transistor trường fet

8/14/2019 transistor trng FET

1/13

Chng 6 1

CHNG 6: TRANSISTOR HIEU NG TRNG FET

6.1 Gii thieu6.2 Ly thuyet hoat ong cua JFET6.3 Ly thuyet hoat ong cua MOSFET6.4 Giai tch o th va phan cc6.5 Giai tch tn hieu ln S sai dang6.6 Giai tch tn hieu nho6.7 M rong


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Chng 6 2

6.1 Gii thieu

Transistor hieu ng trng (Field Effect Transistor FET): JFET: Junction FET MOSFET: Metal-Oxid Semiconductor FET (Insulated-Gate IGFET)

Tnh chat (Phan biet vi BJT) Nhay vi ien ap (voltage-sensitive) Tr khang vao rat cao

6.2 Ly thuyet hoat ong cua JFET6.2.1 Cau tao (n-channel JFET):


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Chng 6 3

6.2.2 Hoat ong:

Gia s S va G noi at; vDS > 0: Dong iD : D S: Phu thuoc vao vDS va ien tr kenh n (Rn-Channel)Dong iChannel Gate 0: Do Diode tao bi tiep xuc pn Channel-Gate phan cc nghch

(a) Khi vDS tang: Vung khuyet (depletion region vung gach cheo) tang Rn-Channel tang

(b) vDS = Vpo (ien ap nghen: pinch-off voltage): Hai vung khuyet cham nhau: iD = Ipo


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Chng 6 4

(c) vDS > Vpo: Va = Vpo = const iD = Ipo = const

(d) vDS = BVDSS: ien ap anh thung.o th:

Gia s vDS = const; vGS thay oi:vGS < 0: Tang vung khuyet i) RChannel tang iD giam

ii) Vpo giamvGS > 0: Giam vung khuyet i) RChannel giam iD tang

ii) Vpo tang


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Chng 6 5

Voltage-Sensitive Device

o th:

Lu y : n-JFET: Phan cc sao chokhongco dong IChannel-Gate (vGS 0 hoac vGS nho > 0)

6.2.3 ac tuyen:ien ap vDS tai iem nghen: vDS-Pinch Off = Vp = Vpo + vGS ien ap anh thung: BVDSX BVDSS + vGS ac tuyen VA trong vungbao hoa (Gia ien ap nghen va anh thung: Vp < vDS < BVDSX)

iD =

++

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23

1 po

GS

po

GS po V

v

V

v I vi vGS < 0

Nhan xet: vGS = 0: iD = Ipo


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Chng 6 6

VGS = - Vpo: iD = 0

Trong vung bao hoa: iD khong phu thuoc vDS Anh hng nhiet o:

iD =

++

2 / 32 / 30 2

31'

po

GS

po

GS po V

v

V

v

T

T I

trong o: Ipo = iD khi vGS = 0 tai nhiet o T0.

6.3 Ly thuyet hoat ong cua MOSFET6.3.1 Cau tao (n-channel MOSFET):

Nhan xet: Ban au cha co kenh dan gia D va S (enhancement mode )Cc cong Gate: Metal Oxide Semiconductor (MOS)


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Chng 6 7

6.3.2 Hoat ong:

Hoat ong loai tang (enhancement mode): vGS > 0: Hnh thanh kenh dan cam ng: vGS > VTN : ien ap ngng Tao kenh dan n cam ng gia S va D

vGS tang Be rong va ien dan (conductivity) kenh dan tang Thay oi vDS: Tng t JFET:

(a) Khi vDS tang Tang vung khuyet Rn-Channel tang: Vung tuyen tnh


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Chng 6 8

(b) vDS = Vp = vGS - VTN: ien ap nghen: Rn-Channel (100 K)


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Chng 6 9

(c) vDS > Vp: iD const: Vung bao hoa

o th:

Lu y : enhancement mode n-MOSFET: Phan cc vGS VTN


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Chng 6 10

6.3.3 ac tuyen:ien ap v

DStai iem nghen: v

DS Pinch Off= V

p= v

GS V

TN= v

GS+ V

po(Vi V

po= - V

TN< 0)

ac tuyen VA trong vung tuyen tnh (vDS < vGS - VTN = Vp):])(2[ 2 DSTN GSn DS vV vk i =

ac tuyen VA trong vung bao hoa (vDS vGS - VTN = Vp):2

2 1][

+==

po

GS poTN GSn DS

V

v I V vk i vi Ipo = knVTN2 va Vpo = - VTN

Nhan xet: n-JFET: vGS 0, Vpo > 0; Enhancement mode n-MOSFET: v GS > 0, Vpo < 0ac tuyen VA: JFET: Bac 3/2 MOSFET: Bac 2

Xem gan ung cho ca hai loai FET:2

2 1][

+==

po

GS poTN GSn DS V

v I V vk i

Anh hng nhiet o:2 / 3

'

=

T

T I I o po po


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Chng 6 11

6.4 Giai tch o th va phan cc

6.4.1 Phan cc JFET:

DCLL: VDD = vDS + iD (Rd + Rs) Phng trnh phan cc: vGS = - iD Rs (Xem iG 0)Nhan xet: Mach t phan cc (self-bias): Do vGS < 0 tao ra bi Rs V du: Thiet ke mach vi tnh iem Q: VDSQ = 15V; IDQ = 3,5 mA

Thay vao DCLL: Rd +Rs = (VDD VDSQ) / IDQ = (30 15) / 3,5 = 4,3 K T ac tuyen VA: VGSQ = -1 V Rs = - VDSQ / IDQ = 1V / 3,5 mA = 286 Rd 4 K Chon Rs = 270 va Rd = 3,9 K


12/13


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Chng 6 13

T phng trnh:22 / 3

0' 1

+

= po

GS po D

V

v

T

T I i

o nhay:

po

S

po

DSGGo po

D DiT

V

R

V

i RV T T I

T dT

idiS D

++

==

1) / (21

2 / 3

/

/

2 / 3'

Nhan xet: Rs 0 lam giam o nhay iD theo t0 Cai thien o on nhe cc tieu DiT S : VGG = 2VGSQ + Vpo

Rs = DQ

poGSQ

I

V V +

6.5 Giai tch tn hieu ln S sai dang

transistor trường fet

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