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Differential Amplifiers and common mode feedback.

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Presentation on theme: "Differential Amplifiers and common mode feedback."— Presentation transcript:

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 Output common mode range (OCMR) = V DD -V SS - V SDPsat - V DSNsat peak-to-peak output voltage 2·OCMR

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 V in V o1 V bb V o2 I2I2 I1I1 Cause of common mode problem V o1 V o1Q actual Q point M2 is in triode V in =V inQ V bb= V bbQ V bb= V bbQ +Δ V in =V inQ +ΔV in Unmatched quiescent currents

11 V in VoVo V xx IxIx IyIy VoVo V yy I y (V o ) I x (V o ) V OCM

12 CM measurement CMFB + - V oCM V oc V o+ V o- V o+ +V o- 2 desired common mode voltage Basic concept of CMFB: e v b

13 CM measurement CMFB + - V oCM V oc V o+ V o- V o+ +V o- 2 Basic concept of CMFB: e v b e Find transfer function from e to V oc, A CMF (s) Find transfer function from an error source to V oc A err (s) V oc error due to error source: err*A err (0)/A CMF (0)

14 example V b2 V i+ V i- V CMFB V b1 CC V o+ V o- + - V o+ V o- V oc V CMFB

15 V oc V oCM Example Need to make sure to have negative feedback ? ?

16 Source follower averager BIAS4 1.5pF M13AM13B 300/3 20K OUT+OUT- M2AM2B 150/3 BIAS3 300/2.25 IN- M3A M3B IN+ 300/2.25 M1AM1B BIAS2 BIAS1 75/3 M7A 75/3 M7B M6C75/2.25 M6AB M11 150/2.25 M8 150/ /2.25 M10 M5 C L =4pF4pF M9A 50/2.25 M4A 50/2.25 M9B 50/2.25 M4B 50/2.25 M12A 1000/2.25 M12B 1000/2.25 V DD V SS Folded cascode amplifier

17 R1R1 R2R2 V o- V o+ Resistive C.M. detectors:

18 V o.c. R1R1 R1R1 V o- V o+ V i- V i+

19 O.K. if op amp is used in a resistive feedback configuration & R 1 is part of feedback network. Otherwise, R 1 becomes part of g 0 & hence reduces A D.C. (v)

20 Buffer V o+, V o- before connecting to R 1. V o+ V o- V oc R1R1 R1R1 Simple implementation: source follower V o- V o+ V o.c. * Gate capacitance is your load to Amp.

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

22 C1C1 C2C2

23 Use buffer to isolate V o node: gate cap is load or resistors

24 Switched cap CMFB V o+ V o- V oCM. Φ2Φ2 Φ1Φ1 Φ1Φ1

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

26 Another implementation Use triode transistors to provide isolation & z(s) simultaneously. V o- V o+ V oc M1M1 M2M2 can be a c.s. M 1, M 2 in deep triode. V GS1, V GS2 >>V T In that case, circuit above M 1, M 2 needs to ensure that M 1, M 2 are in triode.

27 deep triode oper

28 Example: M1M1 M2M2 VbVb V o+ V o- Input state e.g. V o+, V o- 2V at Q & V b 1V, Then M 1&2 will be in deep triode.

29 V o+ V o- V b1 V b2 M1M1 M2M2 VXVX

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

31 V o+ + V o- 2 V oCM. V CMFB V o+ V o- Note the difference from the book accommodates much larger V oCM range

32 M1M1 M3M3 V CM M4M4 M2M2 IBIB IBIB V o+ V o- +Δi-Δi+Δi-Δi -Δi+Δi-Δi+Δi +Δi+Δi+Δi+Δi -Δi-Δi-Δi-Δi V CMFB M5M5 Δi=0 2Δi2Δi Small signal analysis of CMFB Example:

33 Differential V o : V o+ by ΔV o, V o- by ΔV o Common mode V o : V o+ by ΔV o, V o- by ΔV o

34 M1M1 M3M3 V CM M4M4 M2M2 IBIB IBIB V o+ V o- -Δi-Δi +Δi+Δi +Δi+Δi -Δi-Δi V CMFB M5M5 Δi=0 2Δi2Δi M6M6 M7 Δi7Δi7 -2Δi g m6

35

36 CMFB loop gain: example V b2 V i+ V i- V CMFB V b1 CC V o+ V o- + - V o+ V o- V oc V CMFB

37 VoVo v g m5 v -g m5 vr o2 -g m5 vr o2 g m6 Poles: p 1 p 2 z 1 same as before

38 Source follower averager BIAS4 1.5pF M13AM13B 300/3 20K OUT+OUT- M2AM2B 150/3 BIAS3 300/2.25 IN- M3A M3B IN+ 300/2.25 M1AM1B BIAS2 BIAS1 75/3 M7A 75/3 M7B M6C75/2.25 M6AB M11 150/2.25 M8 150/ /2.25 M10 M5 C L =4pF4pF M9A 50/2.25 M4A 50/2.25 M9B 50/2.25 M4B 50/2.25 M12A 1000/2.25 M12B 1000/2.25 V DD V SS Folded cascode amplifier

39 Directly connect V o+, V o- to the gates of CMFB diff amp. V o- V o+ V oCM V CMFB Removing the CM measurement

40 V DD =+1.65V -V SS =-1.65V M 11 M 12 M3M3 M4M4 M 26 M 27 V o1 V o2 M 14 I DC =100υA M 1C M 2C M 13 M1M1 M2M2 V i1 V i2 M 51 M 52 M 21 M 22 M 25 V CM M 23 M 24

41 CMFB with current feedback V oCM CM detect V oc V o+ V o- M3M3 M4M4 M1M1 M2M2 M6M6 M7M7 M5M5 IBIB


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