Differential Amplifiers and common mode feedback

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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

Resistive C.M. detectors: Vo+ Vo-

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

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)

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.

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

C1 C2

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

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

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.

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.

deep triode oper

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.

Vo- Vo+ Vb1 Vb2 VX M1 M2

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

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

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

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

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

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

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

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

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

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

CMFB with current feedback IB Vo+ Voc CM detect VoCM Vo- M5 M6 M7 M1 M2