-
8.
Field Effect Transistors (FET)
-
, .
8.1
(Field Effect Transistor (FET)) . .
, (p n) FET ( (BJT)).
FET BJTS . , .
FET. FE (Junction) (JFET) (w) p-n.
p-n (MEtal)-(Semiconductor) (MESFET). (Insulator) MISFET.
MOSFET.
MOSFET (VLSI, Very-Large-Scale-Integrated circuits) .
FET BJT , 1930, Lilienfeld, , . Barden Brattain , Shockley .
281
-
, .
JFET transistor, ( e-)
() JFET. (source) (drain). .
p JFET n ( ()). (gates).
JFET ( () ())
8.2 Transistor (Junction) s JFET
n
n 1
p p 2
n
n
p p
n
VDD+
-
() () ()
282
-
, .
JFET p-n .
ID -n -p. -p n . -n -p, . n- .
, -p (gates). -p -n, . x . Vx 0 ( ) VDS=VD ( )( )
283
-
, .
(VGS=VG=0). - VD ID . .
VD ID , . VD VG, (pinchoff) . ID VD. .
284
-
, .
(-VG) ID.
VG .
VG I-V JFET. ID VG. , . VG . JFET Field Effect
JFET . JFET GaAs ( ) .
285
-
, .
8.3 FET
286
-
, .
287
-
, .
288
-
, .
289
-
, .
290
-
, .
(source) (drain)
( )
n
(gate)
,
Vp ( ) ID
IDSS JFET
8.3 JFET
n-type p-type
VDDVGG
VDS+
-
VDS
ID
Vp
IDSS
VDS(max)
291
-
, .
8.4 JFET
(drain)
VGS=0 ( )
Vp=4V
30 V
IDSS = 10 mA
(VGS(off))
VGS = VGS(off), . Vp Vp=-VGS(off)
VGS=0
VGS=-1
VGS=-2
VGS=-3 VGS=-4
VDS
ID
4 15 30
0.625 mA
2.5 mA
5.62 mA
10 mA
292
-
, .
1
9/16
1/4
1/16
1 3/4 1/2 1/4
ID
VGS
VGS(off)
IDSS
VDS=15 V
10 mA
5.62 mA
2.5 mA
0.625 mA
-4 -3 -2 -1
ID
VGS
ID = IDSS[1 VGS/VGS(off)]2 ID/IDSS = [1 VGS/VGS(off)]
2
293
-
, .
8.5 BJT vs JFET
JFET Bipolar
. , .
. , .
294
-
, .
(gate bias)
VGG RG. Q.
FET , Q.
:
8.6
RD
RG
+VDD
-VGG 16 mA
4 mA
-8 V -2 V -1 V
ID
VGS
Q1
Q2
IDSS 4 mA 16 mA
VGSS(off) -2 V -8 V
VGS=-1 V, Q Q1 Q2. Q , Q1 Q2.
295
-
, .
VS = VG - VGS
VD = VDD - IDRD
8.6.1
2 DDG1 2
RV = V
R +R
JFET transistor
transistor, VBE 0,7 V transistor . VGS JFET . VG VGS. JFET
transistor.
ID = (VG - VGS) / RS
296
-
, .
transistor. VGS. VSS VGS. VSS RS, ID = VSS/RS.
:
VSS VGS. VGS -1 -5 V. , -10 -15 V
8.6.2
RD
RG
+VDD
RS
-VSS
SS GSD
S
V -VI =
R
297
-
, .
JFET. . RS, -. JFET.
, RS. -, . (ID).
, , - . . (ID).
8.6.3
RD
RG
+VDD
RS
+
-
VS
0 V
298
-
, .
-
IG0. VG = 0. -
VGS=-IDRS , Q (VGSQ, IDQ)
:
IDQ
VGSQ
ID
VGS
Q
GSQGSS
D DQ
-V-VR = =
I I
RS Q.
Q, . RS VGS(off) IDSS RS=-VGS(off)/IDSS
VGS + VS = 0 VGS + IDRS = 0 VGS = -IDRS
299
-
, .
JFET ac gm. :
(transconductance factor) ,
siemens (S), mho.
8.7 AC JFET
Dm
GS
Ig =
V
300
-
, .
gm . gm .
gm , Q (.. ). gm .
gm . gm, .
gm :
JFET
ac JFET.
RGS, M.
gmvgs (vgs = ac - )
.
gm
D
ID
VGS
C
BA
gm
RGS
gmvgsvgs+
-
GS(off)
GS
GS(off)
DSSm
V
V1
V
2Ig
GS(off)
GSmom
V
V1gg
301
-
, .
10.8.1
ac , . . RD , ac .
VGS(off) VGS=0, VGS=0 ID, VD = VDD - IDRD () . , .
ac R1, R2. RGS , : zin=R1||R2 ac : rD=RD||RL.
To : vout=gmvgsrD=gmvinrD
8.8 JFET
RD
R2
+VDD
RS
R1
Vin RL
Vingmvgsvgs
+
-RD||RLR1||R2
Vout
Vout
c
outm D
in
vA= =g r
v
JFET . JFET gmrD RG.
302
8.8.1
-
, .
JFET
303
-
, .
.
ac ac . ac (VS = VG VGS). .
, .
ac :rS=RS||RL
: vgs+gmvgsrS-vin=0 vin=(1+gmrS)vgs RGS , ac : vout=gmvgsrS
:
gmrS
mout S
min S
g rvA= =
v 1+g r
R2
+VDD
RS
R1
Vin
RL
Vingmvgs
RGS
Vout
Vout
c
R1||R2 RS||RL
+ vgs -
304
8.8.2
-
, .
ac : vout=gmvgsrD ac : vin=vgs :
ac : iin=id=gmvgs vgs=vin zin=vgs/iin=1/gm
. ( )
outm D
in
vA= =g r
v
RS
+VDD
RD
Vin
RL
Vingmvgsvgs
+
-rDRS
Vout
Vout
c
1/gm
-VSS
JFET , transistor.
305
8.8.3
-
, .
:
=gmrD=(2000S)(4,7||10)=7,83
:
Vout=AVin=7,83(2mV)=15,7 mV
:
rS=RS||RL=7,5||3=2,14
:
1. gm=2000S, RD=4,7 RL=10. Vin=2mV, Vout;
2. gm=2500S, RS=7,5 RL=3. ;
mout S
min S
g rv (2500S)(2,14)A= = = =0,843v 1+g r 1+(2500S)(2,14)
306
