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S G D ELE6308 Microélectronique analogique et mixte --- Ampop avancés --Mohamad Sawan, Professor Kamal El-Sankary, Ph.D. Laboratoire de neurotechnologies Polystim http://www.cours.polymtl.ca/ele6308/ [email protected] M5418 Advanced Opamps Plan I. II. III. IV. V. VI. VII. VIII. IX. Generality (CMRR, PSRR, …) Input Common Mode Range Noise in CMOS Opamps Cascodes amplifiers Telescopic Opamp Gain boosted opamp Folded cascode amplifier Class AB amplifier Fully-differential Opamp. Page 2 ELE6308- Microélectronique analogique et mixte Advanced Opamps Generality vi = vi ! vi + Vdd Vi/2 ! V+ Vo Vi+ + - vi + + vi ! vcm = 2 vi + vi = vcm + 2 vi ! vi = vcm ! 2 Vi- + - + - + - Vcm Vi/2 + VVss vO = Adm vi + Acm vcm + A+ v + + A! v ! Page 3 ELE6308- Microélectronique analogique et mixte CMRR • The common-mode rejection ratio (CMRR) measures how well the amplifier can reject signals common to both inputs • The differential stage determines how well the entire opamp rejects common mode signals. Page 4 ELE6308- Microélectronique analogique et mixte CMRR (Cont’d) • The common-mode signal appearing on the drains of M3 and M4 will be identical • The most efficient manner in which to increase the CMRR of this amplifier is to increase the resistance ro5 Page 5 ELE6308- Microélectronique analogique et mixte PSRR of 2-Stage Opamp (cont’d) • A– is largely determined by go5 & go7. It is typically ≤ unity at low frequencies and, more importantly, it rolls off with frequency in the same fashion as the open loop gain. • PSRR– remains approximately constant with frequency. • A+ ≈ 1 except at very low frequencies • PSRR+ falls with frequency along with the open loop gain (Adm). • At frequencies above the dominant pole (which is determined by CC), CC “shorts” the gate and drain of M6. Page 6 ELE6308- Microélectronique analogique et mixte PSRR of 2-Stage Opamp (cont’d) Page 7 ELE6308- Microélectronique analogique et mixte PSRR of 2-Stage Opamp (cont’d) • Another significant source of coupling between the supply rails and the output is commonly referred to as “supply” capacitance. This term refers to the coupling from one, or both, of the supply rails into the input nodes of an op amp. It is primarily in circuits such as single-ended sampled-data integrators • The most important contributors to this type of coupling are displacement currents in the capacitances Cgs1 and Cgd1. These currents flow into the summing node and the resulting charge is accumulated on the integration capacitor, Ci. • Interconnect crossovers may also couple supply variations into the summing node. • Fully differential circuits are an obvious means of avoiding the most severe supply capacitance problems. Page 8 ELE6308- Microélectronique analogique et mixte Input Common Mode Range Differential amplifier with a current mirror load. Page 9 ELE6308- Microélectronique analogique et mixte Noise in CMOS Opamps The gain of the input stage in a MOS op amp is usually large enough so that the input-referred noise of the overall amplifier is dominated by the noise contributions from the input-stage transistors. 2 2 2 2 2 2 2 iO = g m1-2 (veq1 + v eq 2 ) + g m3-4 (veq 3 + veq 4 ) 2 2 iO = g m1! 2 v IT 2 Page 10 ELE6308- Microélectronique analogique et mixte Noise in CMOS Op Amps (cont’d) 1/f NOISE For an MOS transistor the input-referred 1/f noise can be modeled as: where Kf is the FLICKER NOISE COEFFICIENT Using this model for each transistor in the input stage, the input-referred 1/f noise for the entire stage is Assuming that L2 = L1, W2 = W1, L4 = L3 and W4 = W3 Page 11 ELE6308- Microélectronique analogique et mixte Noise in CMOS Op Amps (cont’d) Thermal NOISE The input-referred thermal noise for an NMOS transistor is: Page 12 ELE6308- Microélectronique analogique et mixte Cascode amplifiers SIMPLE CASCODE Page 13 ELE6308- Microélectronique analogique