mach dao dong da hai

Chng 3: Mch dao ng a hi

29

Chng 3

MCH DAO NG A HI

3.1 Mch dao ng a hi dng BJT

3.1.1 Trng thi tt, dn bo ha ca BJT

Vcc

Q

BBV

BRCV

CR

K

LED

1

0

CI

BIBEV

CEV

Vcc

QBR

CV

CR

K

LED

0

CI

BIBEV

CEV

Hnh 3.1a Hnh 3.1b

Xt mch hnh 3.1a:

Khi kha K v tr [0], dng 0BI 0CI BJT tt coi nh cc C

v E ca BJT b h mch nh hnh 3.1b. Khi , LED tt, in th VccVC .

Khi kha K v tr [1], dng BI ln,

BJT dn bo ha, ta xem nh cc C v cc E b ni tt.

VVV CEsatCE 2,0 , VVBE 8,0 v

C

B

IKI , vi K l h s dn bo ha su

(K=25), l h s khuch i ca BJT.

Rc

VVVccI CEsatLEDC

, vi LEDV l in

p ri trn LED. Khi , LED sng.

Bi tp:

1/ Tnh Rb, Rc v la chn Q trong mch hnh 3.1a sao cho khi ng kha K qua v tr [1] th transistor dn bo ha. Bit VVBB 5 , Vcc= 12V, VVLED 5,2

v LED sng bnh thng dng qua LED l 20mA.

Vcc

Q

BBV

BRCV

CR

K

LED

1

CI

BIBEV

CEV

Hnh 3.1c


30

3.1.2 Mch dao ng lng n (bistable)

Mch hnh 3.2a l mch dao ng lng n hay cn gi l mch dao ng hai trng thi bn. Trong , mch c thit k sao cho Q1 v Q2 lm vic vng dn bo ha.

Vcc

Q1 Q2

Vi

1OV

1CR

1R 2R2OV

2CR

Vcc

Q1 Q2

Vi

1BEV 2BEV

2CR

1CR

2OV1OV 1R 2R

Hnh 3.2a Hnh 3.2b

Nguyn l hot ng: Gi s ban u Q1 dn, Q2 tt, mch hnh 3.2a tr thnh nh hnh 3.2b.

Lc ny, dng 22

11

RR

VVccI

C

BEB

ti cc B ca transistor Q1 lm transistor Q1 tip

tc dn. ng thi, dng 02 BI ti cc B ca transistor Q2 lm transistor Q2

tip tc tt. Do , nu khng c tc ng bn ngoi th Q1 vn dn, Q2 vn tt. V vy, trng thi Q1 dn, Q2 tt l trng thi n nh ca mch.

thay i trng thi ta cp mt xung m vo Vi, lm 01 BEV Q1

ngng dn lm in th ti 1OV ln 2BEV ln Q2 dn, mch hnh 3.2a

tr thnh nh hnh 3.2c.

Lc ny, dng 11

22

RR

VVccI

C

BEB

ti

cc B ca transistor Q2 lm transistor Q2 tip tc dn. ng thi, dng 01 BI ti

cc B ca transistor Q1 lm transistor Q1 tip tc tt. Do , nu khng c tc ng bn ngoi th Q2 vn dn, Q1 vn tt. V vy, trng thi Q2 dn, Q1 tt l trng thi n nh ca mch.

thay i trng thi ta cp mt

xung dng vo Vi, lm 1BEV ln

Q1 dn lm in th ti VVO 01

VVBE 02 Q2 ngng dn

Vcc

Q1 Q2

Vi

1BEV 2BEV

1CR 2CR

2R1R2OV1OV

Hnh 3.2c


31

T nguyn l hot ng trn, ta thy, mch hnh 3.2 c hai trng thi n nh. V vy, mch c gi l mch dao ng lng n.

