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vs vin- vin+ vbp vbn vbn vbb vbb VDD VDD M9 M12 M1 M2 v- v+ Iref M3 M4
Ri vin- Ri vin+ M1 M2 v- v+ vbp Iref M3 M4 Rf vbn Rf vo- vo+ CL CL vbn R M5 M6 M11 M7 M8 vbb vbb M10 M7t M8t Vocref M10t
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M7t, M8t, and M10t are all sized to operate in triode, as voltage controlled resistor.
When average value of vo+ and vo-, ie voc, is the same as Vocref, everything is as designed for. When voc is higher, M7t and M8t will have smaller resistance. Vd7t and Vd8t will drop. M7 and M8 will have larger Vgs, this will cause vo+ and vo- to drop, hence stabilizing feedback.
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For two stage op amp, detecting Voc and controlling first stage, use Vd from Q6 as feedback.
For single stage op amp, or local, CMFB, use Vd from Q5 as feedback.
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vs vin- vin+ vbp vbn vbn vcmfb vbb VDD VDD M9 M12 M1 M2 v- v+ Iref M3
Ri vin- Ri vin+ M1 M2 v- v+ vbp Iref M3 M4 Rf vbn Rf vo- vo+ CL CL vbn R M5 M6 M11 M7 M8 vcmfb vbb M10
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vb vb cc cc vbn vbb VDD VDD VDD VDD VDD M13 M3 M3 M4 M12 vo+ CL vo- M5
Rc cc Rc vin+ M1 vin- vbn M2 M7 M8 M11 M14 vbb VCMFB M9 M10
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cc cc vbp vbp vbn VDD VDD VDD VDD VDD M16 M4 M3 M4 M13 Rbp vo+ CL vo-
Rc cc Rc M1 vp vn vbn M2 M7 M8 M11 M15 M12 M9 M10 Vcntrl M17
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Vbn and Vbb can be similarly generated as before.
Op amp should be connected in the form: Rf Ri Vi+ Vo- Vp Vn Vi- Vo+ Ri Rf CMFB from two slides back can be used.
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Drawback of previous CMFB circuit is that some of members of the quad transistors turn off when the differential output is > Veff1*8^0.5. This can be very limiting. It is difficult to simultaneously achieve large range and control the gains. The next two makes it easier.
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This allows larger diff OSR.
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This allows rail to rail OSR.
If output stage is not cascaded, can directly use. Vo- Rcmd Rcmd Rcmd Vo+ Ccmd Ccmd Vocdes
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This allows rail to rail OSR.
If output stage is not cascaded, needs buffer before use. Vref = Vocdes – (Veff1+Vt1)
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cc cc vbp vbp vbn VDD VDD VDD VDD VDD M16 M14 M3 M4 M13 Rbp vo+ CL vo-
Rc cc Rc M1 vp vn vbn M2 M7 M8 M11 M15 M12 M9 M10 Vcntrl M17
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All pairs: 1~2, 3~4, 5~6, 7~8, 9~10, 11~12, and 13~14 are matched pairs
Current mirror ratio between 15 and 17 sets input diff pair current Ratio between 3~4 and 16 sets current in folded cascode transistors Ratio between 13~14 and 16 sets second stage current Rbp below 16 sets Vbp, and therefore Vds of 3~4 Can use another R on top of 17 to set Vbn, and therefore Vds of 9~10
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Accuracy of the resistors
Which ones must be accurate? Which ones need not be accurate but must match? Which ones are non critical?
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Vbn Vbn Vbn Vbb Vbb
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VDD VDD VDD Vbn Vbn Vbn Vbb Vbb
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VDD Vbn
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VDD VDD Vbn
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VDD VDD
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