Slide điện tử từ trường lecture 9 mosfet circuits
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Lecture 09
MOSFET Circuits
1
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
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Topic
• Small signal equivalent circuit model
– π model
– T model
– Body effect
• Signal stage MOSFET Amplifier
– CS Amplifier
– CD Amplifier
– CG Amplifier
2
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Small-signal equivalent-circuit model
gm ≡
∂id
∂vGS
k 'n W
(vGS − Vt ) 2
2L
k' W
id
if v gs << 2(VGS − Vt )
iD = n (VGS + v gs − Vt ) 2
2L
k' W
k' W
k' W 2
iD = n (VGS − Vt ) 2 + n (VGS − Vt )v gs + n v gs
2L
L
2L
iD =
ID
CuuDuongThanCong.com
vGS =VGS
3
Microelectronic circuit by
meiling CHEN
/>
k 'n W
id =
(VGS − Vt )v gs
L
id
k 'n W
⇒ gm ≡
=
(VGS − Vt )
v gs
L
vD = VDD − RD iD
vD = VDD − RD ( I D + id )
vD = VDD − RD I D − RD id = VD − RD id
v d = − RD id = − RD g m v gs
vd
Av ≡
= − g m RD
v gs
For the above circuit
ro =
vd
Av ≡
= − g m ( RD // ro )
v gs
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
VA
ID
4
Example 4.10
1. DC analysis
k 'n W
ID =
(VGS − Vt ) 2
2L
= 12 × 0.25m(VGS − 1.5) 2
VGS = VD
VD = VDD − I D RD = 15 − I D × 10k
⎧ I D= 1.06mA
⇒⎨
⎩ VD = 4.4V
Vt = 1.5V
k n' (W ) = 0.25mA / V 2
L
VA = 50V
k 'n W
gm =
(VGS − Vt ) = 0.725mA / V
L
ro =
VA
ID
= 47 k
5
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
2. Small-signal analysis
vo = (i − g m v gs )(ro // RD )
i
vi = v gs
v gs = vo + iRG ⇒ i =
Av =
vo
vi
RL → ∞
v gs − vo
RG
ro // RD
− g m (ro // RD )
RG
=
r // RD
(1 + o
)
RG
Ro
vi = 0
io
RL = 0
Best voltage gain
ii =
V = IRG + vo = IRG + ( I − g mV )(ro // RD )
RG + (ro // RD )
Rin =
1 + g m (ro // RD )
= RG // ro // RD (Q v gs = 0 ∴ g m v gs = 0)
=
− g m v gs
RG
v gs
RG
io
Ais =
is
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
v gs
RL = 0
=1 − g m RG
Best current gain
/>
6
T-model
7
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
8
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Modeling body effect
v gs > 0 , and
vBS > 0 ⇒ Vt ↑⇒ iD ↓
v gs < 0 , and
vBS < 0 ⇒ Vt ↓⇒ iD ↑
9
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
10
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Fixed current Bias
k 'n W
gm =
(VGS − Vt )
L
ro =
VA
ID
k 'n W
(VGS − Vt ) 2
2L
2L × I
− Vt =
k 'n W
I = ID =
VGS
11
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Exercise 4.30
k 'n W
(VGS − Vt ) 2
2L
2L × I
− Vt =
= 2 × 1 × 0 .5 = 1
k 'n W
I = ID =
VGS
k 'n W
gm =
(VGS − Vt ) = 1×1 = 1
L
VA 75
=
= 150k
ro =
0
.
5
m
ID
12
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Single stage MOS amplifiers
• Common source Amplifier
• Common source Amplifier + Source
resistance
• Common Drain Amplifier
• Common Gate Amplifier
13
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
14
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
CS Amplifier
DC voltage source short
DC current source open
Capacitor short
Replace transistor by
small signal equivalent
model
15
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
vo
= − g m v gs (ro // RD )
RL = ∞
vo = − g m v gs (ro // RD // RL )
vi = v gs
Av = − g m (ro // RD // RL )
io
RL = 0
ii =
= − g m v gs
Rout
vs =0
= (ro // RD )
V = v gs
I=
v gs
RG
Rin = RG
v gs
RG
Ais = − g m RG
CuuDuongThanCong.com
16
Microelectronic circuit by
meiling CHEN
/>
CS Amplifier with source resistance
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Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
vs
AC
Rs
g m v gs
vi
+ v gs
vo
−
RG
vi
+ v gs
RG
RL
RD
RS
vo
Av =
ii =
RS
RD
− g m RD
1 + g m RS
RL = 0
I
−
= − g m v gs RD
vi = v gs + vs = v gs + g m v gs RS
io
g m v gs
RL = ∞
vi
V
Ais =
= − g m v gs
RG
=
v gs (1 + g m Rs )
RG
− g m RG
(1 + g m Rs )
v gs = − g m v gs Rs
Q − g m Rs ≠ 0 ∴ v gs = 0
Ro
vi =0
= RD
Ri = RG
18
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
G
vi i = 0
+ v gs −
RG
i
i
1
gm
I
I RD
RD
V
S
RS
i = g m v gs = I + I RD
v gs = −iRs = −( I + I RD ) RS
I + I RD = g m v gs = − g m ( I + I RD ) RS
⇒ (1 + g m RS ) I = −(1 + g m RS ) I RD
⇒ I = − I RD
V
Ro = = RD
I
19
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
CG Amplifier
vo
RL = ∞
= − g m v gs RD
vi = −v gs
Av = g m RD
io
= − g m v gs
RL = 0
ii = − g m v gs
vs
AC
Rs
g m v gs
vi S
−
RD
v gs
+
vo
Ais = 1
RL
G
20
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
vs
Rs
vi
1
i
gm
vo
i
AC
vi
RL
RD
g m v gs
S
−
+
I
RD
v gs
1
Ri =
gm
V
G
Ro
vi =0
= RD
21
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
CD Amplifier
vo
RL→∞
Av =
io
vs
AC
Rs
RG
1
vi
i
vo
gm
i
ro
RL
=
ro
1
ro +
gm
vi
ro
ro +
RL = 0
1
gm
=i
1
i[ro +
]
v
v
gm
g
ii = i =
=
RG RG
RG
Ais =
RG
1
ro +
gm
=
RG g m
1 + ro g m
22
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
vs
AC
Rs
vo
vi
+ v gs
RG
−
ro
g m v gs
RL
vo
RL → ∞
= ro g m v gs
vi = v gs + vo = v gs + ro g m v gs
ro g m
Av =
1 + ro g m
Ro
vi = 0
= (ro // g1m )
io
RL = 0
= g m v gs
v gs + ro g m v gs
vi
=
ii =
RG
RG
Ri = RG
Ais =
RG g m
1 + ro g m
23
Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
Summery & comparisons
Avo = − g m (ro // RD )
CS Amplifier
Ais = − g m RG
Rout
vs =0
= (ro // RD )
Rin = RG
− g m RD
Avo =
1 + g m RS
− g m RG
Ais =
(1 + g m Rs )
Ro
vi =0
= RD
Ri = RG
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Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
CG Amplifier
Avo = g m RD
Ais = 1
Ro
= RD
vi = 0
1
Ri =
gm
CD Amplifier
ro g m
Avo =
1 + ro g m
RG g m
Ais =
1 + ro g m
Ro
vi = 0
= (ro // RD )
Ri = RD
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Microelectronic circuit by
meiling CHEN
CuuDuongThanCong.com
/>
