Analog Integrated Circuits (Jieh Tsorng Wu)

To increase vo m7 m8 and m9 can be biased in the

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Unformatted text preview: + ··· • The input devices, MB1–MB4, must remain in the forward-active region over the voltage range of Vo1 and Vo2. • The variation in Vod can produce an ac component in I3 as well as Voc . • If Voc = VCM , I1 and I2 are nonlinear functions of Vod , but I3 = I1 + I2 is a constant. Opamp-III 15-13 Analog ICs; Jieh-Tsorng Wu CMFB Using Transistors in the Triode Region Common-Mode Feedback VDD M6 VDD M8 IB1 IB2 IB3 Vo1 Vo2 MB3 MB4 MB6 Vx VB1 M7 VB1 M9 MB1 VSS Opamp-III I1 I2 Vy VCM MB5 MB2 VSS 15-14 Analog ICs; Jieh-Tsorng Wu CMFB Using Transistors in the Triode Region MB1, MB2, and MB5 are in the triode region. Let kB1 = kB2 = kB5 = k , 1 I1 = k Vo1 − Vtn − Vx Vx 2 ⇒ 1 I2 = k Vo2 − Vtn − Vx Vx 2 1 I1 + I2 = 2k Voc − Vtn − Vx Vx 2 I1 + I2 = 2IB3 · Voc − Vtn − 1 Vx 2 VCM − Vtn − 1 V 2y = 2IB3 1 IB3 = k VCM − Vtn − Vy Vy 2 Vx ≈ Vy = 1+ IB3 k VCM − Vtn − 1 Vy 2 Voc − VCM VCM − Vtn − 1 Vy 2 • Output swing is reduced, since it is required that Vo1,o2 > Vtn + Vx . • MB1 and MB2 are in the triode region, their effective gm can be small, thus degrading loop gain and bandwidth of the CMFB. Opamp-III 15-15 Analog ICs; Jieh-Tsorng Wu Switched-Capacitor CMFB Common-Mode Feedback VDD M6 VDD M8 IB1 φ1 IB3 IB2 φ2 Vo1 Vo2 VCM VB1 M7 VB1 M9 VCB 1 2 S5 C3 1 S1 C1 2 S7 S3 2 I1 I2 Vx MB1 VSS MB2 S4 VCM S6 C4 1 VCB S8 MB3 VSS Voc − Vx = VCM − VCB Opamp-III S2 C2 2 1 15-16 ⇒ Voc ≈ VCM Analog ICs; Jieh-Tsorng Wu Switched-Capacitor CMFB • The opamp operates in two different modes. It is in the normal mode when φ2 is low. • Assuming ∆Q charges are injected into C3 and C4 when φ1 switches are turned off, Voc − Vx = VCM − VCB + ∆Q C3 ⇒ Voc ≈ VCM + ∆Q C3 • The loop gain of the CMFB is approximately C1 |T | ≈ × gm,B1 · Ro1 C1 + Cgs,B1 • C1 and C2 add differential-mode capacitive loading to the outputs. • The additional common-mode capacitive loading is (C1 + C2) (Cgs,B1 + Cgs,B2). • The value of C3,4 may be between 1/4–1/10 of C1,2 for low-pass filter function. Opamp-III 15-17 Analog ICs; Jieh-Tsorng Wu Folded-Cascode Operational Amplifier VDD M9 M10 M3 M4 VBP1 VBP2 Vo1 Vi1 M1 Vo2 M2 M6 Vi2 I1 M5 M8 M7 VBN2 MB3 VBN1 MB1 MB4 VBN2 CMFB MB2 VSS Opamp-III 15-18 Analog ICs; Jieh-Tsorng Wu Folded-Cascode Operational Amplifier • Frequency compensation is provided by the capacitive loads at the outputs. • Non-dominant poles are determined by M3 and M4, and ≈ ωt3 (ωt4 ). • It is not uncommon that ID1,D2 ID3,D4. • For high-speed designs, use pMOST input stage. The resulting opamps has higher