Dng in p vo, ra ca mch lng n nh sau:

t

0 t1

t1

t2

t2

t

0

Vm2

t1 t2

1OV

2OV

t

0

Vm1

1BEV

t

0

2BEV

t

0

iV

V

t1

t1

t2

V

t3 t4

t3

t3

t3

t3t2

t4

t4

t4

t4

Hnh 3.3

Bi tp 1/ Hy thit k mch hnh 3.2a sao cho mAII CC 2021 . Bit Vcc= 5V.

3.1.3 Mch dao ng n n (monostable)

Hnh 3.4a l mch dao ng n n hay cn gi l mch dao ng mt trng thi bn. Q1 v Q2 c thit k lm vic trong vng dn bo ha.


32

Vcc

Q1 Q2

Vi

1OV

1CR

RC 2OV

2CR1BR

Vcc

Q1Q2

Vi

1OV

1CR

RC 2OV

2CR1BR

2BEV

+-

Hnh 3.4a Hnh 3.4b

Nguyn l hot ng: Gi s ban u Q1 dn, Q2 tt, mch hnh 3.4a tr thnh nh hnh 3.4b.

Lc ny, t C np nng lng t ngun qua 2CR v mi ni BE ca Q1, in p

trn t c chiu nh hnh 3.4b, ngoi dng np qua t dng 1BI cn c cung

cp t ngun qua 1BR . Do , cho d t np y th Q1 vn dn VVO 01

Q2 tip tc tt. Nu khng c tc ng bn ngoi th mch khng thay i trng thi. V vy, trng thi ny l trng thi n nh ca mch.

thay i trng thi ta cp mt xung dng vo Vi, lm 2BEV ln

Q2 dn, t C t in p m vo mi ni BE ca Q1 lm 01 BEV Q1 ngng

dn, mch hnh 3.4a tr thnh nh hnh 3.4c. Lc ny, t C x qua 1BR v Q2 lm

in p trn t gim dn. Sau khi t x ht, t tip tc np nng lng t ngun

Vcc qua 1BR v Q2 in p trn t i

du v tng dn. M CBE VV 1 vi CV l

in p trn t. 1BEV cng tng dn.

Ti mt lc no 1BEV ln, lm Q1

dn VVO 01 Q2 tt

Vcc

Q1Q2

Vi

1OV

1CR

R C2OV

2CR1BR

2BEV1BEV

+-

-

++

Hnh 3.4c


33

T nguyn l hot ng trnh by trn, ta thy trng thi n nh ca mch l trng thi Q1 dn, Q2 tt. Khi c tc ng bn ngoi, mch thay i trng thi Q1 tt, Q2 dn, sau mt thi gian mch t tr v trng thi n nh. Do , trng thi Q1 tt, Q2 dn l trng thi khng n nh ca mch.

Dng in p 1OV , 2OV , 1BEV v 2BEV nh sau:

t

0 t1

t1

t2

t2

t

0

Vcc

t1 t2

1OV

2OV

t

0

Vm

1BEV

t

0

2BEV

t

0

iV

-Vcc

V

t1

t1

t2

OT

V

Hnh 3.5

Gi To l rng xung ra. To ph thuc vo thi gian np, x ca t C. To l thi gian t t1 n t2. Do , tnh To ta xt in p 1BEV trong thi gian t

t1 n t2 v di trc ta nh sau:


34

t

0

Vcc2

VVcc

OT

V

Hnh 3.6

Trong thi gian ny 1BEV cng chnh l in p trn t C v c phng trnh sau:

t

C eVcctV 12)(

Khi oTt th in p VVccVC . Do VVcc VccVC

OT

eVccVcc 12

2

1

2

11

OO TT

ee

Ly logarit hai v ta c:

2

1ln

OT

2lnOT M: CRB1 c gi l thi hng np x ca t

CRCRRCT BBO 11 7.0693.02ln Bi tp 1/ Hy thit k mch hnh 3.4a sao cho mAII CC 2021 v To= 1ms. Bit

Vcc= 5V.

