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Download Lecture 5b: Common-mode feedback
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1 Common-mode feedback CMFB: VCM: common-modefeedbackamplifier(erroramplifier) externalcommon-modevoltage 2 Common-mode feedback DifferenCalinputsignal: Common-modeinputsignal: Common-mode output voltage Vocm midway between the limits of the signal swing (normally power-supply voltages) 3 Using a CMFB amplifier to set output voltages One possible implementaDon of CMFB amplifier • CMFB amplifier is used to amplify the difference between the average of the differenDal amplifiers outputs and Vbiasp. • If the gain of the CMFB is large, the average of the two outputs will be very close to Vbiasp. • Any variaDon in VCMFB affects each output by the same amount. • CMFB amplifier shouldn’t affect the differenDal amplificaDon in the differenDal amplifier. • When the differenDal amplifier outputs are equal, they should be Vbiasp. 4 OperaDon of the CMFB circuit a) No offset Correct outputs, i.e. Vbiasp Needed to stabilize the CMFB loop b) With a 50mV offset: Note how the CMFB isn’t doing anything. Outputs not balanced ⇒ We must use a CMFB that can balance the output over the enDre range of diff-amp output voltage. 5 CompensaDng the CMFB loop • vC is the AC common-mode signal • Unity gain frequency of CMFB loop is fun,cm = A cm ⋅ gmn 2π ⋅ CL • If we want to compensate the CMFB loop with the same load capacitance used to compensate the differenDal forward signal path, then we must ensure that the gain of the CMFB amplifier is A cm ≤ 1 6 CMFB design essenDals • • • • • • • • • • • CMFB is only to adjust DC levels, not for signal quality Should not limit the speed of the amplifier Must be stable Should not limit the signal swing Common-mode range as large as possible No differenDal to common-mode conversion No common-mode to differenDal conversion Must be funcDonal over all signal condiDons As simple as possible (only DC level adjustment) Low power Not accurate (even 100mV error can be tolerable in many cases) 7 Common-mode feedback loop • Gain of common-mode feedback loop: • If M1A and M1B are fully matched, −3 −3 g g 10 10 only common-mode signal is fed back ALOOP = −A CO m1 m2 ≈ −6 −6 = 106 gDS1gDS2 10 10 • If M1A and M1B are fully matched, • Output resistance of common-mode circuit: also differenDal signal will be fed back ⇒ AdM ↓ 1 R o,CM = A COgm1 ΔA diff = −A CO gm1 gm2 gDS1gDS2 ⎡ ΔgDS1 Δg ⎤ + 2 m1 ⎥ ⎢2 gm1 ⎦ ⎣ gDS1 Δgm ~ ΔL ⎫ ⎪ 1 ⎬ ⇒ ca. 1% error ΔgDS1 ~ ΔL ⎪⎭ 8 Common-mode feedback loop • Pole of common-mode feedback loop: pCMFB = A COgm1 CL • Typically pCMFB limits common-mode feedback loop ⇒ ACO ↑ or gm1 ↑ 9 Common-mode detecDon with resistors • CMFB amplifier compares with an external CM-level • Gain of CMFB amplifier: g A CO = m1 > 1 gm4 Limited common-mode range Min{VCM} = VGS1 + VDS,ISS RCM >> ROUT > 1MΩ 10 Common-mode detecDon with transistors only • Common-mode feedback with source follower ACM = 1 • Limited common-mode range min{CM} = VGS,FB1 + VGS6 max{CM} = VDD • Limited differenDal signal range max{ΔVOUT} < VFB,SAT = √2 VSAT,FB1 • VX depends on ΔVOUT ⇒ Δvdiff ⟶ ΔVCM transformaDon! 