e

L C

in

o v r

R // R

v

v A ==

C o R R =

e E in r // R R =

Voltage gain

Input resistance

Output resistance

Current gain

CB amplier with CB

If CB is connected in the Base circuit, RB is shorted by zero reactance of the capacitance . By substituting RB = 0 in the previous formula, we will have the followings :

Analysis of BJT circuits

Documents

Transcript of Analysis of BJT circuits