3.1.4 Mch dao ng bt n (astable) Cho mch dao ng bt n nh hnh 3.7a.

Vcc

Q1 Q2

1OV

1CR

2C

2OV

2CR1BR

1C

2BR

1BEV 2BEV

Vcc

Q1 Q2

1OV

1CR

2C

2OV

2CR1BR

1C

2BR

1BEV 2BEV

+-+-

Hnh 3.7a Hnh 3.7b


35

Nguyn l hot ng: Gi s ban u, Q1 dn, Q2 tt, mch hnh 3.7a tr thnh nh hnh 3.7b

VVO 01 , VccVO 2 . Lc ny, t 2C np nng lng t ngun qua 2CR v

mi ni BE ca Q1, in p trn t c chiu nh hnh 3.7b, ngoi dng np qua t dng 1BI cn c cung cp t ngun qua 1BR . ng thi, t 1C c np qua

2BR v c chiu nh hnh 3.7b. in p trn t 1C , 1CV (in p trn t 1C )

tng dn. M 12 CBE VV 2BEV cng tng dn. Ti mt lc no , 2BEV ln

lm Q2 dn. t 2C t in p m vo mi ni BE ca Q1. 01 BEV lm

Q1 tt. Khi mch trng Q1 tt, Q2 dn, mch hnh 3.7a tr thnh nh hnh 3.7c:

VccVO 1 , VVO 02 . Lc ny, t

1C x nng lng qua mi ni BE ca

Q2. Sau , np nng lng t ngun qua 1CR v mi ni BE ca Q2, in p

trn t o chiu v tng dn xem hnh 3.7c, ngoi dng np qua t dng 2BI cn

c cung cp t ngun qua 2BR . Do ,

Q2 vn c duy tr trng thi dn cho d t 1C np y. ng thi, t 2C

c np qua 1BR v Q2 sau khi x ht

xem hnh 3.7c. in p trn t 2C , 2CV

(in p trn t 2C ) tng dn.

M 21 CBE VV 1BEV cng tng dn. Ti mt lc no , 1BEV ln lm Q1

dn. t 1C t in p m vo mi ni BE ca Q2. 02 BEV lm Q2 tt.

Ta thy, ban u, Q1 dn, Q2 tt, sau mt thi gian mch t ng i qua trng thi Q1 tt, Q2 dn. Khi Q1 tt, Q2 dn sau mt thi gian mch li t i qua trng thi Q1 dn, Q2 tt v c lp i lp li. Do , khng c trng thi n nh. V vy, mch c gi l mch dao ng bt n hay mch dao ng phi n.

Dng in p 1OV , 2OV , 1BEV v 2BEV nh sau:

Vcc

Q1 Q2

1OV

1CR

2C

2OV

2CR1BR

1C

2BR

1BEV 2BEV

Hnh 3.7c


36

t

0 t1

t1

t2

t2

t

0

Vcc

t1 t2

1OV

2OV

t

0

Vcc

1BEV

-Vcc

V

1xT

t1 t2

t

0

V

2BEV

-Vcc

2xT

t3 t4

t3 t4

t3 t4

t3 t4

Hnh 3.8

T hnh 3.8 ta thy, 1xT chnh l thi gian t 2C x v np qua 1BR , 2xT

chnh l thi gian t 1C x v np qua 2BR . Tng t nh mch dao ng n n

ta c cng thc: 211 693.0 CRT Bx

122 693.0 CRT Bx

1xT , 2xT c n v l s. T 1C , 2C c n v l F. 1BR , 2BR c n v l .

Chu k ca in p ra: )(693,0 122121 CRCRTTT BBxx

Tn s ca in p ra: T

f1


37

3.2 Mch dao ng a hi dng op-amp

3.2.1 Trng thi bo ho ca op-amp v mch so snh

Vo

Vi

0

AVcc

AVcc

Vung bao hoa

dng

Vung bao hoa

am

Vung tuyen

tnh

oSatV

oSatV

Hnh 3.9 K hiu op-amp Hnh 3.10 c tuyn truyn t ca op-amp Hnh 3.10 l c tuyn truyn t ca op-amp vi A l li p vng h,

VVVi l in p vo. Da vo c tuyn truyn t ta thy:

Khi VV th osato VV , op-amp lm vic trong vng bo ha dng.

