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Advantages of Using CMOS

Advantages of Using CMOS. Compact (shared diffusion regions) Very low static power dissipation High noise margin (nearly ideal inverter voltage transfer characteristic) Very well modeled and characterized Mechanically robust Lends itself very well to high integration levels

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Advantages of Using CMOS

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  1. Advantages of Using CMOS • Compact (shared diffusion regions) • Very low static power dissipation • High noise margin (nearly ideal inverter voltage transfer characteristic) • Very well modeled and characterized • Mechanically robust • Lends itself very well to high integration levels • “Analog” CMOS process usually includes non-salicided poly layer for linear resistors. • SiGe BiCMOS is very useful but is a generation behind currently available standard CMOS Prof. M. Green / U.C. Irvine

  2. Transistor fT Calculation VDD fT is the frequency at which becomes 1. id ig vgs Cgs VGS fT gives a fundamental speed measure of a technology. 0.25 µm CMOS: fT~ 23GHz (VDD= 2.5V) 0.18 µmCMOS: fT ~ 57GHz (VDD = 1.8V) Prof. M. Green / U.C. Irvine

  3. Assume: Wp = 3Wnfor optimum noise margin. Lp = Ln = Lmin Operation is 4X slower than theoretical maximum due to n-channel & p-channel gates connected in parallel. Static CMOS propagation delay: Vout Vin (Actual  values will be higher due to high diffusion capacitances present in sub-micron transistors.) Prof. M. Green / U.C. Irvine

  4. Verifying with simulation: n-channel ac simulation to determine fT: CMOS inverter transient simulation: IG Vout Vin ID fT = 57GHz Prof. M. Green / U.C. Irvine

  5. Vin Vout ISS IDD Single-Ended Signaling in CMOS VDD IDD Vin Vout sub ISS VSS Series R & L cause supply/ground bounce. Resulting modulation of transistor Vt’s result in pattern-dependent jitter. Prof. M. Green / U.C. Irvine

  6. Effect of Supply/Ground Bounce on Jitter data in data out clock in clock out Rs = 5Ls= 5nH clock out Rs = 0 Ls= 0 clock out Rs = 5 Ls= 5nH data out Prof. M. Green / U.C. Irvine

  7. Advantages of static CMOS gates: • Simple & straightforward design. • Robust operation. • Nearly zero static power dissipation. Disdvantages of static CMOS gates: • Full speed of transistors not exploited due to n-channel & p-channel gate in parallel at load. • Single-ended operation causes current spikes leading to VDD/VSS bounce. • Single-ended operation also highly sensitive to VDD/VSS bounce leading to jitter. Summary of CMOS Gate Performance Prof. M. Green / U.C. Irvine

  8. VDD R R Vout+ Vout- CL CL Vin+ Vin- ISS Current-Mode Logic (CML) CML inverter: • Based on conventional differential pair • Differential operation • Inherent common-mode rejection • Very robust in the presence of common- • mode disturbances (e.g., VDD / VSS bounce) Prof. M. Green / U.C. Irvine

  9. To keep current source transistor in saturation: DC Biasing of CML Inverter VDD + + R R _ _ VS + + _ _ VGS VGS ISS VBIAS Prof. M. Green / U.C. Irvine

  10. Logic Swing & Gain of CML Inverter To achieve full current switching: for correct operation VDD R R VDD-ISSR VDD ISS 0 CL CL VDD VDD-ISSR ISS Prof. M. Green / U.C. Irvine

  11. Small-Signal Behavior of CML Inverter Small-signal voltage gain: rise/fall time constant: (Assuming fanout of 1) Recall for full switching Note: rising & falling time constants are the same Prof. M. Green / U.C. Irvine

  12. Speed vs. Gain in Logic Circuits fast input transition: step response determined by  slow input transition: step response determined by Av Largest possiblegain-bandwidth productis desirable. Prof. M. Green / U.C. Irvine

  13. Relationship between Av,  , and Vswing “large-signal” gain-bandwidth product Larger logic swing preferred for higher gain-bandwidth product Larger Vswing Larger Vmin smaller W/L larger current density Prof. M. Green / U.C. Irvine

  14. Thought Experiment Slower! R R R R ISS ISS Suppose we decrease current density by increasing W/L: Prof. M. Green / U.C. Irvine

  15. Note that the load is only one gate capacitance: CML speed ~ 2.5 times faster than static CMOS n-channel ac simulation to determine fT: CML buffer transient simulation: IG ID fT = 57GHz Prof. M. Green / U.C. Irvine

  16. Once Vswing has been chosen, designer can trade off between gain & bandwidth by parameterizing between R & ISS: Higher speed: ISSR Higher gain: ISSR • Typical Vswing: • Should be large enough to allow sufficient gain-bandwidth product. • Should be small enough to prevent transistors from going into triode. • * CML will still work in triode (unlike BJT), but there is no additional • speed benefit. Prof. M. Green / U.C. Irvine

