Chapter5CurrentMirrors.ppt
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1、Current Mirrors Basic current mirrors (2)For example: If consider channel-length modulation:VX=VGS0=VGST , while VP is dependent on the CM level and input signal of M1 and M2 and may not equal VX.In this case, IT not track IREF accuratelyCurrent Mirrors Cascode current mirrors (1)Use cascode to get
2、more accurate current: 333)(/ombmPYrggVVLargely reduce the change so that VY is more close to VX, and hence Iout more closely track IREF.Refer to P65 Figure 3.23 and eq. (3.63)Current Mirrors Cascode current mirrors (2)More accurate but at cost of higher voltage headroom: Minimum allowable voltage a
3、t node P is equal to:.THTHGSTHGSTHGSGSTHNVVVVVVVVVV)()(1010One Vgs higher than basic current mirror:.Current Mirrors Cascode current mirrors (3)When VX drops from high voltage to low voltage, M3 enter triode region first, and then M2:. M3 enter triode region M2 enter triode regionVX Current Mirrors
4、Cascode current mirrors (4)Less one VTH, So this can be used in low-voltage application.In order to solve the trade-off between accuracy and voltage headroom: Minimum allowable VP becomes: )()(43THGSTHGSVVVVCurrent Mirrors Active current mirrors (1)How about if we use a current mirror as load of a d
5、ifferential pair? outmvRGA |2/ )2/(/11mininminoutmgVVgVIGCurrent Mirrors Active current mirrors (2)Rout is equal to the output resistance of a CS with a degeneration resistance of 1/gm1./1|)2(,|)2/1 (|)/1 (/1 124221421421221mooommoomoomommoutgrifrrggsymmetryifrrgrrgrggR|)2(2|421oomoutmvrrgRGACurrent
6、 Mirrors Active current mirrors (3)Second approach to calculate |Av|:24241211oooomeqeqinPrrrrgRRVV)1 (112422242oomomomeqrrgeffectbodyignoreifrgrgRRefer to eq. (3.110), page 80:Then:Current Mirrors Active current mirrors (4)Second approach to calculate |Av|:11111224222422244222omooomooomoooompoutrgif
7、rrrgrrrgrrrrgVVRefer to eq. (3.104), page 79:Current Mirrors Active current mirrors (5)Second approach to calculate |Av|:|)2(221214224242242222424oomoooomooomooooinPpoutinoutrrgrrrrgrrrgrrrrVVVVVVCurrent Mirrors Differential pair with Active current mirrors (1)Problem of above differential pair:Smal
8、l-signal drain current of M1 is “wasted”Solve method:Use differential pair with active current mirror to combine the small-signal current together.Current Mirrors Differential pair with Active current mirrors (2)Large-signal analysis:Current Mirrors Differential pair with Active current mirrors (3)L
9、arge-signal analysis:v If Vin1-Vin2=-, M1, M3, M4 off, ID4=0, M5 and M2 are in deep triode region, Vout=0;v When (Vin1-Vin2), ID1=ID3=ID4 , Vout ;v When Vin1-Vin2=0, Maximum current ID1=ID2= ID3=ID4, and hence maximum small-signal gain;v When Vin1-Vin20, ID2, M4 enter triode region, the gain decreas
10、e;vWhen Vin1-Vin2=, M2 off and ID2=0, M4 in deep triode region, Vout=VDD.Current Mirrors Differential pair with Active current mirrors (4)Large-signal analysis result:v Always biased in the condition of Vin1=Vin2, so that the gain of differential pair with active current mirrors keeps largest.Curren
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