Khi VV th osato VV , op-amp lm vic trong vng bo ha m.

Khi VV th io VAV .

i vi op-amp l tng th A , VccVosat v in tr ng vo iR . i

vi op-amp thc t th 85 1010 A , osatV nh hn Vcc mt vi volt v in tr

ng vo 136 1010 iR .

Op-amp c dng thc hin mch khuch i, mch cng, mch tr, mch so snh, thc hin mch so snh ta cho op-amp lm vic trong vng bo ha nh hnh 3.11 sau:

+

-

+Vcc

-Vcc

oV

DCV

iV

+

-

+Vcc

-Vcc

oV

DCV

iV

+

-

+Vcc

-Vcc

oViV

Hnh 3.11a Hnh 3.11b Hnh 3.11c

Bi tp Hy v v gii thch dng in p ng ra ca cc mch hnh 3.11abc. Bit

tVi sin10 , vi bt k, DCV c ln bng 5v.

+

-

+Vcc

-Vcc

oVV

V


38

3.2.2 Mch dao ng n n (monostable)

t

0

V

t2

t

0 t3

iV

t1

OT

phT

Vcc

Vcc

Vcc

OV

V

Hnh 3.13

Nguyn l hot ng: ch xc lp, op-amp lm vic trong vng bo ha m, VccVo .

Lc ny, diode dn nn VV , vi V l in p phn cc thun ca diode.

M: CCCCO VVRR

RV

RR

RV

21

1

21

1

Vi 21

1

RR

R

Do CCVV VV , suy ra, op-amp tip tc lm vic trong vng bo ha

m. Nu khng c tc ng bn ngoi th mch khng thay i trng thi. V vy, y l trng thi n nh ca mch.

Khi c xung dng tc ng vo iV , VV , lm op-amp i qua trng

thi bo ha dng, VccVo .

Lc ny, diode ngng dn, t C np nng lng t oV qua R xung mass, lm

V tng dn.

M: CCO VVVRR

RV

21

1

Do V c th tng dn ti Vcc , V c in th nh hn Vcc nn ti mt lc

no VV . Khi ny, op-amp lm vic trong vng bo ha m, VccVo ,

diode dn, t x v in th V c ghim mc V . Op-amp trng thi bo

ha dng l trng thi khng n nh ca mch.

Hnh 3.12

+

-

+Vcc

-Vcc

oVV

R

C

2R

1RiV

V


39

T nguyn l hot ng trn ta thy, mch c mt trng thi n nh nn mch c gi l mch n n hay cn gi mch mt trng thi bn. Hnh 3.13 l dng in p ng ra v dng in p trn t ca mch.

oT l rng xung, n ph thuc vo thi gian np ca t, v vy, tnh oT ta

xt phng trnh sau:

t

C eVVcctV 1)()(

Ti thi im oTt ta c:

VVcceVVccTVOT

oC

1)()(

VVcc

VVcce

OT

1

Vcc

VVcc

V

eOT

1

1

t Vcc

Vk

k

ke

OT

11

k

ke

OT

11

ke

OT

1

1

1

1 ke

OT

1

1ln

kTO

1

1ln

kTO

M: RC

1

1ln

kRCTO

Tng t, ta c:

kRCTph

1

1ln

3.2.3 Mch dao ng bt n (astable) a. Nguyn l:

Mch in hnh 3.14 l s mch dao ng tch thot dng Op-amp cho ra tn hiu vung. S c hai mch hi tip t ng ra v hai ng

vo. C u phn p RC hi tip v ng In

-, cu phn p R1 R2 hi tip v ng In

+.

