Fully differential op amps Widely used for embedded applications Chances are really high that you will be designing fully differential op amps Many advantages, most important two: Robustness to CM errors Doubling signal amplitude  double SNR Great for low voltage applications But more complex, require CM feedback

VDD vs vb M1 M2 M3 M4 M5 M6 M7 M8 M9 R vin vip vo vxx

For single ended, once vin is given, vo is determined. VDD VDD M9 M12 vs Ri vin Iref M1 M2 VDD/2 vb M3 M4 Rf R For single ended, once vin is given, vo is determined. M5 M6 M7 M8

vs vin- vin+ vbp vbn vbn vbb VDD VDD M9 M12 M1 M2 v- v+ Iref M3 M4 vo- 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 M10

Say, Ri = Rf, for gain of -1. For any given vin-, vin+, there are many possible vo+ and vo- combinations that satisfy the correct Vod=-1 but with various voc. Voc values are determined by small changes in Vbb. This can push Voc very high or very low, causing some transistors to go triode.  Need common mode feedback to stabilize Voc at its desired level.

vs vin- vin+ vbp vbn vbn CMFB circuit vbb VDD VDD M9 M12 M1 M2 v- v+ Ri vin- Ri vin+ M1 M2 v- v+ vbp Iref M3 M4 Rf vbn Rf vo- vo+ CL CL vbn R M5 M6 M11 CMFB circuit M7 M8 vbb M10

The CMFB circuit performs three tasks: Measure the common mode voltage Voc Compare it with the desired level Vocref Generate a feedback voltage to adjust Vbb in a way to drive the error toward zero The feedback can be “local”: same stage detect and control; Or “global”: whole op amp output detect, and first stage control.

local

vs vin- vin+ vbp vbn vbn CMFB circuit vbb VDD VDD M9 M12 M1 M2 v- v+ Ri vin- Ri vin+ M1 M2 v- v+ vbp Iref M3 M4 Rf vbn Rf vo- vo+ CL CL vbn R M5 M6 M11 CMFB circuit M7 M8 vbb M10

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

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.

For two stage op amp, detecting Vo and controlling first stage, use Vd from Q6 as feedback. For single stage, or local, CMFB, use Vd from Q5 as feedback.

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

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.

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.

This allows larger diff OSR.

This allows rail to rail OSR.