1. Differential Amplifiers and common mode feedback

2. Differential amplifiers
Cancellation of common mode signals including clock feed-through
Cancellation of even-order harmonics
Increased signal swing
Symbol:

3. Two-Stage, Miller, Differential-In, Differential-Out Op Amp
peak-to-peak
output voltage
≤ 2·OCMR
Output common mode range (OCMR)
= VDD-VSS - VSDPsat - VDSNsat

4. Two-Stage, Miller, Differential-In, Differential-Out Op Amp with Push-Pull Output
Able to actively source and sink output current
Output quiescent current poorly defined

5. Cascode Op Amp with Differential-Outputs, push-pull output

6. Differential-Output, Folded-Cascode
OCMR = VDD -VSS - 2VSDP(sat) -2VDSN(sat)
Quite limited

7. Two-Stage, Differential Output, Folded-Cascode
M11-M13 and M10-M12 provide level shift

8. Common Mode Output Voltage Stabilization
Common mode drift at output causes differential signals move into triode region

9. Common Mode feedback All fully differential amplifier needs CMFB
Common mode output, if uncontrolled, moves to either high or low end, causing triode operation
Ways of common mode stabilization:
external CMFB
internal CMFB

10. Cause of common mode problem
Unmatched quiescent currents
Vbb=VbbQ+Δ
Vbb I2
Vin=VinQ
Vbb=VbbQ
Vo1
Vo2
Vin I1
Vo1Q
Vo1
Vin=VinQ+ΔVin
actual Q point M2 is in triode

11. Vxx Ix Vo
Ix(Vo)
VOCM
Vin
Iy(Vo)
Vyy Iy Vo

12. Basic concept of CMFB: Dvb e desired common mode voltage CM
measurement
Vo+ +Vo- 2
Vo+
Vo-
Voc -
CMFB
Dvb e
VoCM +
desired common mode voltage

13. Basic concept of CMFB: Dvb e e
measurement
Vo+ +Vo- 2
Vo+
Vo-
Voc -
CMFB
Dvb e e
VoCM +
Find transfer function from e to Voc, ACMF(s)
Find transfer function from an error source to Voc Aerr(s)
Voc error due to error source: err*Aerr(0)/ACMF(0)

14. example
Vb2 CC CC
Vi+
Vi-
Vo+
Vo-
VCMFB
Vb1
Vo+
VCMFB
Voc -
Vo- +

15. Need to make sure to have negative feedback
Example
Voc ? ?
VoCM
Need to make sure to have negative feedback

16. Folded cascode amplifier
VDD
M7A
150/3
150/3
M2A
M2B
300/3
300/3
75/3
M13A
M13B
BIAS4
averager
1.5pF
1.5pF
M7B
75/3
M3B
BIAS3
OUT+
OUT-
20K
20K
M3A
300/2.25
300/2.25
300/2.25
300/2.25
M6C
75/2.25
IN-
IN+
Source
follower
M1A
M1B
M12B
M6AB
M12A
1000/2.25
75/2.25
1000/2.25
200/2.25
BIAS2
M11
M10
M9A
M9B
CL=4pF
4pF
150/2.25
50/2.25
50/2.25
BIAS1 M8 M5
200/2.25
M4A
M4B
150/2.25
50/2.25
50/2.25
VSS
Folded cascode amplifier

17. Resistive C.M. detectors:
Vo+
Vo-

18. Resistive C.M. detectors:
Vo.c. R1 R1
Vo+
Vo-
Vi+
Vi-

19. O.K. if op amp is used in a resistive feedback configuration
& R1 is part of feedback network.
Otherwise, R1 becomes part of g0 & hence reduces AD.C.(v)

20. Buffer Vo+, Vo- before connecting to R1.
Voc
Vo+
Vo- R1 R1
Simple implementation:
source follower
Vo.c.
Vo+
Vo-
* Gate capacitance is your load to Amp.

21. Why not:
Vo.c.
Vo+
Vo-
* Initial voltage on cap.

22. C1 C2

23. Use buffer to isolate Vo node:
gate cap is load
or resistors

24. Switched cap CMFB
Vo+ Φ1 Φ2 Φ1
VoCM.
Vo-
VoCM.

25. To increase or decrease the C.M. loop gain:
e.g.
Vo.c.
Vo.c.d.
Vo.c.
Vo.c.d.
VC.M.F.B.
VC.M.F.B.

26. Another implementation
Use triode transistors to provide isolation & z(s) simultaneously.
M1, M2 in deep triode.
VGS1, VGS2>>VT
Voc
Vo+
Vo- M1 M2
In that case, circuit above M1, M2 needs to ensure that M1, M2 are in triode.
can be a c.s.

27. deep triode oper

28. Example:
Input state
Vo+
Vo- Vb M1 M2
e.g. Vo+, Vo-≈2V at Q & Vb ≈1V ,
Then M1&2 will be in deep triode.

29. Vo-
Vo+
Vb1
Vb2 VX M1 M2

30. Two-Stage, Miller, Differential-In, Differential-Out Op Amp
M10 and M11 are in deep triode

31. Vo++ Vo- 2
VoCM.
VCMFB
Vo+
Note the difference from the book
accommodates much larger VoCM range
Vo-

32. Small signal analysis of CMFB
Example: IB IB
VCM M4 M3
Vo+
Vo- M1 M2
-Δi
+Δi
+Δi M5
+Δi
-Δi
-Δi
VCMFB
Δi=0
2Δi

33. Differential Vo: Vo+↓ by ΔVo, Vo-↑ by ΔVo
Common mode Vo: Vo+↑ by ΔVo, Vo-↑ by ΔVo

34. IB IB
VCM M4 M3
Vo+
Vo- M1 M2
+Δi
+Δi M5
-Δi
-Δi
VCMFB
Δi=0
2Δi M7
Δi7 + - 1
gm6
-2Δi
-2Δi M6

36. CMFB loop gain: example
Vb2 CC CC
Vi+
Vi-
Vo+
Vo-
VCMFB
Vb1
Vo+
VCMFB
Voc -
Vo- +

37. -gm5vro2
-gm5vro2gm6 Vo
gm5v v
Poles: p1 p2
z1 same as before

38. Folded cascode amplifier
VDD
M7A
150/3
150/3
M2A
M2B
300/3
300/3
75/3
M13A
M13B
BIAS4
averager
1.5pF
1.5pF
M7B
75/3
M3B
BIAS3
OUT+
OUT-
20K
20K
M3A
300/2.25
300/2.25
300/2.25
300/2.25
M6C
75/2.25
IN-
IN+
Source
follower
M1A
M1B
M12B
M6AB
M12A
1000/2.25
75/2.25
1000/2.25
200/2.25
BIAS2
M11
M10
M9A
M9B
CL=4pF
4pF
150/2.25
50/2.25
50/2.25
BIAS1 M8 M5
200/2.25
M4A
M4B
150/2.25
50/2.25
50/2.25
VSS
Folded cascode amplifier

39. Removing the CM measurement
Vo+
VoCM
Vo-
VCMFB
Directly connect Vo+, Vo- to the gates of CMFB diff amp.

40. VDD=+1.65V
M11
M12 M3 M4
M26
M27
Vo1
Vo2
M21
M22
M23
M24
M1C
M2C
IDC=100υA
VCM
M13
Vi1 M2
Vi2 M1
M14
M51
M52
M25
-VSS=-1.65V

41. CMFB with current feedbackIB
Vo+
Voc CM
detect
VoCM
Vo- M5 M6 M7 M1 M2