gii thch nguyn l mch ta gi s t C cha np in v Op-amp ang trng thi bo ha dng. Lc ny, cu

Hnh 3.14: Mch dao ng tch thot

-

C nap

+

-Vcc

Vo=+Vcc

C

+

-

R1

R2

+Vcc

R


40

phn p R1 R2 a in p dng v ng In+ vi mc in p l: V0 = +VCC

21

2.RR

RVV

CCinVA (

inV > 0V)

Trong khi , ng In- c in p tng dn ln t 0V, in p tng do t C np

qua R theo quy lut hm s m vi hng s thi gian l =RC Khi t C np c

inV <

inV th Op-amp vn trng thi bo ha dng. Khi t C

np n mc in p in

V > in

V th OP-AMP i thnh trng thi bo ha m, ng

ra c V0 = -VCC. Lc ny cu phn p R1 R2 a in p m v ng In+ vi

mc in p l:

21

2.RR

RVV

CCinVB (

inV < 0V )

Trong khi ng In- vn cn ang mc in p dng vi tr s:

21

2.RR

RVV

CCin

do t C ang cn np in. Nh vy Op-amp s chuyn sang trng thi bo ha m nhanh cho cnh xung vung thng ng. T C by gi s x in p dng ang np trn t qua R1 v ti ng ra xung mass. Khi t C x in p dng ang c th

inV vn mc in p m nn Op-amp vn trng thi bo ha m.

Khi t C x ht in p dng s np in qua R c in p m ang c do ng ra ang trng thi bo ha m chiu np by gi ngc vi chiu dng in np trn hnh v. Khi t C np in p m n mc

inV <

inV (ng In

- nh hn ng In+) th Op-amp

li i thnh trng thi bo ha dng v ng ra c V0 = +VCC. Mch tr li trng thi gi thit ban u v hin tng trn c tip din lin tc tun hon.

b. Dng sng cc chn: Mc gii hn in p ng ra l:

V0max +VCC

V0min -VCC Mc gii hn in p hai ngvo l:

VA 21

2.RR

RV

CC

VB 21

2.RR

RV

CC

Dng in ng vo In- l dng tam gic. Thi gian in p ng vo In

- tng t VB ln VA l Op-

amp trng thi bo ha dng, Thi gian in p ng vo In-

VA

Vin-

VB

Vin+

VA

VB

Vo Vcc

-Vcc

t

t

t

Hnh 3.15. Dng sng cc chn.


41

gim t VA xung VB l Op_amp trng thi bo ha dng. Dng in p ng In

+ v ng ra l trng thi xung vung i xng. Chu k ca tn hiu c

tnh theo cng thc

1

21 2.2R

RRCLnRT

Suy ra tn s ca tn hiu xung c tnh theo cng thc

T

f1

Trng hp c bit:

R1 = 2R2 T = 2.R.CLn2 =2.R.C.0,69

f = CRCR ..4,1

1

..69,0.2

1

R1 = R2 T = 2.R.CLn3 =2.R.C.1,1

CR

f

..2,2

1

3.3 Dao ng dng vi mch LM555

3.3.1 Cu to vi mch LM555 Vi mch 555 c ch to thng dng nht

l dng v Plastic nh hnh v 3.16 Chn 1: GND (ni t) Chn 2: Trigger Input (ng vo xung ny) Chn 3: Output ( ng ra)

Chn 4: Reset (hi phc) Chn 5: Control Voltage ( in p iu khin) Chn 6: Threshold (Thm ngng) Chn 7: Dirchage ( x in) Chn 8: +VCC ( ngun dng)

Vi mch nh th 555 c 8 chn, c ngun nui trong khong t +5V n +15V v mc p ti a l +18V. Bn trong vi mch 555 c hn 20 Transistor v nhiu in tr thc hin chc nng nh trong hnh 3.17 gm:

Cu phn p gm 3 in tr 5K ni t ngun +VCC xung mass cho ra hai in p chun l 1/3 VCC v 2/3 VCC .

