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Lecture 12 Digital Circuits (II) MOS INVERTER CIRCUITS

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Lecture 12 Digital Circuits (II) MOS INVERTER CIRCUITS Lecture 12 Digital Circuits (II) MOS INVERTER CIRCUITS Outline • NMOS inverter with resistor pull-up –The inverter • NMOS inverter with current-source pull-up • Complementary MOS (CMOS) inverter • Static analysis of CMOS inverter Reading Assignment: Howe and Sodini; Chapter 5, Section 5.4 6.012 Spring 2007 Lecture 12 1 1. NMOS inverter with resistor pull-up: Dynamics •CL pull-down limited by current through transistor – [shall study this issue in detail with CMOS] •CL pull-up limited by resistor (tPLH ≈ RCL) • Pull-up slowest VDD VDD R R VOUT: VOUT: HI LO LO HI V : VIN: IN C LO HI CL HI LO L pull-down pull-up 6.012 Spring 2007 Lecture 12 2 1. NMOS inverter with resistor pull-up: Inverter design issues Noise margins ↑⇒|Av| ↑⇒ •R ↑⇒|RCL| ↑⇒ slow switching •gm ↑⇒|W| ↑⇒ big transistor – (slow switching at input) Trade-off between speed and noise margin. During pull-up we need: • High current for fast switching • But also high incremental resistance for high noise margin. ⇒ use current source as pull-up 6.012 Spring 2007 Lecture 12 3 2. NMOS inverter with current-source pull-up I—V characteristics of current source: iSUP + 1 ISUP r i oc vSUP SUP _ vSUP Equivalent circuit models : iSUP + ISUP r r vSUP oc oc _ large-signal model small-signal model • High current throughout voltage range vSUP > 0 •iSUP = 0 for vSUP ≤ 0 •iSUP = ISUP + vSUP/ roc for vSUP > 0 • High small-signal resistance roc. 6.012 Spring 2007 Lecture 12 4 NMOS inverter with current-source pull-up Static Characteristics VDD iSUP VOUT VIN CL Inverter characteristics : iD = V 4 3 VIN VGS I + DD SUP roc 2 1 = vOUT vDS VDD (a) VOUT 1 2 3 4 VIN (b) High roc ⇒ high noise margins 6.012 Spring 2007 Lecture 12 5 PMOS as current-source pull-up I—V characteristics of PMOS: + S + VSG _ VSD G B − _ + IDp 5 V + D − V + G − V − D − ID(VSG,VSD) (a) = VSG 3.5 V 300 V = V + V = V − 1 V 250 (triode SD SG Tp SG region) V = 3 V 200 SG − IDp (µA) (saturation region) 150 = VSG 25 100 = 0, 0.5, VSG 1 V (cutoff region) V = 2 V 50 SG = VSG 1.5 V 12345 VSD (V) (b) Note: enhancement-mode PMOS has VTp <0. In saturation: 2 −IDp ∝(VSG + VTp ) 6.012 Spring 2007 Lecture 12 6 PMOS as current-source pull-up: Circuit and load-line diagram of inverter with PMOS current source pull-up: PMOS load line for V =V -V VDD SG DD B -IDp=IDn VDD VB VOUT VIN VIN CL 0 0 VDD VOUT Inverter characteristics: NMOS cutoff PMOS triode VOUT NMOS saturation PMOS triode VDD NMOS saturation PMOS saturation NMOS triode PMOS saturation 0 0 VTn VDD VIN 6.012 Spring 2007 Lecture 12 7 PMOS as current-source pull-up: NMOS inverter with current-source pull-up allows high noise margin with fast switching • High Incremental resistance • Constant charging current of load capacitance But… When VIN = VDD, there is a direct current path between supply and ground ⇒ power is consumed even if the inverter is idle. VDD PMOS load line for VSG=VDD-VB -IDp=IDn VDD VB VOUT:LO VIN VIN:HI CL 0 0 VDD VOUT Ideally, we would like to have a current source that is itself switchable, i.e it shuts off when input is high ⇒ CMOS! 6.012 Spring 2007 Lecture 12 8 3. Complementary MOS (CMOS) Inverter Circuit schematic: VDD VIN VOUT CL Basic Operation: •VIN = 0 ⇒ VOUT = VDD VGSn = 0 < VTn ⇒ NMOS OFF VSGp = VDD > - VTp ⇒ PMOS ON •VIN = VDD ⇒ VOUT = 0 VGSn = VDD > VTn ⇒ NMOS ON VSGp = 0 < - VTp ⇒ PMOS OFF No power consumption while idle in any logic state! 