  17. ISS ISS Other Benefits of CML Gates • Constant current bias VDD / VSS bounce greatly reduced KCL sets this current to be nearly constant. Prof. M. Green / U.C. Irvine

  18. data in data out clock in clock out Rs = 5Ls= 5nH clock out Rs = 0 Ls= 0 clock out Rs = 5 Ls= 5nH data out Prof. M. Green / U.C. Irvine

  19. CML: buffer inverter • Non-inverting buffer available without additional delay: CMOS: tp 2tp buffer inverter Prof. M. Green / U.C. Irvine

  20. Fanout & Scaling of CML Gates R R Vout- Vout+ 1x = Vin+ Vin- ISS R/n R/n Vout- Vout+ nx All voltages unchanged from unit-sized buffer. Currents & power increase by factor of n. = Vin+ Vin- nISS Prof. M. Green / U.C. Irvine

  21. For fanout of n: increases linearly with fanout. Prof. M. Green / U.C. Irvine

  22. to minimize Should set degradation due to interconnect capacitance From interconnect, etc.; assumed not to scale with buffer sizes Power (proportional to n) determined primarily by interconnect capacitance! Prof. M. Green / U.C. Irvine

  23. + VGS _ CML buffer design procedure: • Determine largest allowable ISS (usually limited by electromigration constraints) • Choose “unit-sized” n-channel transistor (typically W/L=20) • Run a series of simulations to determine optimum value of R: • R too small: full current switching not achieved • R too large: slower than necessary • Choose minimum scaling factor after laying out some test buffers of various sizes and determining approximate value of interconnect capacitance Cp. Sub-micron MOSFETs obey square-law characteristics only in a limited region! Mobility reduction (linear) Square-law behavior Weak inversion (exponential) VGS Prof. M. Green / U.C. Irvine

  24. 1. Determine largest allowable ISS standard layout shared drain (1/2 diffusion capacitance) Imax independent of W determined by electromigration limits Prof. M. Green / U.C. Irvine

  25. CML Design Procedure Example R = 900 ISSR = 360mV tp = 10ps R too small Choose: R = 1200 ISSR = 480mV tp = 12ps *R optimum* R = 1500 ISSR = 600mV tp = 14ps R too large Prof. M. Green / U.C. Irvine

  26. Parameterizing Between Gain & Bandwidth ISS = 100 µA R = 4.8 k Av= 9.3 dB BW = 2.6 GHz ISS = 200 µA R = 2.4 k Av= 7.1 dB BW = 5.5 GHz ISS = 400 µA R = 1.2 k Av= 3.9 dB BW = 11.5 GHz Prof. M. Green / U.C. Irvine

  27. Parameterized CML Buffer GSCALE: Global scaling parameter (depends on Cp) MSCALE: Local scaling parameter (depends on fanout or bit rate) GBW: Gain-bandwidth parameter Prof. M. Green / U.C. Irvine

  28. CML with p-channel Active Load Can be used if linear resistors are not available. p-channel load transistors operates in triode region: Increased capacitance and mismatch result Prof. M. Green / U.C. Irvine

  29. Capacitance Comparison (1) gate channel sub Poly resistor: p-channel MOSFET: Prof. M. Green / U.C. Irvine

  30. Capacitance Comparison (2) (Numbers based on TSMC 180nm CMOS process) Cpoly-sub  Cchannel-sub : 0.13 fF/m2 Cdepletion : 1.20 fF/m2 Cchannel-gate : 7.80 fF/m2 Poly resistor: Wpoly= 0.6 Lpoly = 2.5 = 0.1 fF Wchannel= Wdiff= 2.5 µm Lchannel = 0.18 µm Ldiff = 0.3 µm p-channel MOSFET: = 0.9 fF + 1.8 fF + .03 fF = 2.8 fF Prof. M. Green / U.C. Irvine

  31. Capacitance Comparison (3) R = 1.2 k s = 235  Wr = 0.6 µm Lr = 2.5 µm Cres = 0.1 fF Wp = 2.5 µm Ldiff = 0.3 µm Cd2 = 2.8 fF M2 M2 M1 M1 M1 M1 Cd1 = 3.7 fF Cg1 = 5.8 fF Prof. M. Green / U.C. Irvine

  32. Pulse Response ComparisonPWin = 100ps p-channel load (W/L)p = 2.5 µm / 0.18 µm td = 20 ps; PWout = 98 ps resistor load R = 1.2 k td = 16 ps; PWout = 100 ps Prof. M. Green / U.C. Irvine

  33. Eye Diagram Comparisonincluding mismatch effects resistor load = 1.5% mismatch p-channel load = 4% mismatch 160mV gate-referred mismatch DCD ISI Prof. M. Green / U.C. Irvine