Op-amp (1) l mch khuch i so snh c ng In- nhn in p chun 2/3

VCC cn ng In+ th ni ra ngoi chn 6. Ty thuc in p ca chn 6 so vi

in p chun 2/3 VCC m Op-amp (1) c in p mc cao hay thp lm tn hiu R (Reset) iu khin Flip-Flop(F/F).

Hnh 3.16: Hnh dng IC555

6 57

555

2 41

8

3


42

Op-amp (2) l mch khuch i so snh c ng In+ nhn in p chun 1/3

VCC cn ng In- th ni ra ngoi chn 2. Ty thuc in p chn 2 so vi

in p chun 1/3 VCC m Op-amp (2) c p th ra mc cao hay thp lm tn hiu S (Set) iu khin Flip Flop (F/F).

Mch Flip-Flop (F/F) l loi mch lng n kch mt bn. Khi chn Set (S) c in p cao th in p ny kch i trng thi ca F/F l ng Q ln mc cao v ng Q ca xung mc thp. Khi ng Set ang mc cao xung thp

th mch F/F khng i trng thi. Khi chn Reset (R) c in p cao th in p ny kch i trng thi ca F/F lm ng Q ln mc cao v ng Q

xung mc thp. Khi ng Reset ang mc cao xung thp th mch F/F khng i trng thi.

Mch OUTPUT l mch khuch i ng ra tng khuch i dng cp cho ti. y l mch khuch i o c ng vo l chn Q ca F/F nn khi

Q mc cao th ng ra chn 3 ca IC s c in p thp ( 0v) v ngc ti

khi Q mc thp th ng ra chn 3 ca IC s c in p cao (VCC)

Transistor T1 c chn E ni vo 1 in p chun khong 1,4 V v loi Transistor PNP nn khi cc B ni ra ngoi bi chn 4 c in p cao hn 1,4V th T1 ngng dn nn T1 khng nh hng n hot ng ca mch khi chn 4 c in tr tr s nh thch hp (ni mass) th T1 dn bo ha ng thi lm mch OUTPUT cng dn bo ha v ng ra xung thp. Chn 4 c gi l chn Reset c ngha l n Reset IC 555 bt chp tnh trng cc ng vo khc, do chn Reset dng kt thc xung ra sm khi cn. Nu khng dng chc nng Reset th ni chn 4 ln VCC trnh mch b Reset do nhiu.

Transistor T2 l Transistor c cc C h ni ra chn 7 (Discharge = x). Do cc B c phn cc bi mc in p ra Q ca F/F nn khi Q mc

cao th T2 bo ha v cc C ca T2 coi nh ni mass, lc ng ra chn 3 cng mc thp. Khi Q mc thp th T2 ngng dn cc C ca T2 b h,

lc ng ra chn 3 c in p cao. Theo nguyn l trn cc C ca T2 ra chn 7 c th lm ng ra ph c mc in p ging mc in p ca ng ra chn 4. Hnh 3.17 l s cu trc bn trong ca IC 555 v theo kiu s chc nng.


43

3.3.2 Mch dao ng n n (monostable) Hnh 3.18: Mch n n dng IC 555

c th phn tch nguyn l ca mch n n mt cch r rng, d hiu chng ta s dng s hnh 3.18. S hnh v mch p dng IC 555 lm mch n n, s hnh 3.18 kt hp vi s cu trc bn trong IC.

Trong mch ny chn ngng s 6 v chn x s 7 c ni vo im chung ca mch nh th RTC. Chn nhn xung kch s 2 c ni ln ngun + VCC qua in tr 10K sao cho chn ny c in p ln hn 1|3VCC.