non-dominant poles. • Active cascode configuration can be applied to M3, M4, M5, and M6. Opamp-III 15-19 Analog ICs; Jieh-Tsorng Wu Current-Mirror Operational Amplifier VDD M3 M4 M6 M9 M10 Vi1 M12 M5 M11 VBP2 Vo1 M1 M2 Vo2 Vi2 M13 M14 I1 M7 M8 VBN2 MB3 VBN1 MB1 MB4 VBN2 CMFB MB2 VSS Opamp-III 15-20 Analog ICs; Jieh-Tsorng Wu Current-Mirror Operational Amplifier The M3-M5 and M4-M6 current mirrors have a current gain of K . W L = 3 W L = 4 W L 1 K ID1 = ID2 = ID3 = ID4 = = 5 1 K W L 6 1 1 1 ID5 = ID6 = I1 2 K K • The single-ended maximum output current for slewing is Io(max) K = I1 2 • For a general-purpose fully differential opamp, may use large pMOST input stage, K=2, and wide-swing enhanced output-impedance cascode current mirrors. Opamp-III 15-21 Analog ICs; Jieh-Tsorng Wu Current-Mirror Push-Pull Operational Amplifier VDD K:1 1:1 M1 1:1 M2 M4 1:K M3 Vi2 Vi2 Vo1 Vi1 Vi1 I1 Vo2 I1 CMFB CMFB K:1 1:K VSS Opamp-III 15-22 Analog ICs; Jieh-Tsorng Wu Current-Mirror Push-Pull Operational Amplifier • The single-ended maximum output current for slewing is Io(max) = K I1 • The small-signal response is slower due to additional signal paths. Opamp-III 15-23 Analog ICs; Jieh-Tsorng Wu Class-AB Operational Amplifier VDD K:1 1:K M1 Vi1 M2 M3 Vi2 M4 Vo1 Vo2 M5 M7 M8 M6 I I I1 I2 CMFB CMFB K:1 1:K VSS Opamp-III 15-24 Analog ICs; Jieh-Tsorng Wu Class-AB Operational Amplifier If nMOSTs M1–M4 are identical, and pMOSTs M5–M8 are identical, and all current mirrors have a current gain of K , then the bias currents are ID1 = ID2 = ID3 = ID4 1 1 = I1 = I2 = I K K • Low quiescent power and large slew rate. • The input level shifter increases the noise and offset, and adds additional poles. • Not suitable for low-voltage operation. Opamp-III 15-25 Analog ICs; Jieh-Tsorng Wu Fully Differential Operational Amplifiers Telescopic-Cascode VDD VB1 VDD Vb1 M8 VB1 M8 Vo1 M1 M2 C c2 C c1 M5 M5 Vo2 Vo2 VB2 M4 VB3 M3 M1 M2 M6 Two-Stage Vi1 M1 M2 Vi2 Vo1 Vi2 M5 Vb3 Vo2 M6 Vb4 M11 M7 M8 M9 VSS ∆Vo(Two Stage) = VDD − 2VDSAT M4 M3 Vb4 VB4 VSS Vb2 Vo1 M4 Vi1 M3 M10 VB2 M6 Vi2 M9 M7 M9 M7 Vi1 VDD Folded-Cascode VSS ∆Vo(Telescopic) = VDD − 5VDSAT − 3Vmargi n ∆Vo(Folded-Cascode) = VDD − 4VDSAT − 2Vmargi n 2 ∆Vo2 gm/C ∆Vo Speed ∝ ∝ SNR · · VDD Power kT /C VDD · I Opamp-III 15-26 Analog ICs; Jieh-Tsorng Wu Active-Cascode Telescopic Operational Amplifier VDD • Have the best speed/power ratio. VB1 M8 M7 VPC M6 • A1 and A2 auxiliary amplifiers are used to increase output impedance and the dc voltage gain, Av (0). M5 A1 Vo2 Vo1 A2 M4 M3 VNC Vi1 M1 VB4 M2 M9 Vi2 • Explicit compensation capacitors may be required at the outp...
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