+ ), %
1
I
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% F % |
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6 *6
2(, .6
6 $ ,
;H =
;-( @ &: # @ & =
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/ :# / # |
F % |
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(-# & ("'
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0 3 ,,, J++ # ( # ''8(')$# H 8')
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/ # / # / # / 3 /F2
/ ≡ / ≡ / ≡ /
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0 3 ,,, J++ |
# ( !# )(!& # F 8'8 |
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−
M # # 3 F % / # / / / # / 3 /F2
; <G # <( !K# L !N
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@ =25μA
; H /-+",A * /
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/< # /< 5 3 |
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−
&( ")' *+ ,-,.
/ 0 0 11 1 0 /1 , ,
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= 6 55 55 4 |
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≈ − α " |
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1/ .9/ − α 6 55 4 |
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" |
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" ↑ |
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#$8 /1, *+ 6$*+4 ,-,.
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' ") "" &"$% !
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&φ φ / , 2 > / ?-, 0
6 - 1
CHUNG-YU WU
Chapter 6 Frequency Response of MOS Amplifiers
§6-1 Single-Stage Amplifier
§6-1.1 Source follower
High-frequency small-signal equivalent circuit
+VDD
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Ii |
RS |
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Cgd1 |
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RS |
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-gmb1Vo |
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M1 |
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Vi |
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Cgs1 |
gds1 |
gm1(Vi-Vo) |
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Vo |
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gds2 |
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Cdb2 |
Csb1 |
CL |
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VBIAS |
M2 |
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Zo |
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-VSS |
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AV(s) |
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≡ Vo(s) = |
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sCgs1 + gm1 |
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Vi(s) |
Rs(C gs1CL eq |
+ Cgs1Cgd1 |
+ Cgd1CLeq )s |
2 |
+ ( |
gm1 |
RsC gd1 |
+ CLeq + Cgs1)s + gm1 / α1 + G Leq |
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á 1 |
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where GLeq=gds1+gds2 , CLeq=CL+Csb1+Cdb2
* Left-Half-Plane (LHP) pole: fp = G Leq + g m1 / α1
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2π(Cgs1 + CLeq ) |
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LHP zero: fz = |
gm 1 |
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2πCgs1 |
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In general, |
fp fz |
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CLeq Cgs1 |
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* If |
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−1) |
+ |
G Leq |
, we have |
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Cgs1 |
α1 |
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gm 1 |
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fp = fz and Av(s) |
Cgs1 |
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1 indep. of s. |
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Cgs1 + CLeq
>Better high frequency response. How to achieve this?
Adding an extra capacitor Cx such that
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Cx+Cgs1=CLeq ê |
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G Leq ú |
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Zi(s) º |
Vi (s) |
= |
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gs1 |
s +g |
m1 |
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Ii(s) |
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Cgs1s(G Leq + gmb 1 + sC Leq ) ú |
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* If gmb1+GLeq<< CLeqS and Cgd1 is neglected, |
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gm1 |
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Zi(s) @ |
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Cgs1S CLeqS Cgs1CLeqS2
The input impedance consists of the series connected Cgs1, CLeq, and the negative resistance
gm 1
Cgs1CLeqw2
6 - 2
CHUNG-YU WU
Thus oscillation is possible.
* If gmb1+GLeq is neglected, the equivalent input capacitances
Cin=Cgd1||Cin'
æ |
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Cin' CLeq ç |
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CLeq |
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m1 |
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gs1 |
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gs1 |
ø |
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For large gm1, Cin' CLeq
The large load capacitance CL is well blocked or buffered from the preceding stage.
Zo(s) º |
Vo(s) |
|Vi=0 = |
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Rs Cgd1s +1 |
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Io(s) |
+ gmb 1 + sCLeq |
+(sC gs1 + gm1 ) |
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G Leq |
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(C s + C |
gs1 |
s) +1 |
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gd1 |
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* If s = 0, Zo = Ro = |
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gm1 + gmb 1 |
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If s→∞, Zo'(without CLeq) Rs for Rs |
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and Cgs1>>Cgd1 |
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G Leq + gm1 |
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Since usually Rs |
1 |
,we |
have |
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gm1 +g mb1
6 - 3
CHUNG-YU WU
|Zo| |
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Rs |
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α1/gm1 |
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> Zo |
α ω > Inductive load |
ω |
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Zo(s) |
RsCgs1s + 1 |
L |
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gm 1 / α1 + Cgs1s |
R1 |
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α1 |
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Zo |
R1=Rs |
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gm1 |
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α1 |
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R2= |
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gm1 |
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Cgs1α1 |
α |
1 ) |
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L= |
(Rs |
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gm1 |
gm1 |
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L and CL causes output signal ringing. |
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* Two source followers in cascade might cause oscillation because |
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First SF L in Zo 1 |
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Second SF -R and Cin Z i 2 |
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§6-1.2 Enhancement load NMOS common-source gain stage +VDD
M2 |
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Cgs2 |
gmb2Vo |
gds2 |
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gm2Vo |
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Vo |
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V1 |
Cgd1 |
Vo |
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RS |
+ |
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gm1V1 |
Cdb1 Csb2 CL |
gds1 |
CL |
Vi |
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Cgs1 |
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M1 |
_ |
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-VSS |
RS |
Cgd1 |
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V1 |
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Vo |
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gm1V1 GLeq |
CLeq |
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