Hnh 3.18: Mch n n dng IC 555

c im ca mch n n l khi c xung m hp tc ng tc thi ng vo Trigger chn hai mch s i trng thi v ti ng ra chn 3 s c xung dng ra. rng xung ng ra c thi gian di hay ngn ty thuc mch nh th RTC, sau mch s tr li trng thi ban u. Nguyn l mch n n c gii thch nh sau: Khi m in t C ni chn 6 v 7 xung masse lm OP- AMP (1) c ng In

+

nh hn ng In- nn ng ra V01 = 0v, ng R mc thp. Lc OP-AMP (2) c

Hnh 3.17 Cu trc ca IC 555.

2

Control

Voltage

T2

Vcc

1

+

-15

Trigger Input

S

GND

5K

R OUTPUT

3

Threshold

3

5K

6

F/F

8

2

Vcc

5K

Vr = 1.4 V

+

-2

T1

3

4

1

Vcc

Q

Dirchage

Reset

OUTPUT

NOT

7

+Vcc01

K

C

K

Rt

555

12

3

45

6

7

8

Vo

10 K

K

+Vcc


44

ng In+ cng nh hn ng In

- nn ng ra V02 = 0v, ng s cng thp. Mch F|F c

hai ng R v S u mc thp v nh cu trc ca mch chi tit n F\F c ng ra Q mc cao, qua mch o ng ra chn 3 s c mc thp gn 0v. khi Q mc

cao o phn cc bo ha cho T2 lm T2 dn ni chn 7 xung mass, chn 6 cng b ni mass nn t C khng np in c, mch s n nh trng thi ny nu khng c tc ng khc bn ngoi. Khi ng kha K s c xung m kch vo chn Trigger s 2 lm OP-AMP (2) i trng thi ng S ln mc cao. Mc cao ca ng S iu khin lm F|F i trng thi, lm ng Q xung mc thp, ng ra qua mch o s tng ln mc cao

v xung dng ra. Lc , Q mc thp nn T2 ngng dn t C np in qua

RT. Trong thi gian t C np in mch vn gi trng thi ny nn ng ra tip tc ng cao. in p np trn t c tr s tng theo hm s m v khi in p t ga tr 2|3 VCC th OP-AMP (1) i trng thi,ng R tng ln mc cao. Ng Rc mc cao s iu khin F|F tr li trng thi c, ng Q ln mc cao lm ng ra qua mch

o s xung mc thp chm dt xung dng ra. ng thi lc T2 c phn cc bo ha nn chn 7 ni mass lm t C x in, mch s n nh trng thi ny cho n khi no c xung m khc tc ng vo chn Trigger (s 2). Thi gian xung dng ra tc l thi gian np in t 0v ln 2|3 VCC c tnh theo nh sau: in p np trn t tng theo hm s m l:

VC = VCC ( 1- x

t

e )

Trong = RT. C Thi gian t np c in th t 0V ln 2|3 VCC l tx c tnh bi:

VC = VCC ( 1 - x

t

e ) = 2|3VCC

Suy ra: ( 1- x

t

e ) = 3

2hay l 1 -

3

2 =

xt

e

3

1 =

xt

e =

xt

e

1

x

t

e = 3

Cui cng ta c hm s ngc ca hm s m l Ln.

Nh vy: tx = . Ln3 ( Ln3 = 1,1 ) tx = 1,1 RT. C


45

3.3.3 Mch dao ng bt n (astable)

S mch hnh 3.19 l ng dng ca IC 555 l mch a hi phi n to xung vung.

Trong mch chn ngng (Threshold) so 6 c ni vi chn Trigger s 2 nn hai chn ny c chung in p l in p trn t C so vi in p chun 2/3 VCC v 1/3 VCC bi Op- amp (1) v Op- amp (2).