et mixte Cascode amplifiers (cont’d) Telescopic Opamp Gain comparable to a 2-stage op amp can also be obtained in a cascoded single stage Advantages • Single bias current • High speed Disadvantage • Reduced output swing • Reduced common-mode input range Page 14 ELE6308- Microélectronique analogique et mixte Cascode amplifiers (cont’d) Two-stage opamp with simple cascode • The second-stage gain has greatly increased improving the Miller compensation • The overall gain is approximately (gmrds)3 or very large • PSRR+ is improved because Cc can be smaller. Page 15 ELE6308- Microélectronique analogique et mixte Gain boosted opamp • The “gain-boosted” cascode boosts the output resistance, and thus the low-frequency gain, by AB. Page 16 ELE6308- Microélectronique analogique et mixte Gain boosted opamp (cont’d) • Assume a single-pole response for the gain boosting amplifier with the pole at ω2 and a unity gain bandwidth of ω4. • The unity-gain bandwidth, ω5, of the overall cascode is unchanged and still determined by the load capacitance: ω5 = gm1/CL • The total gain, Atot, decreases above ω1 due to CL. Page 17 ELE6308- Microélectronique analogique et mixte Gain boosted opamp (cont’d) • Feedback amplifier realized by commonsource amplifier • Assuming output impedance of IB1 is equal to rds3, loop gain will be gm3rds3/2 resulting in • Circuit consisting of Q4, Q5, Q6. Also Iin and IB2 operate like a diodeconnected transistor results in accurate matching of Iout to Iin • Note that shown amplifier is NOT wide-swing requires output to be 2Veff + Vthn above lower supply. Page 18 ELE6308- Microélectronique analogique et mixte Gain boosted opamp (cont’d) Improved output voltage swing • Q3 and Q7 are biased at 4 times current density 2Veff • Requires roughly twice power dissipation. Page 19 ELE6308- Microélectronique analogique et mixte Folded-Cascode amplifier High gain structure Advantage: • By using a folded cascode configuration, the headroom problem could be alleviated. Drawback: • By using PMOS transistor, the nondominant pole is at lower frequency (PMOS has lower fT). Page 20 ELE6308- Microélectronique analogique et mixte Folded-Cascode Opamp (cont’d) • The output resistance of the cascoded single-stage amplifier is increased by gmro relative to the basic 2-stage opamp with a common-source second stage. • The transconductance of the stage is simply the gm of the input pair. Thus, the voltage gain is comparable to that of the two-stage opamp. Gm = g m ,n1 RO = RO ,up RO ,dn ( RO ,up = g m , p 6 ro , p 6 ro , p 4 ro ,n 2 ) + RO ,dn = g m ,n8 ro ,n8 ro ,n10 Vin - 2 A = Gm RO = g m ,n1 RO ! (g m ro ) Page 21 + gm1 Vin rout Vo - ELE6308- Microélectronique analogique et mixte Folded-Cascode Opamp (cont’d) gmn1Ro • CL provides the frequency compensation. An increase in CL lowers the amplifier’s unity-gain bandwidth but does not compromise its phase margin. • The second nondominant pole in the folded cascode is associated with the cascode devices M5 and M6. This pole is located near the transistor ωt and is much greater in magnitude than the lowest nondominant pole in the 2-stage opamp, |p2| = gm6/CL. Page 22 Gain • The dominant pole of the folded cascode amplifier is associated with the output node. P1= 1/RoCL P2=gmp6/Cp6 0 w1=gmn1/C L f (Hz) 0 -90 -180 ELE6308- Microélectronique analogique et mixte Folded-Cascode Opamp (cont’d) Advantages • Simple compensation, since CC is CL • High speed, since the nondominant poles are near ωt • Good PSRR since there is no pole splitting • Output swing higher than one-stage opamp such as telescopic. Disadvantages • Reduced output swing compared to 2stage opamp • Higher noise compared to 2-stage opamp • Higher power consumption compared to telescopic. Page 23 ELE6308- Microélectronique analogique et mixte Class AB amplifier • “Class AB”: an amplifier that can have an output current which is larger than its DC