11 CMFB with double differenDal amplifier • Common-mode detecDon with source-coupled pair MFB1A and MFB1B • Common-mode cpmparison and feedback with sourcecoupled pair formed by MFB1A+MFB1B and MFB1C 1⎛W⎞ ⎛W⎞ ⇒⎜ ⎟ = ⎜ ⎟ ⎝ L ⎠MFB1A,MFB1B 2 ⎝ L ⎠MFB1C 12 • Gain and pole of CMFB: A CO = gm,MFB1C gm,MFB2C pole = gm,MFB2 C GS,MFB2 Fast common-mode sehling • Limited common-mode range • Limited differenDal output swing, if |Vo1 – Vo2| > VFB,SAT ACO → no common-mode feedback! → 13 Source degeneraDon common-mode feedback loop • Common-mode feedback is performed by M3 and M4 • M3 and M4 work as source degeneraDon resistors controlled by output common-mode voltage • M3 and M4 in linear region i.e. parallel conductance is g CM = gDS3 + gDS4 W (VDD − VOUT + − VT ) L W gDS4 = μC OX (VDD − VOUT - − VT ) L ass. VOUT + = Vo + ΔV, VOUT - = Vo − ΔV W ⇒ g CM = μC OX (VDD − Vo − ΔV − VT + VDD − Vo + ΔV − VT ) L W = 2μμ OX (VDD − Vo − VT ) L gDS3 = μC OX • Gain of common-mode feedback: A CO = gm3 < 1 ; VDS3 < VDS,SAT gDS3 ⇒ slow! 14 SC common-mode feedback • • • • Op-amp is used only during the φ2 phase Cmbias is an input that determines common-mode output voltage Phase φ1: both capacitors Ccm are charged to the desired value of output voltage Vocm Phase φ2: both capacitors Ccm (charged to Vocm) are connected between the differenDal output nodes and Cmbias • The average voltage applied to Cmbias node will be Vocm • The voltage across Ccm does not change when phase periods are small enough 15 SC common-mode feedback Start-up sehling to correct commonmode bias values: VB4 VB1 CMF1 • Fast sehling to new common-mode level i.e. only charge sharing between capacitors • Switched capacitors to refresh transistor bias voltages CMF2 t 16 Folded cascode OTA with rail-to-rail common-mode levels rail-to-railinputstage Cascodegainstage VB4 rail-to-railCMFB VB6 VB3 IN+ IN- OUT- OUT+ VB2 VB1 • NMOS and PMOS input stages in parallel ⇒ rail-to-rail input common-mode range • NMOS and PMOS common-mode feedback circuits ⇒ rail-to-rail common-mode detecDon range VB5 17 Fully differenDal Miller compensated two stage amplifier CMFB-circuit Inputstage 2nd-stage 2nd-stage common-mode sensingcircuit VB4 VB5 CM IN- OUT- OUT+ VB3 IN+ VB2 VB1 VB1 • • • • Folded cascode input stage Miller compensated 2nd stage Common-mode sensing with resistors Common-mode feedback with differenDal amplifier • Limited common-mode range • Stability – speed trade-off 18 Fully differenDal Miller compensated two stage amplifier 1st stage VDD 2nd stage VDD M4 M5 M6 M7 2nd stage CMFB V1 M14 M13 VBB V2 M37 M35 C1 M8 M9 M2 OUTP 4p V3 M1 C2 GND VDD R4 100k GND M16 M17 OUTM 100k VDD 4p M36 M12 M36 M15 R3 INM INP M10 M11 V4 M3 VBD GND • • • • GND Folded cascode 1st stage Miller compensated 2nd stage Current steering common-mode feedback Common-mode sensing with resistors • Stability-speed trade-off in CMFB (CMFB over both amplifier stages) • Limited common-mode range min{CM} = VGS16 + VDS10 = VT + 2VDS,SAT 19 Fully differenDal 2-stage amplifier with double CMFB circuit Problem: • One global CMFB from the output to input stage is difficult to design fast enough • Also stability might be a problem SoluDon: • Separate local CMFB circuits for the input stage and output stage ⇒ Increase complexity and power consumpDon 20