Chn 5 c t nh 0.01F ni mass lc nhiu tn s cao c th lm nh hng in p chun 2/3 VCC. Chn 4 ni ngun +VCC nn khng dng chc nng Reset, chn 7 x in c ni vo gia 2 in tr RA v RB to ng x in cho t. Ng ra chn 3 c in tr 1,2 K hn dng cho Led v Led biu th mc in p ra (ch c th dng trong trng hp tn s dao ng c tr s thp t 20HZ tr xung cn tn s cao hn 40HZ trng thi sng tt ca Led kh c th nhn bit bng mt thng). phn tch nguyn l ca mch cn phi hp mch ng dng hnh 3.19 v s cu trc hnh 3.17. Khi mi ng in t C bt u np t 0V ln nn: - Op-amp (1) c VI

+< VI

- nn ng ra c V01 = mc thp, ng R= 0 (mc thp).

- Op-amp (2) c VI+ > VI

- nn ng ra c V02 = mc cao, ng S=1 (mc cao).

Mch F/F c ng S=1 nn Q =1 v Q = 0. Lc ng ra chn 3 c V0 VCC (do

qua mch o ) lm Led sng. - Transistor T2 c VB2 = 0 do Q = 0 nn T2 ngng dn t C c np in.

T C np in qua RA v RB vi hng s thi gian khi np l:

)(BAnap

RR C

Khi in p trn t tng n mc 1/3 VCC th Op-amp (2) i trng thi, ng ra c V02 = mc thp, ng S = 0 ( mc thp). Khi S xung mc thp th F/F khng i trng thi nn in p ng ra vn mc cao, Led vn sng. Khi in p trn t tng n mc 2/3 VCC th Op-amp (1) di trng thi, ng ra c V01 = mc cao, ng R = 1.

Mch F/F c ng R= 1 nn Q= 1. Lc ra chn 3 c V0 0V lm Led tt. Khi ng Q=1 s lm T2 dn bo ha v chn 7 ni mass, lm t C khng np tip in p c m phi x in qua RB v Transistor T2 xung mass. T C x in qua RB vi hng s thi gian l:

x = RB. C Khi in p trn t (tc la in p chn 2 v chn 6) gim xung di 2/3 VCC th Op-amp(1) i tr li trng thi c l V01 = mc thp, ng R = 0. Khi R xung mc thp th F/F khng i trng thi nn in p ng ra vn mc thp,

Hnh 3.19: Mch a hi phi n

+VCC=12V

.01

C

R96

Ra

R=1,2K

Rb

555

12

3

4

5

6

78


46

Led vn tt. Khi in p trn t gim xung n mc 1/3 VCC th Op- amp(2) li c VI

+ > VI

- nn ng ra c V02 = mc cao, ng S1 = 1. Mch F/F c ng S=1 v

0Q , ng ra chn 3 qua mch o c V0 + VCC lm Led li sng, ng thi

lc T2 mt phn cc do 0Q nn ngng dn v chm dt giai on x in

ca t. Nh vy mch tr lai trng thi ban u v t li np in t mc 1/3 VCC nn n 2/3 VCC, hin tng ny s tip din lin tc v tun hon.

Lu : Khi mi m in t C s np in t 0V ln 2/3 VCC ri sau t x in l 2/3 VCC xung 1/2 VCC ch khng x xung 0V. Nhng chu k sau t s np t 1/3VCC ln 2/3VCC ch khng np t 0V na.

Thi gian t np l thi gian V0 + VCC, Led sng. Thi gian t x l thi gian

V0 0V, Led tt. Thi gian np v x t c tnh theo cng thc:

Thi gian np: tnp = 0,69. np tnp = 0,69.( RA + RB ) C

Thi gian x: tx = 0,69.x tx = 0,69. RB. C

in p ng ra chn 3 c dng hnh vung vi chu k l: T = tnp +tx

T = 0,69 ( RA + 2RB ) C

Do thi gian np v thi gian x khng bng nhau ( tnp > tx ) nn tn hiu hnh vung ra khng i xng. Tn s ca tn hiu hnh vung l:

CRRT

f

BA)2(69,0

11

3.4 Dao ng dng cc cng logic

mach dao dong da hai

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