  34. MA MA MA MA MB MB Series-Gated CML Topology XOR gate: Common-mode voltage of BP/N critical: • Too low  current source transistor biased in triode • Too high  Transistors MB biased in triode Prof. M. Green / U.C. Irvine

  35. Series-Gated CML (2) VS I1 I2 BP BN ISS Transistors should be biased in saturation to realize maximum gm . Especially important when gate voltages exhibit slow slew rates Slope = gm -ISS Prof. M. Green / U.C. Irvine

  36. VB(cm) = 1.3 VB(cm) = 1.0 VB(cm) = 1.6 DC current: VB(cm) = 1.3 VB(cm) = 1.0 Transient response: (400mV amplitude sine wave applied to BP/BN) VB(cm) = 1.6 t Prof. M. Green / U.C. Irvine

  37. Level-Shifting CML Buffer Used to drive clock inputs of series-gated CML gates VDD Output levels: + Rcm _ R R ISS • DC levels shifted down by ISSRcm • Vswing unchanged Prof. M. Green / U.C. Irvine

  38. CML Select Circuit R R OUTP OUTN AP AN BP BN SELA SELB ISS Be reassigning the inputs, the XOR can be transformed into a Select circuit. Used in a 2:1 multiplexer. SELA AP/N BP/N OUTP/N Prof. M. Green / U.C. Irvine

  39. OUTP OUTN DP DN CKP CKN ISS CML Latch By setting BP/N = OUTP/N, we can construct a CML latch: Prof. M. Green / U.C. Irvine

  40. CML D Flip-Flop XP OUTP XN OUTN DP DN XP XN CKP CKN CKN CKP CKP/N DP/N OUTP/N Output OUTP/N is synchronized with CKP/N falling edge. Prof. M. Green / U.C. Irvine

  41. R R dc operating points VGG IGG CML Latch Design Considerations IGG slope=1/rgg VGG Necessary criterion for bistability: at middle operating point (Equivalent to loop gain = gmR > 1) Prof. M. Green / U.C. Irvine

  42. Avoiding Latch Transparency XP/N “transparent” latch Prof. M. Green / U.C. Irvine

  43. XP XN XP XN GBW parameter can be increased to ensure bistability. R=1000 R=800 R=600 Prof. M. Green / U.C. Irvine

  44. Buffering Clock Signals (1) Clock signals (generated from VCO or clock divider) often drive large capacitive loads. 1x 1x C … n 1x C C Fanout = n For a large fanout, attenuation of clock amplitude will occur. Prof. M. Green / U.C. Irvine

  45. Buffering Clock Signals (2) ktp n x k2 x … k x 1x m stages less attenuation at each stage Now  is increased by k << n Delay = mktp Power = P1(1 + k + k2 + … + n) Power dissipated by first stage As fclock 1/tp then k  1; number of stages and total power become very large. Prof. M. Green / U.C. Irvine

  46. Buffering Clock Signals (3) Since clock signal is made up of a single frequency (+ harmonics), resonance can be used to increase gain with greatly reduced power dissipation. Resonant frequency: at resonance If lossless inductors were available, we could achieve high gain at any frequency simply by choosing the correct inductor value. Prof. M. Green / U.C. Irvine

  47. l pH/m l l On-Chip Passive Elements l l Resistor: t w l Capacitor: l (+ fringing) w d substrate l Inductor: t w Inductance calculation much more complicated! Prof. M. Green / U.C. Irvine

  48. l pH/m l l l t w Special case of Greenhouse result Note for l>> w, L is a weak function of w To increase effective inductance per unit length, we make use of mutual inductance via spiral structure: Prof. M. Green / U.C. Irvine

  49. Modeling of Spiral Inductor 1 2 • Accurate lumped model should include: • Series inductance (self + mutual) & resistance • Skin effect (frequency dependent series resistance) • Interwinding capacitance • Capacitance to substrate • Substrate capacitance & loss number of turns n = 2 • Procedure for constructing lumped model: • 2-port s-parameters over frequency range of interest • (this comes from the inductor simulator) • Choose lumped circuit topology. • Run simulations to find the optimal lumped circuit • element values such that the the circuit s-parameters • are sufficiently close to the inductor’s s-parameters • (can use .net and .optimize in HSPICE) • Design of inductor requires: • inductor simulation package • (e.g., asitic) • trial and error • conversion to lumped element model Prof. M. Green / U.C. Irvine

  50. Modeling of Spiral Inductor (cont.) Link to “asitic” web pages: http://rfic.eecs.berkeley.edu/~niknejad/asitic.html Inductor magnitude impedance vs. frequency • Parameters most relevant to circuit designers: • Inductance • Series resistance • Self-resonant frequency Prof. M. Green / U.C. Irvine

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