1. 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

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

3. 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

4. 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

5. 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.

6. 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

7. 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.

8. local

9. 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

10. vs vin- vin+ vbp vbn vbn vbb vbb VDD VDD M9 M12 M1 M2 v- v+ Iref M3 M4Ri
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

11. 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.

12. 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.

13. vb vb cc cc vbn vbb VDD VDD VDD VDD VDD M13 M3 M3 M4 M12 vo+ CL vo- M5Rc cc Rc
vin+ M1
vin-
vbn M2 M7 M8
M11
M14
vbb
VCMFB M9
M10

14. 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.

15. 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.

16. This allows larger diff OSR.

17. This allows rail to rail OSR.