quiescent current. • M6 and M7 act as a level shifter, M8 and M9 act as a class AB push-pull amplifier. Page 24 ELE6308- Microélectronique analogique et mixte Class AB amplifier (cont’d) • When Vin = 0, the quiescent currents are set by VB1 & VB2 • As Vin goes positive, the current in M1 & M4 increases until Vin = VDD, while M2 & M3 turn off. Therefore, the mirror circuit M5-M7 turns off, while a very strong drive is supplied to the mirror M8M10. Page 25 ELE6308- Microélectronique analogique et mixte Fully-differential Opamp • High PSRR since supply variations appear as commonmode signals • Effective output swing is doubled • The input-referred noise is unchanged 6 dB increase in the SNR • Stable DC bias points at the output of a fully differential amplifier requires commonmode feedback. Page 26 ELE6308- Microélectronique analogique et mixte Fully-differential Opamp (cont’d) Common-mode feedback has three key components: • Sampling of the common-mode output voltage: • Comparison with reference voltage • Feedback of common-mode error to adjust output common-mode voltage Page 27 ELE6308- Microélectronique analogique et mixte Fully-differential Opamp (cont’d) • The common-mode output voltage is sensed using RC filter which takes the average of Vo+ and Vo• The common mode voltage is explicitly compared with a reference voltage Vrefo • The error signal is used to adjust the common-mode bias current via a current mirror (M4). • The opamp must be able to drive resistive loads. Source followers may be used to buffer the resistive divider if needed. Page 28 ELE6308- Microélectronique analogique et mixte Fully-differential Opamp (cont’d) Two PMOS differential pairs used for sensing the common-mode output voltage. • The common mode voltage is explicitly compared with a reference voltage Vrefo. • The error signal is used to adjust the common-mode bias current via a current mirror (M5). • Maximum output common-mode range is limited by M1- M4. Page 29 ELE6308- Microélectronique analogique et mixte Fully-differential Opamp (cont’d) Typical differential opamp circuit. Common-mode feedback modulates I3 so as to maintain VCM Page 30 ELE6308- Microélectronique analogique et mixte Fully-differential Opamp (cont’d) Common-mode voltage sensing using transistors in the linear region. • M11 and M12, which are biased in the linear region, sense the common-mode output voltage and correspondingly modulate the current in M9 and M10 so as to maintain the desired common-mode output level. Page 31 ELE6308- Microélectronique analogique et mixte Fully-differential Opamp (cont’d) Dynamic Common-Mode Biasing • C1 and C2 are nominally equal capacitors that form a voltage divider to sense the common-mode output voltage and level shift that voltage to node A. • Capacitors C3 and C4, together with the switches controlled by φ1 and φ2, are used to establish and maintain the desired DC level shift voltage on C1 and C2. • CMO = desired common-mode output voltage • BIAS4 = corresponding bias voltage needed at node A • This circuit provides a full rail-to-rail common-mode output range and is widely used in switched-capacitor circuits. Page 32 ELE6308- Microélectronique analogique et mixte Fully-differential Opamp (cont’d) Fully differential class AB If M1, M2, M5 and M6 are identical and M3, M4, M7 and M8 are identical, then when Vin(+) = Vin(–) => I1 = I2 = I • Furthermore, if the current mirrors have a gain of 1, then Id20 = Id16 = I And IDD = 6I. Page 33 ELE6308- Microélectronique analogique et mixte Fully-differential Opamp (cont’d) Fully-differential class AB operation • For a large positive differential input, half of the circuit is cut off and the active portion of the circuit becomes: Page 34 ELE6308- Microélectronique analogique et mixte Fully-differential Opamp (cont’d) Very high gain differential folded cascode opamp Page 35 ELE6308- Microélectronique analogique et mixte