BJT 1 Examples

8/17/2019 BJT 1 Examples

1/31

BJT’s

pn

Collector

CiC

BiB

n

Base

EiE

Active Base Region

Emitter

Emitter-BaseJunction Collector-Base

Junction

Figure 5.1(a) - Simplifie cross-section of an npn transistor !it" currents t"at occur uring

#normal# operation

p$

p$

p$

p$

p

p

Active %ransistor

Region

iB

iC

iE

p

p

p

p

Figure 5.1(&) - Cross-section of an integrate npn &ipolar 'unction transistor

(&)

Base (B)

Collector (C)

Emitter (E)

iC

iE

iB

n

Collector

pBase

n

Emitter

vBC

vBE

iC

iB

iE(a)

Figure 5 . - (a) eali*e npn tr ansis tor s tr ucture for a general &ias conition

(&) Circuit s+m&ol for t"e npn transistor


2/31

Forward Characteristics

n

p

n

Collector

Base

Emitter

iE

iB

iC

vBE

iF

C

B

E

Fβ

iF

n

p

n

Collector

Base

Emitter

iE

iB

iC

vBE

iF

C

B

E

Fβ

iF

n

p

n

Collector

Base

Emitter

iE

iB

iC

vBE

iF

C

B

E

Fβ

i F

Figure 5., - npn transistor !it" vBE applie an vBC .


3/31

Reverse Characteristics

n

p

n

Collector

Base

Emitter

iE

i B

iR

i C

vBC

C

E

B

i R

R β

Figu re 5./ - %ran sistor !it" vBC ap plie an vBE .

Table 5.1

Common-Emitter and Common-Base

Current Gain Comparison

αF or αR

.1 .11

.5 1

.0 0

.05 10

.00 00

.00 /00

βF = αF

1 − αF or βR =

αR

1 − αR


4/31

ENGR 311 – Electronic Devices and Circuits Octoer !"# !$$$

Transistor %odel& Current '()li*ier

A Summar+ For Clarification (assume npn for t"e follo!ing general rules2properties 3 for pnp reverse

polarities)

Rules 2 4roperties

1 3 %"e collector must &e positive t"an t"e emitter.

3 %"e &ase-emitter an &ase-collector circuits &e"ave lie ioes. 6ormall+ t"e &ase-emitter ioe isconucting an t"e &ase-collector ioe is reverse-&iase

, 3 7"en 1 an are o&e+e c is proportional to & (c &eta . &)Bot" & an c follo! to t"e emitter.

6ote8 t"e collector current is not ue to for!ar conuction of t"e &ase-collector ioe9 t"at ioe is reverse-

&iase. Just t"in of it as :transistor action.;

4ropert+ , gives t"e transistor its usefulness8 a small current flo!ing into t"e &ase controls a muc" larger

current flo!ing into t"e collector.

6ote t"e effect of propert+ . %"is means +ou can (?& ?e $ ?&e) (for an npn).

et me emp"asi*e again t"at +ou s"oul not tr+ to t"in of t"e collector current as ioe conuction. t isn


5/31

+c – ,ce Characteristic *or an n)n Transistor


6/31

+c- ,e Characteristics


7/31

Biasin.

For common emitter amplifier


8/31

ENGR 311 - BJTs – E/ercises - Octoer !0# !$$1

Eamples Solution

Eample 1 - Beta 1= vBE .? atiC 1mA. Design circuit so t"at a

current of mA flo!s t"roug" t"e

collector an a voltage of $5? appears att"e collector.

Eample - n t"e circuit &elo! vC -.?. f Beta 5= fin E= B= C an

?C.


9/31

Eample , 3 n t"e circuit &elo!= ?&

1?= ?E 1.?. 7"at are alfa an &etafor t"is transistor 7"at voltage ?C o

+ou epect at t"e collector.

Eample / - Beta 1 3 Determine all

noe voltages an &ranc" currents.


10/31


11/31

Eample 5

Determine t"e voltages at all noes an current t"roug" all &ranc"es.

Assume &eta 1 an &eta . Assume 1 is in t"e active moe.


12/31

ENGR 311 - Gra)hical Re)resentation o* Transistor Characteristics - Octoer 31# !$$1

Conceptual circuit for measuring t"e iC -vCE c"aracteristics of t"e BJ%. 2 %"e iC -vCE c"aracteristics of a practical BJ%.

%"e iC -vCB c"aracteristics for an npn transistor in t"e active moe


13/31

Determine t"e voltages at all noes an t"e currents at all &ranc"es in t"e circuit &elo!.

Solution


14/31

The Transistor 's 'n '()li*ier – DC Conditions


15/31

The Collector Current and The Transconductance

The Base Current and the +n)ut Resistance at the Base

(a) Conceptual circuit to illustrate t"e operation of t"e transistor of an amplifier. 2 %"e circuit of

(a) !it" t"e signal source vbe eliminate for c (&ias) anal+sis.


16/31

Transistor as 'n '()li*ier - (all i.nal '))ro/i(ation

%ransconuctance (gm)= nput Resistance at t"e Base (r π)= nput Resistance at t"e Emitter (re)= ?oltage Gain

Eercise /. an /.,


17/31

Small-Signal EHuivalent Circuits Ioels

Amplifier Circuit 7it"out DC Sources

+&ri-π


18/31

%"e % Ioel

Application of t"e Small-Signal EHuivalent Circuits

1

,

/

5


19/31

Eample /.0

DC Anal+sis

Small-Signal Anal+sis


20/31

Eample /.11

Determine voltage gain in t"e circuit &elo!