6.012 Spring 2007 Lecture 12 9 VOUT V 1 DD 2 CMOS Inverter (Contd.): 3 Output characteristics of both transistors: 4 5 VDD VIN = − − = IDn IDp IDp IDn VIN 3 3 4 2 4 2 5 1 5 1 VDD VOUT VDD VOUT n-channel p-channel (a) (b) Note: VIN = VGSn = VDD -VSGp ⇒ VSGp=VDD -VIN VOUT = VDSn = VDD -VSDp ⇒ VSDp=VDD -VOUT IDn = -IDp Combine into single diagram of ID vs. VOUT with VIN as parameter 6.012 Spring 2007 Lecture 12 10 CMOS Inverter (Contd.): ID VDD-VIN VIN 0 0 VOUT • No current while idle in any logic state Inverter Characteristics: NMOS cutoff PMOS triode VOUT NMOS saturation PMOS triode VDD NMOS saturation PMOS saturation NMOS triode PMOS saturation NMOS triode PMOS cutoff 0 0 V V +V V Tn DD Tp VDD IN • “rail-to-rail” logic: logic levels are 0 and VDD • High |Av| around logic threshold ⇒ good noise margins 6.012 Spring 2007 Lecture 12 11 2. CMOS inverter: noise margins VOUT NML VDD Av(VM) VM 0 0 V V V V IL M IH VDD IN NMH • Calculate VM • Calculate Av(VM) • Calculate NML and NMH Calculate VM (VM = VIN = VOUT) At VM both transistors are saturated: Wn 2 I Dn = µ nCox()VM − VTn 2Ln Wp 2 −IDp = µ pCox()VDD − VM + VTp 2Lp 6.012 Spring 2007 Lecture 12 12 CMOS inverter: noise margins (contd.) Define: Wn Wp kn = µnCox; kp = µpCox Ln Lp Since : I Dn = −IDp Then: 1 2 1 2 kn()VM − VTn = kp (VDD − VM + VTp ) 2 2 Solve for VM: k V + p V + V Tn k ()DD Tp V = n M k 1+ p kn Usually, VTn and VTp fixed and VTn = - VTp ⇒ VM engineered through kp/kn ratio. 6.012 Spring 2007 Lecture 12 13 CMOS inverter: noise margins (contd..) • Symmetric case: kn = kp V V = DD M 2 This implies: Wp Wp µ pCox µp k L L W W p =1 = p ≈ p ⇒ p ≈ 2 n k Wn Wn L L n µ nCox 2µ p p n Ln Ln Since usually Lp ≈ Ln = Lmin ⇒ Wp ≈ 2Wn Wn Wp • Asymmetric case: kn >> kp, or >> Ln Lp VM ≈ VTn NMOS turns on as soon as VIN goes above VTn. Wn Wp • Asymmetric case: kn << kp, or << Ln Lp VM ≈ VDD + VTp PMOS turns on as soon as VIN goes below VDD + VTp. 6.012 Spring 2007 Lecture 12 14 CMOS inverter: noise margins (contd…) Calculate Av(VM) • Small signal model: S2 + v =-v sg2 in gmpvsg2 rop - G2 D2 G1 D1 + + + vin vgs1 gmnvgs1 ron vout - - - S1 G1=G2 D1=D2 + + vin gmnvin gmpvin ron//rop vout - - S1=S2 Av =−()gmn + gmp (ron // rop ) This can be rather large. 6.012 Spring 2007 Lecture 12 15 CMOS inverter: calculate noise margins (contd.) VOUT NML VDD Av(VM) VM 0 0 V V V V IL M IH VDD IN NMH • Noise-margin low, NML: VDD − VM VIL = VM − Av VDD − VM NML = VIL − VOL = VIL = VM − A v • Noise-margin high, NMH: ⎛ 1 ⎞ ⎜ VIH = VM 1 + ⎟ ⎝ Av ⎠ ⎛ 1 ⎞ ⎜ NMH = VOH − VIH = VDD − VM 1+ ⎟ ⎝ Av ⎠ 6.012 Spring 2007 Lecture 12 16 What did we learn today? Summary of Key Concepts • In NMOS inverter with resistor pull-up, there is a trade-off between noise margin and speed • Trade-off resolved using current source pull-up – Use PMOS as current source. • In NMOS inverter with current-source pull-up: if VIN = High, there is power consumption even if inverter is idling. • Complementary MOS: NMOS and PMOS switch- on alternatively. – No current path between power supply and ground – No power consumption while idling • Calculation of CMOS –VM – Noise Margin 6.012 Spring 2007 Lecture 12 17.
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