DC Anal+sis

Small-Signal Ioel Small-Signal Anal+sis Directl+ on Circuit


21/31

Grap"ical Anal+sis

Grap"ical etermination of t"e signal components vbe= ib= ic= an vce !"en a signal component vi is

superimpose on t"e c voltage V BB.


22/31

Biasing %"e BJ% For Discrete-Circuit Design

Basic Single-Stage BJ% Amplifier Configurations


23/31

%inorit4-Carrier Trans)ort +n the Base Re.ion

(&)

7

n( )

i%= + H A D n

n

n()

n(7)

6 6

iC

Collector BaseEmitter

4

Space C"arge Regions

iE &o

&o(p = n )

i%

vBE

vBC

Fβ

F β

R

R

$ $iB

(a)

REC

Figure 5.15 - (a) Curren ts in t" e &ase region of a n pn tr an sistor

(&) Iin orit+ carrier concen tra tion in t" e &ase of t" e npn tra ns istor

i% HADn. n2 -HADnn. (n&o27&). Kep(v&e2?t) 3 ep(v&c2?t)L

s HADn.(n&o27&) (HADn.niM )26a&.7&

n&o eHuili&rium electron ensit+

A cross-sectional area of t"e &ase region

7& &ase !it"Dn iffusit+ (cmM2s)

6a& oping concentration in &ase of transistor<

ni intrinsic-carrier concentration (1M1.cmM,)

n&o niM 2 6a&


24/31

Base Transit Ti(e

%o turn t"e BJ% minorit+-carrier c"arge must &e introuce into t"e &ase to esta&lis" t"e graient.

%"e for!ar transit time tau-f represents t"e time constant associate !it" storing t"e reHuire c"arge in t"e &ase region an is efine &+

2%

Di**usion Ca)acitance

For t"e &ase-emitter voltage an "ence t"e collector current in t"e BJ% to c"ange= t"e c"arge store in t"e &ase

region also must c"ange.

7

n( )

n()

n(7) n &on &o

Figure 5.1>(a) - Ecess minorit+ c"arge store in t"e &ipolar &ase region

7

n( )

n(7) n &o

n &o

n(= ? )BE1

n(= ? )BE

∆

Figure 5.1>(&) - Store c"arge c"an ges as vBE c"an ges

%"is c"ange in c"arge !it" v&e can &e moele &+ a capacitance CD

CD (c2?%). τf


25/31

Fre5uenc4 De)endence o* the Co((on-E(itter Current Gain

B

C

E

iB

iC

iE

B

iB

E

vBE

C

$

-

i β iC F B

E Fi ( β $ 1) i

B

ep ( )S

vBE

?%i

C

Figure 5. - Simplifie moel for t"e npn transistor for t"e for!ar-active region

B

i B

iE

E

C

i β iC F B$-. ? vBE

i C

Figure 5.1 - Furt"er simplification of t"e npn moel for t"e for!ar-active region

Beta-cutoff FreHuenc+

1 01 1 1 >1 51 /1

-1

1

11

1

1,

FreHuenc+ (J*)

C o m m o n -

E m i t t e r C u r r e n t G a i n

f %

Figure 5. - Common-emitter current gain β vs. freHue nc+


26/31

Transconductance

Relates c"anges in ic to c"anges in v&e

gm ic2v&e (N-point)

gm c 2?%

CD gm.τf

The Earl4 E**ect (James Earl+ form Bell a&s)

Eperimentall+ emonstrate t"at !"en t"e output curves are etrapolate &ac to a point of *ero collector

current= t"e curves all intersect at a constant voltage point vce -?A

-15? -1? -5? ? 5? 1? 15?

Collector-Emitter ?oltage

/.mA

.mA

A

Collector

Cur

r ent

1 uA

uA

> uA

/ uA

uA

A-?

B

B

B

B

B

Figure - 5 ., %ransistor outp ut c "aracter ist ics ientif+ing t" e ear l+ voltage ?A

Ioeling t"e Earl+ Effect

ic

Betaf

i&


27/31

Tolerances in Bias Circuits

7orst Case Anal+sis

R C

R E

R 1

R ,> Ω

1 Ω

Ω

1> Ω

? $1 ?CC

1

)C

?EF

R EF

$1( ?

)B

R E

1> 5 Ω

1( 5 Ω

R C

((5 Ω

/ ?

)E

?CC

Stu+ t"e operation of t"e transistor consiering tolerances (!orst case ana+sis) in t"e circuit. Assume t"at t"e

1? po!er suppl+ "as a 5O tolerance an t"e resistors "ave 1O tolerance. Assume also t"at t"e voltage rop

in R E can &e neglecte= an &eta is large.

?E (ma= min)

C (ma= min)

?CE (ma= min)


28/31

%onte Carlo 'nal4sis

4erform Ionte Carlo Anal+sis on previous circuit assuming t"e ranom values to

?cc= R1= R= Rc= Re= an &eta. (Pse Ecel an2or 4spice).

Calculate

?E

R E

B

C

E

?C ?CC 3 C.R C 3 E.R E


29/31

Electronic Devices and Circuits – 11676$$

%onte Carlo 'nal4sis – 8sin. 9s)ice


30/31


31/31

4ro&e Qutput

c()= &()= ?ce

BJT 1 Examples

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